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Plant Cell. Jan 2009; 21(1): 216–233.
PMCID: PMC2648098

ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1), ADPG2, and QUARTET2 Are Polygalacturonases Required for Cell Separation during Reproductive Development in Arabidopsis[W]

Abstract

Cell separation is thought to involve degradation of pectin by several hydrolytic enzymes, particularly polygalacturonase (PG). Here, we characterize an activation tagging line with reduced growth and male sterility caused by increased expression of a PG encoded by QUARTET2 (QRT2). QRT2 is essential for pollen grain separation and is part of a small family of three closely related endo-PGs in the Arabidopsis thaliana proteome, including ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1) and ADPG2. Functional assays and complementation experiments confirm that ADPG1, ADPG2, and QRT2 are PGs. Genetic analysis demonstrates that ADPG1 and ADPG2 are essential for silique dehiscence. In addition, ADPG2 and QRT2 contribute to floral organ abscission, while all three genes contribute to anther dehiscence. Expression analysis is consistent with the observed mutant phenotypes. INDEHISCENT (IND) encodes a putative basic helix-loop-helix required for silique dehiscence, and we demonstrate that the closely related HECATE3 (HEC3) gene is required for normal seed abscission and show that IND and HEC3 are required for normal expression of ADPG1 in the silique dehiscence zone and seed abscission zone, respectively. We also show that jasmonic acid and ethylene act together with abscisic acid to regulate floral organ abscission, in part by promoting QRT2 expression. These results demonstrate that multiple cell separation events, including both abscission and dehiscence, require closely related PG genes.

INTRODUCTION

Cell separation events that lead to organ abscission or dehiscence play important roles in plant development, particularly during reproductive processes. Examples include the abscission of leaves and outer floral organs and several processes related to pollination and seed set, fruit maturation, and seed dispersal (Lewis et al., 2006). Depending on individual plant species, entire flowers can abscise in the absence of fertilization and seed set or in unfavorable conditions. Abscission generally occurs at the completion of fruit maturation, often as an aid to seed dispersal, but can also occur during the early stages of fruit development even if seed set is successful. Undesirable abscission events are a major issue in the commercial production of many crops, including fruit loss from premature abscission (e.g., in many perennial horticultural crops) and seed loss from field crops (e.g., canola [Brassica napus]).

Pod shatter is a specialized type of fruit dehiscence event in which the fruit breaks into parts and, because it can be studied in Arabidopsis thaliana, represents one of the best understood types of cell separation. In the family Brassicaceae, which includes Arabidopsis and canola, cell separation generally occurs along the sites of fusion between carpels that compose the fruit. A second separation event then occurs to allow the seed to detach from the maternal plant. Dehiscence of the Arabidopsis silique is very similar to the process in canola (Spence et al., 1996) and has been used as an effective model in which to study pod shatter. During ovary and fruit development, cell fate specification must occur to form the dehiscence zone (DZ), a specialized layer in which cell separation occurs to allow the silique to open. In Arabidopsis, the DZ consists of a few cell layers separating the replum from the edges of the two fused carpels (Spence et al., 1996). Genetic approaches have revealed that several genes encoding transcription factors are required for DZ differentiation (Ferrándiz et al., 2000; Liljegren et al., 2000; Rajani and Sundaresan, 2001; Liljegren et al., 2004). Once the DZ is correctly specified and established, other essential processes include secondary wall formation at the valve margins (Mitsuda and Ohme-Takagi, 2008) and degradation of cell walls, including the middle lamella, in the separation layer. This is thought to occur by the action of proteins involved in cell wall loosening, including polygalacturonases (PGs), β-1,4-glucanase, and expansin (Bonghi et al., 1993; Taylor et al., 1993, 1994; Lashbrook et al., 1994; del Campillo and Bennett, 1996; Cho and Cosgrove, 2000), although this hypothesis is supported by only limited direct evidence.

Other important cell separation events occur in male flower organs during flower development. During pollen development, a separation event is required after meiosis of the pollen mother cell to separate the four microspores. The three quartet (qrt) mutants (qrt1, qrt2, and qrt3) of Arabidopsis are defective in this process and produce tetrad pollen in which microspores fail to separate during pollen development (Preuss et al., 1994). Immunohistochemical analyses suggest that QRT1 and QRT2 are required for pectin degradation of the cell wall surrounding the pollen mother cell during pollen development (Rhee and Somerville, 1998). Recent molecular studies have revealed that QRT1 and QRT3 encode a pectin methylesterase (PME) and an atypical PG, respectively (Rhee et al., 2003; Francis et al., 2006). Although QRT2 has been mapped to the top of chromosome 3 (Preuss et al., 1994), the affected gene has not yet been identified.

Pollen function also requires a second cell separation event in which the mature anthers dehisce to release functional pollen grains at anthesis. Anther dehiscence requires breakdown of the stomium, specialized cells that keep the anther locules closed, and genetic analysis has shown that the plant hormone jasmonic acid (JA) is required for this process. For example, plants lacking ALLENE OXIDE SYNTHASE (AOS) are JA deficient and do not shed pollen (Park et al., 2002; von Malek et al., 2002). At the cellular level, anther dehiscence is similar to silique dehiscence and, like microspore separation, is thought to involve similar cell wall degrading enzymes (Roberts et al., 2002). Later in Arabidopsis flower development, additional cell separation events occur in the floral tissues of the outer three whorls. Several days after anthesis, the sepals, petals, and stamens detach from the flower base to reveal either an unfertilized ovary or a developing silique containing immature seeds.

The cell separation events described above are all thought to involve the degradation of pectin by PGs, although this hypothesis has not been confirmed by genetic evidence. It is also not clear to what extent, if any, the same PGs function in different abscission/dehiscence events, and this uncertainty has contributed to the complex naming system, based on expression in different abscision zones (AZs) and DZs, sometimes used for PGs. Homogalacturonan-rich pectin is commonly found in the middle lamella region of the cell wall where two adjacent cells abut and pectin integrity is important for cell adhesion (MacDougall et al., 1996; Ridley et al., 2001). Endopolygalacturonases (endo-PGs) catalyze random hydrolysis of α-1,4-glycosidic linkages in polygalacturonic acid (GalUA), a polymer that constitutes the main chain of the homogalacturonan region of pectin (Biely et al., 1996). Although there is only limited direct genetic evidence for the physiological importance of individual PGs, correlations have been reported between increasing PG activity and cell separation in fruit ripening and in the shedding of leaves, flowers, and fruit (Taylor et al., 1993; Kalaitzis et al., 1995; Brown, 1997; Kalaitzis et al., 1997). More recently, silencing of tomato (Solanum lycopersicum) abscission-related PGs was shown to increase the break strength of the leaf abscission zone and delay abscission in explants treated with ethylene (Jiang et al., 2008), and a putative Arabidopsis PG has been shown to promote floral organ abscission (González-Carranza et al., 2007). The importance of PG is also illustrated by the ‘Flavr savr’ tomato (Hadfield and Bennett, 1998) and peach (Prunus persica) lacking a functional Melting flesh/Freestone locus (Peace et al., 2005), both of which have reduced expression of a fruit ripening-associated PG and delayed fruit softening.

The best-characterized (fungal) endo-PG enzyme requires four to five consecutive runs of unesterified GalUA residues for cleavage (Benen et al., 1999; Pařenicová et al., 2000). However, little is known about the enzyme activity and substrate specificity of most plant PGs. There are at least 69 and 59 predicted PGs in the Arabidopsis and rice (Oryza sativa) genomes, respectively (Kim et al., 2006; González-Carranza et al., 2007), and it has been suggested that one group of related PGs tend to be expressed in flowers and flower buds, while PGs expressed in vegetative tissues generally belong to other groups (Torki et al., 2000; Kim et al., 2006). The implication is that the diverse potential physiological roles of PGs may be a consequence of differential expression in specific tissues rather than or in addition to differences in enzyme substrate specificity. These questions have been investigated to a limited extent using transgenic plants with altered expression of endo-PGs. For example, overexpression of an apple (Malus domestica) endo-PG in transgenic apples resulted in silvery colored leaves and premature leaf shedding due to reduced cell adhesion in leaves and in the leaf abscission zone (Atkinson et al., 2002). By contrast, ectopic expression of a tomato fruit-specific endo-PG (pTOM6) in tobacco (Nicotiana tabacum) revealed no leaf phenotype or detectable alterations in cell wall pectins, although the pTOM6 protein was properly processed and localized in the cell wall of tobacco leaves (Osteryoung et al., 1990). While these results suggest that individual PGs may have different substrate specificities or requirements for activity, it is not clear how overall PG activity is regulated in different plant tissues in vivo.

