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Plant Cell. Jan 2003; 15(1): 79–92.
PMCID: PMC143452

Altered Cell Cycle Distribution, Hyperplasia, and Inhibited Differentiation in Arabidopsis Caused by the D-Type Cyclin CYCD3

Abstract

CYCD3;1 expression in Arabidopsis is associated with proliferating tissues such as meristems and developing leaves but not with differentiated tissues. Constitutive overexpression of CYCD3;1 increases CYCD3;1-associated kinase activity and reduces the proportion of cells in the G1-phase of the cell cycle. Moreover, CYCD3;1 overexpression leads to striking alterations in development. Leaf architecture in overexpressing plants is altered radically, with a failure to develop distinct spongy and palisade mesophyll layers. Associated with this, we observe hyperproliferation of leaf cells; in particular, the epidermis consists of large numbers of small, incompletely differentiated polygonal cells. Endoreduplication, a marker for differentiated cells that have exited from the mitotic cell cycle, is inhibited strongly in CYCD3;1-overexpressing plants. Transcript analysis reveals an activation of putative compensatory mechanisms upon CYCD3;1 overexpression or subsequent cell cycle activation. These results demonstrate that cell cycle exit in the G1-phase is required for normal cellular differentiation processes during plant development and suggest a critical role for CYCD3 in the switch from cell proliferation to the final stages of differentiation.

INTRODUCTION

The formation of a plant body depends on the coordinated generation of cells followed by their expansion and functional specialization (den Boer and Murray, 2000). Cell differentiation often is correlated or coordinated with the reduction or cessation of division activity (Donnelly et al., 1999; De Veylder et al., 2001), although attempts to define the molecular links between cell cycle control and differentiation have not identified the plant division regulators that control the timing of cell cycle exit in relation to cell differentiation. Rather, manipulation of a variety of cell cycle components, including cyclin-dependent kinase (CDK) (Hemerly et al., 1995; Porceddu et al., 2001), CDK inhibitor proteins (Wang et al., 2000; De Veylder et al., 2001), and mitotic cyclins (Doerner et al., 1996), have been found variously to affect cell cycle phase length, the number of cell cycles, or the final cell size. However, in most of these studies, neither architectural modifications of the plant nor changes in the developmental timing of cell division and differentiation were observed. Thus, these regulators affect primarily the cell cycle itself and do not appear to significantly disturb the process of cell differentiation. Upregulation or downregulation of a CDK-activating kinase decreased CDK activity and promoted the differentiation of root meristem cells, but differentiation preceded cell cycle arrest and could not be mimicked by cell cycle blockers (Umeda et al., 2000), suggesting the involvement of mechanisms that control differentiation independently of the cell cycle.

Therefore, the relationship between cell proliferation and differentiation in plants is unclear. In mammals, cell cycle exit has been shown to be required for the proper execution of various differentiation pathways, including skeletal myogenesis (Skapek et al., 1995; Zacksenhaus et al., 1996; Guo and Walsh, 1997) and lens fiber cell differentiation (Zhang et al., 1998), and the retinoblastoma (Rb) pathway appears to play a critical role in coordinating proliferation and differentiation.

In plants, the cyclin D/Rb pathway is present (Xie et al., 1996; Huntley et al., 1998) and is proposed to mediate G1/S entry according to a mechanism that appears to be conserved in its key elements in all higher eukaryotes. D-type cyclins are stimulated by mitogenic growth signals and, in common with all cyclins, form a kinase complex with a CDK subunit. A key phosphorylation target of D-cyclin kinases appears to be the Rb protein. Rb binds a family of heterodimeric transcription factors called E2F/DP and is localized to promoters that contain E2F binding sites. Many E2F-regulated genes are required for cell growth and cell cycle progression. Rb then recruits histone deacetylase activity to promotor-bound E2Fs, inhibiting the transcription of E2F-regulated genes. Phosphorylation of Rb causes it to lose its association with E2Fs, resulting in the release of the transcriptional silencing of E2F-regulated genes and subsequent entry into S-phase (de Jager and Murray, 1999).