Here, we report on the characterization of an activation tagging line with reduced growth and male sterility caused by increased expression of a PG encoded by At3g07970. This gene has been previously named QRT2 based on tetrad pollen production in mutant qrt2 plants (Preuss et al., 1994). QRT2 is part of a small family of three closely related endo-PGs in the Arabidopsis proteome, and our genetic analysis demonstrates that two of these PGs are required for silique dehiscence (ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 [ADPG1] and ADPG2), two are required for normal floral organ abscission (ADPG2 and QRT2), and all three are required for normal anther dehiscence. Expression analysis based on transcriptional β-glucuronidase (GUS) constructs is consistent with the observed mutant phenotypes. Based on the transcriptional regulation of all three genes by JA, and of ADPG2 by ethylene, we show that JA and ethylene act together with abscisic acid (ABA) to regulate floral organ abscission, in part by promoting QRT2 expression.

RESULTS

Activation Tagging of the At3g07970 Gene

From a population of several hundred independent activation tagging lines, we identified one line, designated activation tagging line no. 3 (AT3), in which most anthers failed to shed pollen and the majority of flowers did not set seeds (see Supplemental Figure 1 online). The selectable marker (the bar gene) cosegregated with the mutant phenotype through several generations, suggesting that there was a single T-DNA insertion site that might be causing activation of an adjacent gene. The T-DNA insertion site adjacent to the left border was identified using thermal asymmetric interlaced–PCR (TAIL-PCR) (Liu et al., 1995) and found to be located within the second exon of At3g07960, which encodes a predicted protein similar to 1-phosphatidylinositol-4-phosphate 5-kinase (Figure 1A). The genes closest to the T-DNA enhancer sequence were At3g07950 (predicted to encode an unknown protein) and At3g07970 (predicted to encode a PG), and the T-DNA was located ~3.5 and 5.7 kb from their putative translational start codons, respectively (Figure 1A). RT-PCR analysis of these three genes revealed that compared with the wild-type, only At3g07970 was clearly overexpressed in flower buds and rosette leaves of AT3 plants (Figure 1B). The reduced expression of At3g07960 in AT3 flower buds is presumably due to the T-DNA insertion in this gene. Since the AT3 phenotype is genetically dominant, it is likely that reduced expression of At3g07960 does not contribute to the AT3 phenotype (see below). Quantitative real-time RT-PCR (qRT-PCR) revealed that the transcript level of At3g07970 in AT3 flower buds was ~350 times higher than in wild-type flower buds (Table 1).

Figure 1.
Activation Tagging of At3g07970.
Table 1.
Change in QRT2 Transcript Levels in Flower Buds of AT3 and 35S Enhancer-ProQRT2:QRT2 Transgenic Plants Relative to the Wild-Type

To determine if the AT3 mutant phenotype is caused by activation of the At3g07970 gene, we generated transgenic plants with the 4x 35S enhancer sequence fused to a genomic fragment including introns, exons, and the putative promoter (2085 bp 5′ from the predicted translation initiation site) of the At3g07970 gene (Figure 1C). Of 30 independent transgenic lines, 14 exhibited anther defects and a dwarf rosette with curled leaves similar to the AT3 line and failed to set seeds (Figure 1D). These lines could be further divided into those with either weak (seven lines) or strong (seven lines) phenotypes. The weak lines had a less severe rosette phenotype and flowers similar to those of AT3. The strong lines had severely dwarfed rosettes with extremely curled leaves and defective flowers that often failed to fully open with abnormal petals and completely indehiscent anthers (Figure 1E).

To confirm overexpression of At3g07970 in these transgenic plants, we performed qRT-PCR using flower buds from selected lines and found that relative to wild-type flower buds, At3g07970 expression in lines with a strong phenotype was >2000 times higher, while expression lines with a weak phenotype was between 10 and 40 times higher (Table 1). For the original AT3 line, the increase in At3g07970 expression was intermediate between the weak and strong lines (Table 1). Thus, At3g07970 expression was strongly correlated with the observed phenotypes, demonstrating that the AT3 phenotype is caused by activation of the At3g07970 gene.

At3g07970 Is QRT2

To examine the biological role of At3g07970, we identified loss-of-function alleles of At3g07970 using T-DNA insertion lines obtained from the ABRC (Figure 2A). These two mutant lines are likely to represent null or near-null alleles because expression could not be detected by qRT-PCR using 40 cycles and gene-specific primers on either side of the T-DNA insertion sites (see Supplemental Figures 2 and 3 online). Plants homozygous for either of the two independent mutant alleles of At3g07970 had no apparent phenotype compared with the wild-type, with the exception that mature pollen grains were arranged in a tetrad similar to the phenotype of the qrt mutants (Preuss et al., 1994). Although the QRT2 gene was not identified, previous mapping experiments revealed that this locus is linked to GAPC at the top of chromosome 3 (Preuss et al., 1994), in the same region as At3g07970. In addition, microscopy observations using an antibody that recognizes pectin indicated that QRT2 may be required for cell type–specific pectin degradation to separate microspores (Rhee and Somerville, 1998). As these results are consistent with At3g07970 encoding QRT2, we performed allelism tests by crossing the original qrt2-1 allele with the two At3g07970 T-DNA insertion mutants (hereafter called qrt2-2 and qrt2-3). Pollen grains of all F1 progeny exhibited the tetrad phenotype, confirming that At3g07970 is QRT2. Consistent with this result, F1 progeny from control crosses between qrt2-2 and the wild-type or between qrt2-3 and the wild-type produced only monad pollen.

Figure 2.
adpg1, adpg2, and qrt2 Loss-of-Function Phenotypes.

The qrt2-1 allele was generated using ethyl methanesulfonate (Preuss et al., 1994). To identify the mutation present in qrt2-1, a 2735-bp genomic fragment, corresponding to the QRT2 open reading frame with 359 bp upstream of the translational start codon and 10 bp downstream of the stop codon, was cloned from qrt2-1 and wild-type Landsberg erecta (Ler) plants and sequenced. A single nucleotide mutation was found in qrt2-1 that changed the predicted amino acid sequence from Val (GTG) to Ala (GCG) at position 372 (Figure 2A; see Supplemental Figure 2A online). Val is a hydrophobic amino acid, and sequence analysis revealed that a hydrophobic amino acid (Val, Leu, or Ile) in this position is highly conserved in PGs from plants and fungi (Markovic and Janecek, 2001; Kim et al., 2006). qRT-PCR from flower buds revealed no detectable change in QRT2 expression level between wild-type Ler and qrt2-1 (see Supplemental Figure 2B online), consistent with the qrt2-1 mutant phenotype being caused by a single amino acid substitution that decreases protein activity.

Loss of PG Function Prevents Pod Shatter

Phylogenetic analysis of predicted Arabidopsis PGs revealed that there are two PGs closely related to QRT2, encoded by At3g57510 and At2g41850 (Kim et al., 2006). The putative PG encoded by At3g57510 has previously been given several names, including ADPG1, PGA9, pga1;6, and SAC70 (Jenkins et al., 1999; Sander et al., 2001). Although the putative PG encoded by At2g41850 has been referred to as PGAZAT (González-Carranza et al., 2002), we refer to this protein as ADPG2 based on its expression pattern and biological role (see below).

The similarity between QRT2 and ADPG1 is potentially revealing because the latter has previously been identified as a putative ortholog of RAPESEED DEHISCENCE ZONE POLYGALACTURONASE1 (RDPG1) from canola. RDPG1, also called SAC66, has been proposed to be involved in breakdown of the middle lamella of the separation zone during pod shatter, although functional analysis of ADPG1 or RDPG1 in planta has not been reported (Jenkins et al., 1996, 1999; Petersen et al., 1996; Sander et al., 2001). Based on the similarities between the predicted proteins and the observation that ADPG1 is also expressed in the silique DZ, a similar role has been proposed for ADPG1 in Arabidopsis (González-Carranza et al., 2007). Consistent with possible roles in fruit abscission processes, expression data also revealed that ADPG1 and ADPG2 are expressed predominantly in floral tissues (Grennan, 2006; Kim et al., 2006; González-Carranza et al., 2007; see below).

To examine the biological role of ADPG1 and ADPG2, we identified two independent loss-of-function T-DNA insertion mutant alleles for each gene obtained from ABRC (SALK lines) and the Max-Planck Institute (GABI line) (Figure 2A). With the possible exception of adpg1-1, these lines are likely to represent null or near-null alleles because expression could not be detected by qRT-PCR using 40 cycles and gene-specific primers on either side of the T-DNA insertion sites (see Supplemental Figure 3 online). For adpg1-1, the presence of the T-DNA inserted into the third intron did not completely prevent expression based on a similar analysis. However, qRT-PCR revealed that the transcript level in adpg1-1 was 18 times less than the wild-type (see Supplemental Figure 4A online). In addition, sequence analysis of RT-PCR products revealed that the entire 4th exon, consisting of 21 nucleotides and encoding seven amino acids, was missing from the adpgp1-1 mRNA (Figure 2A, missing exon in adpg1-1 in red; see Supplemental Figure 4B online). Since this sequence is still present in genomic DNA of adpg1-1, it is likely that the T-DNA insertion in the third intron causes incorrect splicing which results in exon 3 being joined directly to exon 5. The effect, if any, that this has on protein activity is not known.

Plants homozygous for mutant alleles adpg1-1 or adpg1-2 were indistinguishable from the wild-type, including the release of monad pollen, with the exception of impaired pod shatter due to failure of the valve to properly detach from the central part of the silique (Figure 2B). However, pod shatter did occur when mature, dry adpg1 siliques were gently compressed to increase pressure on the valve DZs. Once opened in this way, seed abscission appeared normal. By contrast, under normal growing conditions, single adpg2 mutants appeared similar to wild-type plants in terms of pod shatter and also produced monad pollen. However, reduced pod shatter was observed in adpg2-1 and adpg2-2 plants (and in double mutants with qrt2) when watering was ceased before overall plant senescence was complete (see Supplemental Table 1 online). To determine if the ADPG1, ADPG2, and QRT2 genes are functionally redundant, double and triple mutants were generated by crossing. Siliques of the double mutants adpg1-1 adpg2-1 and adpg1-2 adpg2-2 exhibited a more severe phenotype than did those of the adpg1 single mutants and failed to dehisce even if compressed. When siliques were cut open, seed abscission again appeared normal, as did pollen grain separation. Triple mutants were also constructed with the two qrt2 T-DNA alleles. In terms of pod shatter and seed abscission, adpg2 qrt2 double mutants were similar to adpg2 single mutants, double mutants lacking both ADPG1 and QRT2 appeared identical to adpg1 single mutants, and the adpg1-1 adpg2-1 qrt2-2 and adpg1-2 adpg2-2 qrt2-3 triple mutants resembled the adpg1 adpg2 double mutants (see Supplemental Table 1 online). These results suggest that ADPG1 and ADPG2 have partially redundant roles in Arabidopsis pod shatter. As expected, all plants homozygous for qrt2 produced tetrad pollen (Figure 2E).

As pod shatter represents a well-characterized cell separation event (Roberts et al., 2002; Lewis et al., 2006), the silique DZ was used to investigate pectin levels associated with reduced PG function. Consistent with a role for ADPG1 and ADPG2 in cell separation in the final stages of pod shatter, transverse sections of maturing siliques just prior to pod shatter (stage 17b; Roeder and Yanofsky, 2006) revealed that adpg1 adpg2 qrt2 triple mutants were morphologically similar to the wild-type (Figure 2C). At stage 18, when silique dehiscence normally takes place (Roeder and Yanofsky, 2006), cell separation occurred in wild-type siliques but not in adpg1 adpg2 qrt2 triple mutant siliques (Figure 3). Although it has been reported previously that cell separation occurs between cells without cell rupture (Spence et al., 1996), we observed separation between intact cells in addition to breaking of nearby cells (Figure 3B) in the DZ of wild-type siliques. Pectic polysaccharides, including potential PG substrates, can be visualized using monoclonal antibodies that recognize unesterified (JIM5) and esterified (JIM7) regions of pectin (e.g., Francis et al., 2006). For example, JIM5 has been used previously to show that, in contrast with the wild-type, the primary cell wall that surrounds the microspores is not degraded in qrt2-1 anthers (Rhee and Somerville, 1998). If ADPG1 and ADPG2 degrade the majority of pectin present in the silique DZ, clear differences would be expected in the pectin levels of wild-type and mutant siliques. However, comparison between transmission electron microscopy (TEM) sections of wild-type and adpg1 adpg2 qrt2 triple mutant siliques at stage 18 did not reveal any obvious differences in the overall levels of pectin (Figures 3D and 3E), suggesting that the degradation of only a small proportion of the total JIM5-recognized pectin is required for pod shatter.

Figure 3.
ADPG1 and ADPG2 Are Required for Pod Shatter.

Loss of PG Function Delays Anther Dehiscence

In contrast with the single and double mutants, the adpg1 adpg2 qrt2 triple mutants consistently exhibited delayed anther dehiscence in early flowers (Figure 2D) due to failure of the stomium to separate at stage 13 (Figure 2F; Sanders et al., 1999). The anther phenotype progressively became weaker as the plant continued to grow and produced more flowers. The delay in pollen release and pollination in early flowers did not prevent seed set in the triple mutants, although it appeared that a small proportion of flowers were not pollinated. These results suggest that functional redundancy exists between ADPG1, ADPG2, and QRT2 in anther dehiscence. Furthermore, it is clear that a common PG-dependent mechanism is involved in cell separation events in the anther and silique DZ and during microspore development.

Loss of PG Function Delays Floral Organ Abscission

After investigating several methods to assess floral organ abscission, firmly pressing each flower on intact plants grown under standard conditions was chosen as the most reliable and consistent (see Supplemental Figure 5 online). There was a clear delineation between younger flowers with floral organs that did not detach when pressed and older flowers that lost their floral organs easily. In particular, this approach resolved the issue of floral organs being lost during plant growth and handling. Analysis of the adpg1 adpg2 and qrt2 single mutants in this manner revealed that ADPG2 and QRT2 promote floral organ abscission (Figure 4), consistent with previous reports that ADPG2 promotes this cell separation event (González-Carranza et al., 2007). The adpg2 qrt2 double mutant exhibited a slightly greater delay than either single mutant, while the adpg1 adpg2 qrt2 triple mutant appeared similar to the adpg2 qrt2 double mutant, consistent with no role for ADPG1 in this process.

Figure 4.
ADPG2, QRT2, Ethylene, JA, and ABA Are Required for Normal Floral Organ Abscission.

ADPG1, ADPG2, and At1g48100 Encode PGs

Based on the similarity of their predicted amino acid sequence to plant PGs for which functional assays have been reported (Hadfield et al., 1998; Degan et al., 2001), ADPG1, ADPG2, QRT2, and a less closely related putative PG encoded by At1g48100 are expected to encode PGs. However, as there is no direct evidence of PG activity for these proteins, we attempted to directly measure PG activity of heterologous proteins in vitro. Truncated versions of these four proteins, lacking the N-terminal hydrophobic region predicted to function as a signal peptide, were expressed in Escherichia coli as His-tagged recombinant fusion proteins. His-GFP (green fluorescent protein), His-ADPG1, His-ADPG2, and His-At1g48100 proteins of the expected size were successfully expressed in E. coli based on SDS-PAGE followed by Coomassie blue staining. These His-tagged recombinant proteins were purified using a nickel-nitrilotriacetic acid agarose (Ni-NTA) column and their activity assayed using polygalacturonic acid as the substrate (Table 2). All three proteins possessed PG activity, and the specific activity of His-ADPG2 was ~30 times higher than that of His-ADPG1, whereas activity of His-GFP could not be detected. The activity of the His-At1g48100 recombinant protein was 40 and 100 times higher than that of His-ADPG2 and His-ADPG1, respectively.