Several lines of evidence support an analogous system operational in plants. In Arabidopsis, a family of 10 genes encoding D-type cyclins (CYCD) is present, and homologs have been identified in a variety of other species. Cyclin D levels and activity respond to a number of extrinsic signals, including hormone and Suc levels (Soni et al., 1995; Riou-Khamlichi et al., 1999, 2000; Hu et al., 2000). A multigene family encoding E2Fs has been identified in Arabidopsis, consensus E2F binding sites have been recognized in promoters of several genes, and interaction between plant Rb proteins and E2F has been shown in a number of cases (de Jager and Murray, 1999; de Jager et al., 2001; Mariconti et al., 2002). Although correlative links between differentiation and Rb protein accumulation have been shown in maize leaves, in which basal proliferative regions contain low levels of Rb and differentiating distal leaf segments contain high Rb levels (Huntley et al., 1998), a possible molecular role for the cyclin D/Rb pathway in mediating the switch between cell division and differentiation has not been defined in plants.

Analysis of CYCD-related genes in the Arabidopsis genome shows 10 genes that fall into six or seven groups (Oakenfull et al., 2002; Vandepoele et al., 2002), most of which have not been analyzed in detail. The CYCD3 group includes three genes, of which CYCD3;1 is the best studied. In cell cultures, CYCD3;1 mRNA levels do not depend strongly on the position of cells in the cell cycle, in contrast to the expression of mitotic cyclins such as CYCB1;1 (Menges and Murray, 2002). Rather, CYCD3;1 expression depends on the availability of Suc and plant hormones (Riou- Khamlichi et al., 2000). Readdition of Suc to Suc-deprived cell cultures results in the induction of CYCD3;1 in late G1-phase (Menges and Murray, 2002), with the mRNA subsequently remaining at a relatively constant level in cycling cells. In addition to the Suc response, CYCD3;1 is induced in both cell cultures and in plants by cytokinin (Riou-Khamlichi et al., 1999) and, to a lesser extent, by brassinosteroid (Hu et al., 2000) and other “mitogenic” plant hormones, including auxin and gibberellin (Oakenfull et al., 2002). Moreover, leaf explants that constitutively express CYCD3;1 can produce calli in the absence of exogenous cytokinin (Riou-Khamlichi et al., 1999). By contrast, CYCD2;1 transcripts show no regulation by hormones.

The activation of CYCD3;1 during G1-phase, together with its response to extrinsic factors, including hormones such as cytokinin, auxin, brassinosteroid, and gibberellin, which are known to be key regulators of both proliferation and differentiation, suggested that it could play a role in integrating proliferative and other signals. Moreover, the correlation of its associated kinase activity with the abundance of its mRNA and protein (Healy et al., 2001), a feature not observed for CYCD2;1, also suggested that overexpression could be used to increase CYCD3;1-associated kinase activity.

We investigated the expression pattern of CYCD3;1 in Arabidopsis in the shoot apex and found it to be associated with proliferating tissues and absent in fully differentiated tissues. To analyze a possible role for CYCD3;1 in proliferation/differentiation decisions, we generated transgenic Arabidopsis overexpressing CYCD3;1 under the control of a constitutive 35S promoter of Cauliflower mosaic virus (CaMV). Overexpression of CYCD3;1 reduced the proportion of cells in the G1-phase of the cell cycle and resulted in hyperproliferation of leaf epidermal tissues. Furthermore, overexpression of CYCD3;1 led to differentiation defects in leaf tissues. These results show that cell cycle exit, which was observed previously to be correlated with later stages of leaf development (Donnelly et al., 1999), is a requirement for the normal differentiation of many leaf cell types, and they suggest that the downregulation of CYCD3;1 is an important factor in the onset of cellular expansion and differentiation in plants.