Table 2.
Specific Activity of PGs in Vitro

Expression of ADPG1, ADPG2, and QRT2

Previous work has investigated the expression pattern of both ADPG1 and ADPG2. In Arabidopsis, ADPG1 expression has previously been localized to the DZ of anthers and maturing siliques by immuno-electron microscopy with an anti-RDPG1 antibody, by comparison with an RDPG1:GUS reporter (Sander et al., 2001) and by using an ADPG1:GUS reporter (González-Carranza et al., 2007). The expression domain for ADPG2 was examined using transcriptional constructs in which either GUS or GFP was under the control of the presumed ADPG2 promoter (González-Carranza et al., 2002, 2007). Based on this analysis, ADPG2 has been reported to be expressed in roots and in the abscission zone of the sepals, petals, and stamens of flowers and is upregulated by ethylene in these tissues (González-Carranza et al., 2007). ADPG2 expression has not been reported in anthers or silique DZs. Previous attempts to monitor At3g07970 (QRT2) expression using GUS were not successful (González-Carranza et al., 2007), although this gene is expressed in flowers undergoing floral organ abscission (Kim and Patterson, 2006).

Expression of ADPG1, ADPG2, and QRT2 was initially examined using qRT-PCR with RNA from different plant tissues (Figure 5). Consistent with previous reports, ADPG1 was the most highly expressed of the three genes and was detected predominately in flower buds (which included anthers prior to anthesis) and siliques just before pod shatter. ADPG2 was also expressed in flower buds and mature siliques as well as in roots. QRT2 was expressed predominantly in roots with lower expression levels in rosette leaves, flower buds (see also Figure 1B), and siliques. In addition, in mature siliques harvested just after they turned yellow, but before pod shatter, the seeds were removed from the remaining fruit tissue and RNA extracted separately. For all three genes, the majority of expression occurred in the non-seed tissue (Figure 5B).

Figure 5.
Tissue-Specific Expression of ADPG1, ADPG2, and QRT2.

To further investigate the expression patterns of ADPG1, ADPG2, and QRT2, promoter:GUS transcriptional constructs were transformed into wild-type plants. Initially, we tested GUS staining from 8 to 12 independent transgenic lines for each construct to confirm that GUS activity was consistently detected in the same tissues. At least two representative lines were selected for further analysis.

Consistent with the loss-of-function phenotypes and the expression analysis described above, the ADPG1:GUS reporter was predominantly expressed in silique DZs (Figure 6A) and in anthers just prior to anthesis (Figure 6D). Expression was also observed in seed abscission zones (Figure 6A), although we have not observed any obvious defect in seed abscission in adpg1 mutants. GUS staining in the silique DZ was first detected in fertilized pistils and increased throughout silique development until reaching a maximum in fully matured siliques just prior to pod shatter, consistent with the results obtained by qRT-PCR (Figure 5). In elongating siliques, GUS staining was first detected in the DZ at the base of siliques and spread along the entire DZ as the silique matured. Cross sections of anthers from ADPG1:GUS lines confirmed that ADPG1-GUS was expressed in stomium cells, which allow anther dehiscence to occur (Keijzer, 1987), the apical tip of the anther filament (Figure 6J), and the DZ of siliques (Figure 6K).

Figure 6.
ADPG1, ADPG2, and QRT2 Are Expressed at Sites of Cell Separation.

To further investigate the expression of ADPG1 in the silique DZ and the seed AZ, mutants with defects in these processes were examined. Several genes that are required for correct formation of the silique DZ have been identified, including INDEHISCENT (IND). IND encodes a putative basic helix-loop-helix transcription factor and forms part of a large gene family in Arabidopsis. Siliques on ind mutants do not shatter due to a failure to develop specialized cells at the junction between the carpels and the replum that later form the DZ (Liljegren et al., 2004). Compared with wild-type siliques, ADPG1:GUS expression was not detectable at the junction between a carpel and replum, where the DZ fails to form in this mutant (Figures 6O and 6P). By contrast, ADPG1:GUS expression was still present at the site of the seed AZ, consistent with normal seed abscission in the ind mutant. Unfertilized ovules also expressed ADPG1:GUS in the funiculus, despite the fact that only fertilized seeds abscise.

HECATE3 (HEC3) encodes a protein closely related to IND that is required for normal functioning of female reproductive tissues (Gremski et al., 2007). In addition to these phenotypes, hec3 mutants also fail to form a seed AZ, and seed abscission does not occur (Figure 6O). Consistent with a role for ADPG1 in seed abscission, ADPG1:GUS was not expressed in the funiculus of fertilized seeds or unfertilized ovules in hec3 plants (Figure 6P). However, pod shatter occurs normally in hec3 plants, and ADPG1:GUS was expressed in the DZ of hec3 siliques (Figures 6O and 6P). These results suggest that IND and HEC3 are required for normal expression of ADPG1 in the silique DZ and seed AZ, respectively.

In agreement with previous reports and qRT-PCR analysis (Figure 5), ADPG2:GUS was expressed in the AZs of sepals, petals, and stamens in flowers just prior to floral organ abscission (Figure 6B) and at the site of lateral root emergence (Kim and Patterson, 2006; González-Carranza et al., 2007). ADPG2:GUS was also expressed early in anther development, at around the time of microspore separation (Figure 6L), later in the anther DZ just prior to dehiscence (Figure 6B), and in the DZ of maturing siliques (Figure 6E). These observations, obtained using less-stringent GUS staining conditions to increase sensitivity, are consistent with the observed role of ADPG2 in anther dehiscence and pod shatter and the qRT-PCR analysis (Figure 5) described above but are inconsistent with previously published results (González-Carranza et al., 2007). A likely explanation for this discrepancy is differences in the presumed promoter sequence used. We used 2177 bp upstream from the predicted translation initiation site, whereas González-Carranza et al. (2007) used 1476 bp of the ADPG2 promoter which lacked the 25 bp of 5′ untranslated ADPG2 sequence. Based on GUS analysis, ADPG2 was also expressed in germinating seeds, at the point at which the radicle broke through the seed coat during germination (Figure 6I), a process that involves cell separation (Roberts et al., 2002). No defect in seed germination was observed, presumably because other PGs are also expressed in this tissue (González-Carranza et al., 2007).

The QRT2:GUS reporter was predominantly expressed in the abscission zones of sepals, petals, and stamens in flowers just prior to floral organ abscission (Figure 6C), similar to the reported expression pattern of ADPG2 and consistent with previous results obtained from a time course of floral organ abscission (Kim and Patterson, 2006). A previous attempt to drive GUS expression by the presumed At3g07970 (QRT2) promoter did not result in detectable GUS activity (González-Carranza et al., 2007), presumably because this construct used 1492 bp, while we used 2085 bp of 5′ sequence. QRT2:GUS was also expressed early in anther development, at around the time of microspore separation (Figure 6N), and later just prior to anther dehiscence (Figure 6M). GUS staining was also observed in mature siliques at pod shatter, consistent with qRT-PCR analysis (Figure 5). In contrast with ADPG1 and ADPG2 expression in mature siliques, and consistent with no detectable role for QRT2 in pod shatter, QRT2:GUS was expressed only in the vascular bundles in the replum and at the central region of the valve. These data are consistent with the roles for QRT2 in microspore separation, anther dehiscence, and floral organ abscission described above.

Although expression of both ADPG2 (González-Carranza et al., 2007) and QRT2 (Figure 6H) was detected in at the site of lateral root emergence, where cell separation occurs (Roberts et al., 2002), no obvious root phenotype was observed in either single mutant or in the adpg2 qrt2 double mutant. Finally, the expression of QRT2 throughout the plant, albeit in very localized regions, is consistent with the defects observed in AT3 flowers and vegetative tissues (Figure 1; see Supplemental Figure 1 online).

Complementation of the Pod Shatter Defect

To further confirm the biological role of the PG encoded by ADPG1, we introduced a construct containing the putative ADPG1 promoter fused to the ADPG1 cDNA into the adpg1-1 adpg2-1 qrt2-2 triple mutant. As the adpg2 qrt2 double mutant does not have a detectable pod shatter phenotype under normal conditions, complete complementation of the adpg1 lesion will restore pod shatter to the wild-type. Twenty-one of 24 independent transgenic lines showed normal pod shatter (Figure 7), suggesting that the promoter region used is sufficient for functional expression of the ADPG1 cDNA. This result also suggests that the intron and 3′ untranscribed sequences of ADPG1 (Figure 2A) are not essential for gene function. To investigate possible functional specificity between PGs, we used the same ADPG1 promoter sequence to drive expression of either the QRT2 cDNA or the cDNA of At1g48100, which encodes a putative PG from a related clade to ADPG1/ADPG2/QRT2 (BLAST score for ADPG1 versus the At1g48100 gene product is 6 × 10−68; Kim et al., 2006). Both constructs were transformed into the adpg1-1 adpg2-1 qrt2-2 triple mutant and assayed for their ability to restore pod shatter (Figure 7). In 15 of 22 independent ADPG1:QRT2 transgenic lines, the pod shatter phenotype of the adpg1 adpg2 qrt2 triple mutant was partially complemented. By contrast, none of the 28 ADPG1:At1g48100 transgenic lines assayed were complemented for pod shatter. To eliminate the possibility that the level of complementation was limited by the transcript level of QRT2 or At1g48100 driven by the ADPG1 promoter, we performed qRT-PCR using transgene-specific primers. Compared with transcript levels of ADPG1 in the triple mutant, those of QRT2 and At1g48100 were similar or higher (Figure 7). For the two functional constructs, the transcript levels of ADPG1 and QRT2 were correlated with the degree of complementation of the pod shatter phenotype, although a lower level of ADPG1 expression was required for complementation compared with QRT2. Thus, based on this assay for pod shatter, ADPG1 is more active than QRT2 and At1g48100 is not active.

Figure 7.
Complementation of the Pod Shatter Defect.

Hormonal Regulation of Floral Organ Abscission

Cell separation events are highly controlled processes, and plant hormones have often been implicated in regulating organ abscission. Consistent with previous reports that ethylene mutants have delayed floral organ abscission (Bleecker and Patterson, 1997; Chao et al., 1997; Patterson, 2001; Patterson and Bleecker, 2004), ADPG2 expression is increased by exogenous ethylene in the AZ of floral organs (González-Carranza et al., 2002) and genetic analysis revealed that ADPG2 promotes floral organ abscission (Figure 4; González-Carranza et al., 2007).

Genetic analysis of JA biosynthesis and signal transduction mutants has shown that this hormone plays an important role in anther development and dehiscence. Using a JA-deficient mutant, a recent microarray study revealed that the transcript levels of ADPG1, ADPG2, and QRT2 were upregulated ~10-fold after JA treatment of anthers (Mandaokar et al., 2006). This result, together with the delayed anther dehiscence phenotype of the adpg1 adpg2 qrt2 triple mutant and induction of QRT2:GUS expression in aos anthers treated with 1 mM methyl jasmonate (Figure 6F), suggests that the defective anther phenotype of JA mutants is partly due to reduced expression of these PG genes.

The results described above raise the possibility that JA and ethylene may act together to regulate different cell separation events, such as floral organ abscission, in part by promoting expression of ADPG1, ADPG2, or QRT2. To test this hypothesis, we examined floral organ abscission in mutants lacking ETHYLENE INSENSITIVE2 (EIN2; Alonso et al., 1999) and AOS as described above (Figure 4). Floral organ abscission was delayed in the JA-deficient aos and ethylene-insensitive ein2-1 single mutants and in the ein2 aos double mutant, confirming that both hormones promote this cell separation process. The fact that floral organ abscission eventually occurred in the ein2 aos double mutant demonstrates that other signals can also promote abscission in the absence of ethylene and JA. Because of its proposed role in a range of senescence/abscission events, the hormone ABA is a good candidate for at least a component of this signal. ABA-deficient mutants, such as those that lack ABA DEFICIENT2 (ABA2) (Cheng et al., 2002; González-Guzmán et al., 2002), display normal floral organ abscission, consistent with important roles for ethylene and JA (Figure 4). To test the relative contributions of these three hormones, we generated the ein2-1 aba2-2 double and ein2-1 aos aba2-2 triple mutants. Floral organ abscission was delayed in the double mutant (similar to the ein2-1 single mutant) and extremely delayed in the triple mutant (Figure 4), demonstrating partially redundant roles for all three hormones in this abscission event. Apart from delayed floral organ abscission, flowers of the ein2, aba2, and ein2 aba2 double mutant appeared to develop and senesce normally. By contrast, flower development and senescence was delayed in the aos, ein2 aos, and ein2 aos aba2 mutants compared with wild-type plants (Park et al., 2002; von Malek et al., 2002; see Supplemental Figure 5 online).

To determine if ethylene, JA, and ABA can regulate PG expression in floral organ AZs, a segregating F2 population from a cross between the ein2 aos aba2 triple mutant and a representative QRT2:GUS line was examined. Consistent with segregation of the three hormone-related genes, plants in this population exhibited floral organ abscission phenotypes ranging from similar to wild-type to similar to the ein2 aos aba2 parent (Figure 4E). Approximately 75% of the progeny chosen for analysis carried the QRT2:GUS reporter, consistent with the segregation of a single transgene locus. This approach enabled the relationship (if any) between QRT2 expression and abscission to be determined over a wide range of floral organ abscission phenotypes. A very strong correlation was observed between the position of the youngest flower expressing QRT2:GUS in the floral AZ and the number of flowers retaining floral organs after firm pressing (Figure 4E). In all plants examined, QRT2:GUS was first expressed either in the floral AZ of the youngest flower in which floral organ abscission occurred or in the AZ of the next-youngest flower just prior to abscission of the floral organs. Supporting a causal role for QRT2 in cell separation, no plants were observed in which floral organ abscission preceded expression of QRT2:GUS in the floral organ AZ. These results suggest that all three hormones are required for normal QRT2 expression in floral organ AZs. While ethylene may directly regulate QRT2 expression in the floral organ AZ, JA and ABA may act indirectly via roles in promoting flower senescence.

DISCUSSION

PGs have been suggested to play critical roles in cell separation in plants during a number of physiological processes involving abscission or dehiscence of plant organs and tissues. For example, an increase in the activity and protein level of PGs can be detected in the cell separation zone just prior to pod dehiscence (Sander et al., 2001). However, despite a range of indirect evidence, relatively little experimental data using loss-of-function alleles of genes known to encode PGs have been reported (Lewis et al., 2006). In addition to ‘Flavr Savr’ tomato, which has reduced PG expression and delayed fruit softening (Dellapenna et al., 1986; Sheehy et al., 1988; Smith et al., 1990), QRT3 has been shown to encode a PG required for separation of microspores during pollen development (Rhee et al., 2003). Based on a single loss-of-function allele, González-Carranza et al. (2007) have also recently reported that ADPG2 promotes floral organ abscission, although these authors did not confirm PG activity for ADPG2.

Using recombinant proteins, we have demonstrated that ADPG1, ADPG2, and At1g48100 encode functional PGs. The ability of an ADPG1:QRT2 construct to partially complement the adpg1 adpg2 qrt2 pod shatter phenotype also confirms that QRT2 is a PG, consistent with its proposed role in pectin degradation (Rhee and Somerville, 1998). Supporting an important role for pectin in plant growth, overexpression of QRT2 causes dwarfism and male sterility. The simplest explanation for this result is that increased QRT2 expression leads to abnormal cell–cell adhesion, which inhibits growth and flower function.