RESULTS

Expression Analysis of CYCD3;1 in the Shoot Apex

To investigate a possible role for CYCD3;1 in cell division and differentiation control, we examined its expression pattern in the Arabidopsis shoot apex. The shoot apex contains the meristem itself, which consists of a central zone with less rapidly dividing stem cells and a surrounding peripheral region within which new organ primordia are initiated. The growth of primordia is driven initially by cell division, and further leaf development is a combination of cell division and cell expansion accompanied by progressive differentiation. Therefore, the apical region consists of a variety of cell types that exhibit different proliferation rates and degrees of differentiation.

CYCD3;1 transcripts were localized by in situ RNA hybridization on sections from 2-month-old vegetative short-day-grown Arabidopsis plants and from floral long-day-induced plants (Riou-Khamlichi et al., 1999) (Figure 1). CYCD3;1 probes do not cross-hybridize under the conditions used with the other CYCD3 genes present in Arabidopsis (Soni et al., 1995), indicating that the expression pattern observed is specific for CYCD3;1.

Figure 1.
Localization of CYCD3;1 Transcripts in Vegetative and Flowering Arabidopsis Shoot Apices.

We found that CYCD3;1 mRNA is abundant in the vegetative shoot meristem and inflorescence, particularly in peripheral meristem areas and young primordia, and also is present at high levels in young developing leaves, procambium, vascular tissues, and axillary floral buds. In more mature developing leaves, a strong signal persists in adaxial cells. In cells undergoing endoreduplication, such as the mesophyll of maturing leaves and the pith, no signal above background was observed (Figures 1A and 1B).

Using short-day-grown plants induced to flower by several long days, we were able to follow the expression of CYCD3;1 through the floral transition and early inflorescence development. Soon after induction, CYCD3;1 expression was observed on the flanks of the meristem (Figure 1C), and as elongation of the floral spike was initiated, CYCD3;1 mRNA was abundant in the pith flanking the meristem (Besnard-Wibaut, 1977), the meristem, and the initiated buds (Figure 1D). As the floral stalks grew further, mRNA levels decreased in the pith (Figure 1F, cf. control Figure 1E). Again, strong expression also was observed in axillary buds (Figure 1G).

Constitutive Overexpression of CYCD3;1 Retards Plant Development

To investigate a possible role for CYCD3;1 in the control of cell proliferation during shoot and leaf development, we examined the effect of its ectopic constitutive expression in Arabidopsis plants (Riou-Khamlichi et al., 1999). We have described previously the generation of these lines, in which CYCD3;1 is not expressed initially and then is activated by FLP site-specific recombinase–mediated excision of a β-glucuronidase (GUS) gene-blocking sequence (Riou-Khamlichi et al., 1999). The advantage of this approach is that the selection of transformants is independent of any effects of transgene expression. Three independent single-copy lines were subjected to FLP-mediated activation of CYCD3;1, selected for germinal transmission of the activated CaMV35S-CYCD3;1 construct, and analyzed further. All three lines showed similar phenotypes, including the cytokinin-independent generation of callus from leaf explants described previously (Riou-Khamlichi et al., 1999), and line G54 was used for more detailed phenotypic analysis.

Plants of line G54 expressing CYCD3;1 contained increased levels of CYCD3;1 protein in leaves, flowers, roots, and shoots (Figure 2A). Furthermore, the protein kinase activity of CYCD3;1-containing immunoprecipitates increased by a factor of 8 in the overexpressing plants (Figure 2B).

Figure 2.
CYCD3;1 Protein Levels and Associated Kinase Activity upon Constitutive CYCD3;1 Overexpression.

Striking developmental effects were observed in these lines. In young seedlings, cotyledons were enlarged, and subsequent rosette leaves curled soon after their formation over the abaxial surface and remained somewhat smaller than their wild-type equivalents (Figures 3A and 3B). The emergence of leaves was slower in CYCD3;1-overexpressing (CYCD3;1 OE) plants than in wild-type plants; CYCD3;1 OE plants produced a flower stalk ~10 days later than wild-type plants under long-day conditions. The developmental timing of the floral transition was not affected, because both CYCD3;1 OE and wild-type plants initiated flowering after producing approximately seven rosette leaves (Figure 3E). The floral stalk of CYCD3;1 OE plants was shorter and bore curled cauline leaves and fewer flowers compared with that of wild-type plants (Figures 3C and 3D). These results indicate a possible effect of CYCD3;1 expression, particularly on leaf initiation in the meristematic region and subsequent leaf development.