As overexpression studies provide only limited information on normal gene function, we have analyzed loss-of-function alleles for ADPG1, ADPG2, and QRT2 and identified important physiological roles for these genes in several cell separation events during reproductive development (Table 3). In addition, Jiang et al. (2008) have recently shown that PGs are also involved in leaf abscission in tomato. Taken together, these results support the conclusion that multiple, and possibly all, cell separation/abscission/dehiscence events in plants use a common PG-dependent mechanism. In Arabidopsis, combinations of ADPG1, ADPG2, and QRT2 appear to be involved in all cell separation processes associated with reproductive development so far examined.

Table 3.
Distinct and Overlapping Roles of ADPG1, ADPG2, and QRT2 in Separation Events during Arabidopsis Development

PGs Are Required for Microspore Separation

Previous genetic analyses have shown that like QRT1 and QRT3, QRT2 is required during microsporogenesis for the separation of the developing pollen grains (Preuss et al., 1994; Rhee and Somerville, 1998), and we have confirmed this result with two additional loss-of-function qrt2 alleles. QRT1 and QRT3 encode a PME and a PG relatively distantly related to QRT2, respectively (Rhee et al., 2003; Francis et al., 2006). Pectins are synthesized in Golgi bodies and secreted from cells in a highly methylesterified form (Micheli, 2001; Schols and Voragen, 2002). PME catalyzes pectin demethylesterification and appears to increase the ability of PG to cleave pectin (Tucker and Seymour, 2002). Consequently, an attractive model for microspore separation is that pectin is first demethylated by QRT1 and subsequently degraded by PGs, such as QRT2 and QRT3. Although both QRT2 and QRT3 encode PGs, they are not functionally redundant because loss of either gene prevents microspore separation. Structural comparison of pollen grain tetrads using scanning electron microscopy of the qrt mutants has revealed that qrt1 and qrt2 are structurally similar to the wild-type, with the exception of the pollen fusion phenotype. By contrast, qrt3 pollen grains frequently had a layer of material deposited on the surface of the distal region of the pollen grain tetrads (Rhee et al., 2003), suggesting that QRT2 and QRT3 have distinct roles during pollen development. Since the structure of qrt1 pollen is similar to that of qrt2, it is possible that pectin demethylesterified by QRT1 may be used as a substrate by QRT2 rather than by QRT3.

The qrt2-1 allele is predicted to encode a mutant protein with a single amino acid substitution (Val to Ala) at position 372 compared with the wild-type QRT2 protein, suggesting that this amino acid is important for enzyme activity. Val is a highly hydrophobic and aliphatic amino acid, as are Leu and Ile. Phylogenetic analysis reveals that Val, Leu, or Ile at the position corresponding to the qrt2-1 lesion is highly conserved in PGs from plants and fungi, supporting an important role for this residue in enzyme function. Although the three-dimensional structure of PGII from the phytopathogenic fungus Aspergillus niger has been determined, and site-directed mutagenesis used to identify several critical amino acid residues required for activity and substrate binding (van Santen et al., 1999; Armand et al., 2000; Páges et al., 2000), the role of this Val/Leu/Ile residue has not yet been clarified.

PGs Are Required for Pod Shatter and Normal Anther Dehiscence

The two Arabidopsis proteins most closely related to QRT2 are encoded by the duplicate genes ADPG1 and ADPG2 (Kim et al., 2006). Based on its expression pattern in the DZ of maturing siliques, ADPG1 has previously been suggested to be involved in pod shatter (González-Carranza et al., 2007). Genetic analysis revealed that both ADPG1 and ADPG2 are partially functionally redundant for silique dehiscence (Figures 2 and and3),3), a conclusion supported by the observation that ADPG2 is also expressed in the silique DZ prior to pod shatter (Figure 6E). A role for ADPG1 in pod shatter is also supported by the absence of detectable expression of this gene at the valve margin of the nonshattering ind mutant. In addition, although no defect in seed abscission was observed in the adpg1 adpg2 qrt2 triple mutant, ADPG1 was not detected in funiculi of the seed abscission-defective hec3 mutant. This result suggests that ADPG1, along with PGs other than ADPG2 and QRT2, are required for seed abscission.

Functional redundancy between related PGs is also observed in anther dehiscence such that the adpg1 adpg2 qrt2 triple mutant has delayed pollen release, while single and double mutants have no detectable anther defect. The fact that anthers of the triple mutant do eventually dehisce suggests that other PGs may also contribute to this process. Based on phylogenetic and expression analyses (Grennan, 2006; Kim et al., 2006; González-Carranza et al., 2007), At1g80170 is a good candidate to encode an additional PG involved in anther dehiscence.

PGs Are Required for Normal Floral Organ Abscission

Floral organ abscission was delayed, but not completely prevented, in the adpg2 and qrt2 single mutants and in the adpg2 qrt2 double mutant (Figure 4), consistent with the expression of both genes in the floral organ AZ (Figure 6). By contrast, ADPG1 is not expressed in this tissue and does not appear to be required for this abscission process. Based on expression analysis, At2g43890 and At2g43880 (Kim et al., 2006) may also be involved in floral organ abscission together with ADPG2 and QRT2.

Hormones and Floral Organ Abscission

Plant hormones have a number of important roles during flower development, fertilization, and seed development and throughout fruit set, growth, and senescence. Based on available data for the regulation of ADPG1/ADPG2/QRT2 by JA and ethylene, we investigated the role of these two hormones in organ abscission using the ethylene-insensitive ein2 and JA-deficient aos mutants. Analysis of the ein2 aos double mutant revealed that these plant hormones act in a partially redundant manner to promote floral organ abscission. This conclusion is consistent with the expression of ADPG2 (Figure 6; González-Carranza et al., 2002, 2007) and QRT2 (Figure 6; Kim and Patterson, 2006) in the floral organ abscission zone, delayed flower organ abscission in plants with reduced ADPG2 expression (Figure 4; González-Carranza et al., 2007), and the known role of ethylene in promoting floral organ senescence and detachment (Bleecker and Patterson, 1997; Chao et al., 1997; Patterson, 2001). ABA also promotes floral organ abscission in combination with JA and ethylene as this process was further delayed in an ein2 aos aba2 triple mutant (Figure 4). As floral organ abscission eventually occurred in the ein2 aos aba2 triple mutant, and other cell separation events appeared normal in these plants, additional signals that promote cell separation must also exist. Auxin is a good candidate for this signal as it has been suggested to be involved in many, and perhaps all, abscission events. For example, application of auxin or auxin-like molecules can inhibit pod shatter in canola (Chauvaux et al., 1997), and auxin analogs have been shown to inhibit RDPG1 activity by blocking its secretion into the cell wall (Degan et al., 2001). Finally, auxin mutants have been reported to have delayed floral organ abscission and silique dehiscence in Arabidopsis (Ellis et al., 2005; Okushima et al., 2005). Thus, floral organ abscission is controlled by the combined action of at least four different plant hormones that act in part by regulating PG expression.

Functional Specificity of Arabidopsis PGs

Many genes encoding putative PGs have been identified in a number of species, including Arabidopsis, tomato, and rice (Hadfield et al., 1998; Kim et al., 2006; González-Carranza et al., 2007). Other cell wall proteins, such as expansin and xyloglucan endotransglycosylase, also consist of multiple gene families in plants (Rose et al., 2002; Sampedro and Cosgrove, 2005). However, in the absence of comprehensive biochemical analyses, it is not known whether members of these gene families, which often display different expression patterns, possess different substrate specificities (Rose et al., 2002; Sampedro and Cosgrove, 2005). For example, cytochrome P450 monooxygenases are also encoded by multiple gene families, with individual genes encoding enzymes with strict substrate specificity (Schuler and Werck-Reichhart, 2003). Our cDNA swapping experiments using the ADPG1 promoter in the adpg1 adpg2 qrt2 triple mutant background suggest that closely related PGs can also exhibit substrate specificity (Figure 7). While the ADPG1:ADPG1 construct was able to fully restore pod shatter, ADPG1:QRT2 (BLAST score for ADPG1 versus the QRT2 gene product is 10−104) only partially restored pod shatter in adpg1 adpg2 qrt2 plants. Furthermore, when the same promoter was used to drive expression of the At1g48100 cDNA (BLAST score for ADPG1 versus the At1g48100 gene product is 6 × 10−68), no restoration of pod shatter was observed. Although we cannot formally exclude defects in translation or posttranslational processing in these experiments, these results suggest the existence of strict substrate specificity or substrate accessibility for closely related PGs. Consistent with limited pectin substrates for ADPG1 and ADPG2 in planta, it appears that the majority of JIM5-recognized pectin in cell walls of the silique DZ is not degraded during pod shatter (Figure 3).

Agricultural Importance of Abscission Events

The roles for PGs and plant hormones described here have important practical implications for agriculture and horticulture. For example, preventing or delaying floral organ abscission is of great potential interest to the cut flower industry. More generally, abscission/dehiscence associated with the harvest of seeds and/or fruit is also a major production issue in a wide range of crops. One of the best known examples is pod shatter, which can cause seed loss prior to harvest and is an important problem for several crops, particularly canola (oil seed rape). Reducing the activity of canola genes closely related to ADPG1 and ADPG2 (Sander et al., 2001; González-Carranza et al., 2002) may lead to siliques that do not shatter as readily as normal siliques, reducing seed losses.

METHODS

Plant Materials and Growth Conditions

Arabidopsis thaliana ecotypes Col-0 or Ler were used in this study. The aos (accession number CS6149) and ein2-1 (CS3071) mutant seeds were obtained from the ABRC (http://www.biosci.ohio-state.edu/~plantbio/Facilities/abrc/abrchome.htm). The aba2-2 seeds were obtained from Eiji Nambara (University of Toronto, Canada). Plants were grown under long-day conditions (16 h light/8 h dark) in a 22°C growth room either on soil (equal volumes of seed raising mix [Debco] and perlite) with standard Arabidopsis nutrient solution (http://www.biosci.ohio-state.edu/pcmb/Facilities/abrc/handling.htm), on Growool (Swain et al., 2004), or on agar medium consisting of 0.5× MS salts, 1% (w/v) sucrose, and 0.8% agar for selection of transgenic plants or 0.6% phytagel (Sigma-Aldrich) for detection of GUS staining in roots.

Activation Tagging Mutant Screening and TAIL-PCR

We generated several hundred activation tagging lines in the Ler background using Agrobacterium tumefaciens–mediated transformation and the vacuum infiltration method (Bechtold et al., 1993) with the activation tagging vector pSKI015 (Weigel et al., 2000). Seeds from infiltrated plants were sown on Growool and sprayed with Basta (Swain et al., 2004) to identify transgenic seedlings that were screened for novel phenotypes. To identify the T-DNA insertion site, TAIL-PCR was performed according to Liu et al. (1995) using genomic DNA extracted from AT3 inflorescences with the PhytoPure plant DNA extraction kit (Amersham Life Sciences) and primers SKIL1 (5′-ACGACGGATCGTAATTTGTCG-3′), SKIL2 (5′-TTCATTTTATAATAACGCTGCGG-3′), or SKIL3 (5′-CTTTCTTTTCCTCCATATTGACC-3′) for the pSKI015 left border sequence and the AD2 degenerate primer (Liu et al., 1995).

Identification of T-DNA Insertion Mutants

T-DNA insertion mutants were obtained from the ABRC for SALK lines (Alonso et al., 2003) or the Max-Planck institute for the GABI-Kat line (Rosso et al., 2003). Alleles were named as follows: adpg1-1 (SALK_034714), adpg1-2 (SALK_057704), adpg2-1 (SALK_035098), adpg2-2 (GABI_289C07), qrt2-2 (SALK_132478), and qrt2-3 (SALK_031337). Homozygous T-DNA insertion mutants were identified using PCR with gene-specific primers (see Supplemental Table 2 online). The primer complementary to the T-DNA was SALK LBa1 (5′-TGGTTCACGTAGTGGGCCATCG-3′) for SALK lines and GABI LB (5′-CCCATTTGGACGTGAATGTAGACAC-3′) for the GABI-Kat line. Genomic DNA was isolated by standard methods (Neff et al., 1998). PCR was performed using rTaq DNA polymerase (Invitrogen).

RNA Extraction and Analysis

RNA was extracted using the phenol/SDS method (Naito et al., 1994). Total RNA (1 μg) was treated with DNase I (Promega) to eliminate genomic DNA contamination. After inactivation of DNA polymerase I activity by adding EDTA (2.5 mM final concentration) and heating at 65°C for 10 min, first-strand cDNA was synthesized from DNase I (Amplification Grade; Invitrogen) treated total RNA with random hexamers using a SuperScript III reverse transcriptase according to the manufacturer's instructions (Invitrogen). For RT-PCR, PCR was performed using first-strand cDNA as a template with gene-specific primers (see Supplemental Tables 3 and 4 online) followed by agarose gel electrophoresis to quantify the amount of PCR product. qRT-PCR with the SYBR Green I dye method was performed using the first-strand cDNA as a template with gene-specific primers (see Supplemental Tables 3 and 4 online) on a sequence detector system (My iQ single color real-time PCR detection system Model: MyiQ Optical Module; Bio-Rad). The mean value of three biological replicates was normalized using an 18S rRNA as the internal control with primers 18S rRNA For (5′-GGGACAGTCGGGGGCATTCG-3′) and 18S rRNA Rev (5′-TCCGCTGATCCCTGGTCGGC-3′).

Plasmid Construction and Transformation

We used the Gateway system (Invitrogen) for plasmid construction, and the sequences of all plasmids used in this study were examined to ensure that no mutations were introduced during plasmid construction.

The QRT2 cDNA and ADPG1 cDNA were cloned into the pENTR vector (Invitrogen) to create pENTR-QRT2 and pENTR-ADPG1, respectively. The 35S enhancer sequence was fused upstream of the QRT2 genomic fragment consisting of 2085 bp of QRT2 promoter sequence from the predicted translational initiation site and the QRT2 gene with introns and exons to create pENTR-35S enhancer-ProQRT2:QRT2. The ADPG1 promoter, 2433 bp upstream from the predicted translational initiation site, was fused upstream of the ADPG1 cDNA, QRT2 cDNA, and At1g48100 cDNA to create pENTR-ProADPG1:ADPG1, pENTR-ProADPG1:QRT2, and pENTR-ProADPG1:At1g48100, respectively. The GUS coding region was fused downstream of the ADPG1 promoter (2433 bp upstream from the translation initiation site), the ADPG2 promoter (2177 bp upstream from the translation initiation site), and the QRT2 promoter (2085 bp upstream from the predicted translation initiation site) to create pENTR-ProADPG1:GUS, pENTR-ProADPG2:GUS, and pENTR-ProQRT2:GUS, respectively. Precise procedures for constructing these plasmids are described in the Supplemental Methods online.

To generate binary vectors for plant transformation, an LR reaction was performed with the binary vector for the Gateway system, pGWB1, in which a target gene from pENTR can be introduced before the nopaline synthase terminator. The resulting binary vectors, 35S enhancer-ProQRT2:QRT2, ADPG1:GUS, ADPG2:GUS, QRT2:GUS, ADPG1:ADPG1, ADPG1:QRT2, and ADPG1:At1g48100, were transformed into wild-type Col-0 or the adpg1-1 adpg2-1 qrt2-2 mutant using the Agrobacterium-mediated floral dip method (Clough and Bent, 1998). Plants carrying a T-DNA insertion were selected on agar plates containing 20 mg/L kanamycin (Sigma-Aldrich), 20 mg/L hygromycin (Sigma-Aldrich), and 50 mg/L carbenicillin (Sigma-Aldrich).

Scanning Electron Microscopy of Pollen Tetrad

Pollen from the adpg1 adpg2 qrt2 triple mutant was mounted on double-sided carbon tabs, coated with ~20 nm gold using an Emitech K550X sputter coater, and viewed at 15 kV accelerating voltage in a Zeiss EVO LS15 scanning electron microscope.