Figure 3.
CYCD3;1 Overexpression Causes Leaf Abnormalities and Retarded Development.

Overexpression of CYCD3;1 Reduces the Proportion of G1 Cells and Uncouples the Cell Cycle from Growth in the Shoot Apex

The shoot apex contains the meristematic stem cells and their immediate derivatives, which undergo little endoreduplication, allowing any effects of CYCD3;1 expression on the cell cycle to be observed. We examined the cell cycle distribution of the nuclei of cells from dissected shoot apices using flow cytometry. We observed a striking reduction of 30% in the proportion of G1 nuclei in CYCD3;1 OE plants and an accompanying increase in G2 nuclei compared with wild-type apices (Figure 4A). No change in the proportion of S-phase nuclei was observed. This finding suggests that CYCD3;1 has a primary effect of increasing the tendency of cells to exit from G1-phase, resulting in their accumulation in G2-phase, but it does not promote the G2/M transition.

Figure 4.
Effects of CYCD3;1 Overexpression on Cell Division in Shoot Apices and Leaves.

In mammals and yeast, the G1-phase of the cell cycle is the major period of cellular growth, and commitment to division during G1-phase generally is subject to cell size control (Neufeld and Edgar, 1998; Polymenis and Schmidt, 1999). Arabidopsis meristematic cells do not undergo the cell expansion associated with their more differentiated progeny in lateral organs and stem tissue and are relatively uniform in size. Consistent with CYCD3;1 promoting precocious G1 exit, we found a reduction in the average interphase cell size in the shoot apical meristem of CYCD3;1 OE plants. Cell areas were measured from thin sections, and in the wild type, the shoot apical meristem was found to vary from 14 to 64 μm2, with a median value of 39 μm2 (Figure 4B, left), whereas CYCD3;1 OE meristematic cells were significantly smaller, with a size range from 10 to 55 μm2 in cross-sectional area and a median size of 26 μm2 (Figure 4B, right). Cells flanking the shoot apical meristem of CYCD3;1 OE plants (Figure 4B, arrowheads) also were smaller than comparable cells bordering wild-type meristems. These results indicate that CYCD3;1 expression causes precocious G1 exit and, in doing so, overrides the normal size control for cell division and results in a shift in the cell cycle distribution of meristem cells from G1- to G2-phase.

CYCD3;1 Overexpression Inhibits Cell Cycle Exit in Leaves and Cotyledons

Cell division in plants normally is concentrated in the meristematic regions and young organs (Fobert et al., 1994). To determine whether the effects of CYCD3;1 on the G1/S transition also resulted in a modification of the spatial localization of cell division, we assessed the overall distribution of cell proliferation using a fusion of GUS to the promoter and destruction box of the mitotic cyclin CYCB1;1 (Colon-Carmona et al., 1999). The CYCB1-GUS fusion protein is present only in cells in G2/M and is destroyed rapidly when cells passed through mitosis, so GUS staining patterns indicate regions containing cells engaged in active cell division. CYCD3;1 OE seedlings carrying this construct stained for GUS over a wider region than wild-type control seedlings (Figure 4C), showing that cells do not exit from the cell cycle with normal developmental timing when overexpressing CYCD3;1, resulting in ectopic divisions.

Cell division normally ceases during cotyledon and leaf development (Donnelly et al., 1999; De Veylder et al., 2001) and is not observed in mature organs. However, in the epidermis of mature cotyledons of CYCD3;1 OE seedlings, small nuclei and scattered cells expressing CYCB-GUS (data not shown) and mitotic figures continued to be observed, whereas in wild-type cotyledons, larger nuclei were observed and no mitotic figures could be found (Figure 4D). Because cotyledons of CYCD3;1 OE plants were larger than those in wild-type plants but contained smaller cells, this finding implies an increase in the number of cells present in cotyledons.