Light Microscopy

Tissue was fixed in 2% glutaraldehyde in 25 mM sodium phosphate buffer at pH 6.8, vacuum infiltrated, and incubated at 4°C overnight. Tissue was briefly rinsed in 25 mM sodium phosphate buffer, pH 6.8, and dehydrated in an ethanol series (25, 50, 70, 95, and 100%), with 2 h in each solution. Tissue was run through an LR white resin/ethanol mix (25:75, 50:50, 75:25, 100:0 ×2) and finally embedded in LR white resin. Using a microtome, 2-μm cross sections were cut and placed on glass slides. These were stained with 0.3% Toluidine Blue in 1% sodium tetraborate, pH 9.0, for 30 s or in 0.02% ruthenium red, then water mounted and covered with a cover slip for observation by light microscopy.

For GUS assays, plant material was gently fixed by incubation in 90% acetone on ice for 15 min, and the tissue washed in sodium phosphate buffer, pH 7.2. The tissue was then incubated in a 3 mM GUS staining solution consisting of 3 mM K3Fe(CN)6, 3 mM K4Fe(CN)6, 2 mM 5-bromo-4-chloro-3-indolyl-β-d-glucronic acid, and 50 mM NaPO4, pH 7.2, vacuum infiltrated, and incubated at 37°C for 16 h. For detection of ADPG2:GUS and QRT2:GUS in anthers and siliques, 0 mM K3Fe(CN)6/0 mM K4Fe(CN)6 was used to increase sensitivity. After staining, tissues were fixed in 50% ethanol, 5% acetic acid, and 3.7% formaldehyde and then washed several times with 70% and then 85% ethanol. For making cross sections after staining, the tissues were processed through an ethanol series (30, 50, 70, 85, 95, and 100%), with the 50% step incorporating a 1-h fixative wash (50% ethanol, 5% acetic acid, and 3.7% formaldehyde). Tissue was run through a Lemosol/ethanol gradient (25:75, 50:50, 75:25, 100:0 ×2) and finally embedded in Paraffin. Using a microtome, 7-μm cross sections were cut and placed on glass slides. The tissue was examined using a microscope.

Preparation of Siliques for Light and Electron Microscopy

Arabidopsis siliques were fixed in 2.5% glutaraldehyde (EM Grade) in PBS, pH 7.2, for 2 h at room temperature and stored overnight at 4°C. Siliques were washed three times in distilled water and then dehydrated in a graded ethanol series. Over several days, the samples were infiltrated with LR White resin, and then individual siliques were placed in gelatin capsules, filled with fresh resin, and polymerized in a 55°C oven overnight.

Sectioning and Toluidine Blue Staining of Dehiscing Siliques

Individual siliques in resin blocks were sectioned (1 μm) transversely on a Leica Ultracut R microtome. The sections were collected and dried onto glass microscope slides and stained with a 2% (w/v) aqueous Toluidine Blue solution for 30 s, washed, and dried on a hotplate.

Immunolabeling with Gold-Labeled Pectin Antibodies for TEM

Ultrathin sections (80 nm) were cut on a Leica Ultracut R microtome using a diamond knife and collected on 100 mesh formvar-coated gold grids. The grids were preincubated in a blocking buffer (1% BSA in PBS) for 30 min at room temperature to prevent nonspecific antibody binding. Two monoclonal antibodies raised against pectic polysaccharides, JIM5 and JIM7 (Knox et al., 1990; Plant Probes) were diluted 1:50 in blocking buffer. The grids were incubated in drops of the diluted primary antibody for 1 h at room temperature and then overnight at 4°C. The grids were washed three times in PBS and two times in blocking buffer and then incubated in a 1:20 dilution of an anti-rat secondary antibody conjugated to 18-nm colloidal gold (Jackson ImmunoResearch). Grids were washed in PBS and several times in water before poststaining in 2% (w/v) aqueous uranyl acetate. The sections were viewed on a Philips BioTwin transmission electron microscope and images captured on a Gatan multiscan digital camera. In control experiments, the primary antibody was omitted and no labeling was observed.

Functional Assay of PG Proteins

Truncated versions of the PG proteins, lacking the hydrophobic N-terminal region encoding a signal peptide predicted by the SOSUI program (Hirokawa et al., 1998; http://bp.nuap.nagoya-u.ac.jp/sosui/) were expressed as His-tagged fusion proteins in Escherichia coli. GFP and N-terminal truncated versions of ADPG1, ADPG2, QRT2, and At1g48100 were cloned into the pENTR vector (Invitrogen) to create pENTR-GFP, pENTR-tADPG1, pENTR-tADPG2, pENTR-tQRT2, and pENTR-tAt1g48100, respectively. GFP was used as a negative control. Details of these plasmid constructions are provided in the Supplement Methods online. A Gateway LR reaction was performed using pENTR-GFP, pENTR-tADPG1, pENTR-tADPG2, pENTR-tQRT2, and pENTR-tAt1g48100 with pDEST17, which is Gateway E. coli expression vector that provides a 6x Histidine tag at the N terminus of the expressed target gene driven by the T7 promoter (Invitrogen). The resulting plasmids, pDEST17-His-GFP, pDEST17-His-tADPG1, pDEST17-His-tADPG2, pDEST17-His-tQRT2, and pDEST17-His-tAt1g48100, were transformed into E. coli strain origami B (DE3) (Novagen) that is able to provide a soluble and active enzyme with proper disulfide bond formation (Shimizu et al., 2005). Soluble PG protein was purified using a Ni-NTA column (Ni-NTA agarose; Qiagen) according to the manufacturer's protocol. Purified PG solution was dialyzed against a 150 mM sodium chloride/50 mM sodium acetate, pH 5.0, solution. The standard curve was determined by the use of galacturonic acid (Sigma-Aldrich), and PG activity was measured according to Gross (1982) using polygalacturonic acid from citrus fruit (Sigma-Aldrich) as the substrate. One unit of the endo-PG activity was defined as the amount of enzyme that liberated 1 μmol of reducing end groups per 1 min at pH 5.0 and 30°C. Specific activity was calculated from three independent samples.

Accession Numbers

Sequence data for the genes described in this article can be found in the Arabidopsis Genome Initiative under the following accession numbers: ADPG1 (At3g57510), ADPG2 (At2g41850), QRT2 (At3g07970), and GAPC (At3g04120).

Supplemental Data

The following materials are available in the online version of this article.

  • Supplemental Figure 1. Phenotype of the AT3 Line.
  • Supplemental Figure 2. qrt2-1 Causes a Single Amino Acid Substitution in the Predicted QRT2 Protein without Altering the Transcript Level.
  • Supplemental Figure 3. Reduced Transcript Levels of ADPG1, ADPG2, and QRT2 in the adpg1 adpg2 qrt2 Triple Mutants.
  • Supplemental Figure 4. adpg1-1 Reduces Transcript Levels and Causes an In-Frame Deletion.
  • Supplemental Figure 5. Delayed Abscission of Floral Organs in PG and Hormone Mutants.
  • Supplemental Table 1. Phenotypes of adpg1, adpg2, and qrt2 Single, Double, and Triple Mutants.
  • Supplemental Table 2. Primers for Genotyping T-DNA Insertion Mutants.
  • Supplemental Table 3. Primers for RT-PCR to Examine the Transcript Level in T-DNA Insertion Mutants.
  • Supplemental Table 4. Primers for RT-PCR to Examine Target Gene Expression.
  • Supplemental Methods. Plasmid Construction.

Supplementary Material

[Supplemental Data]
[Author Profile]

Acknowledgments

We thank ABRC and the Max-Planck institute for providing the Arabidopsis T-DNA insertion mutants, Tsuyoshi Nakagawa (Shimane University) for providing the Gateway destination binary vector (pGWB1), and Michael Groszmann and Mark Talbot for help with sectioning for GUS visualization and scanning electron microscopy analysis. M.O. was supported by a CSIRO Office of the Chief Executive (OCE) Fellowship. P.K. was supported by a Riverlink PhD studentship from La Trobe University and CSIRO. S.W. was supported by the Grains Research and Development Corporation, Australia.

Notes

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantcell.org/) is Stephen M. Swain (steve.swain/at/csiro.au).

[W]Online version contains Web-only data.

www.plantcell.org/cgi/doi/10.1105/tpc.108.063768

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