To quantify the effects of CYCD3;1 on cell proliferation in leaves, we performed a detailed analysis of cell number and size in the adaxial epidermal surface of rosette leaves at three stages of development (~10 mm2, 30 to 50 mm2, and fully mature) (Figure 4E). These data show that CYCD3;1 overexpression had a dramatic effect on the number of cells present in leaves, because the adaxial epidermis of CYCD3;1 OE leaves of ~10 mm2 already contained 20 to 30 times more cells than the epidermis of wild-type leaves. Mature rosette leaves reached half the final surface area of the wild-type leaves but contained ~18-fold more cells in their adaxial epidermis. We conclude that cell proliferation is the dominant mechanism for the growth of CYCD3;1 OE leaves, whereas in wild-type leaves, a combination of cell division followed by expansion is involved.

Inhibition of Differentiation in the Leaves and Vasculature of CYCD3;1 OE Plants

Because cell proliferation continued in CYCD3;1 OE leaves at a time when it had ceased in wild-type leaves, we examined the effect of this hyperproliferation on leaf structure and cell differentiation. Mature leaf structure in dicotyledonous plants is characterized by distinct adaxial and abaxial (lower) epidermal layers. The palisade layer lies below the adaxial epidermis and consists of tightly packed elongated cells arranged with their long axes perpendicular to the leaf surface, below which the spongy mesophyll consists of more rounded, loosely packed cells with frequent intercellular air spaces (Bowman, 1993). In young leaves (≤6 mm in length) of the CYCD3;1 OE plants, this typical dorsoventral organization of the leaf parenchyma into palisade and spongy mesophyll was not distinguishable, with the palisade layer apparently replaced by smaller spherical parenchyma cells and fewer intercellular air spaces present (Figure 5A). In more mature leaves of the CYCD3;1 OE plants, palisade and spongy mesophyll layers did become recognizable, but both consisted of smaller cells than in the wild type (Figure 5B). The morphological distinction between the adaxial and abaxial epidermis was largely lost, and the venation pattern of CYCD3;1 OE leaves also was less symmetric than in wild-type leaves (data not shown).

Figure 5.
CYCD3;1 Overexpression Affects Differentiation in Leaves.

Cross-sections through the bases of floral stalks revealed retarded formation of lignified secondary xylem elements in CYCD3;1 OE plants; this result was either a direct consequence of higher CYCD3 levels or connected with reduced radial growth (Figure 5E). It remains unclear how causal the relationship between these two processes is. Although the density of stomata was lower in CYCD3;1 OE plants, CYCD3;1 apparently does not profoundly influence differentiation after the specification of specific cell types such as stomatal guard cells (data not shown).

Mature leaf epidermal cells normally exhibit a characteristic sinuous outline and a large size. Histological examination of CYCD3;1 OE leaves revealed that both the adaxial and abaxial epidermis of transgenic leaves were characterized by the presence of small polygon-shaped pavement cells (Figure 5C, right), in contrast to the expanded sinusoid-shaped pavement cells in the wild type (Figure 5C, left). By plotting cell area against a “shape factor” (4Π area/perimeter2), which is unity for a perfect circle and decreases for more complex shapes, we can see that for wild-type epidermal cells, an increase in cell area is accompanied by the adoption of more complex shapes as cells differentiate (Figure 5D). In CYCD3;1 OE plants, cell sizes and shape factors were clustered at higher shape factor values (Figure 5D), suggesting that they are unable to develop their mature shape. We also noted that the adoption of a mature epidermal cell size was affected dramatically, because even in mature rosette leaves, the cell area of transgenic pavement cells did not exceed 350 μm2, whereas the wild-type cells expanded from 350 to 22,000 μm2.

Endoreduplication is a marker for the differentiated state of cells in many aerial plant tissues, and in leaves it occurs only after the cessation of normal mitotic cycles (De Veylder et al., 2001). Comparison of ploidy levels showed that CYCD3;1 OE cells were largely deficient in this differentiated phenotype and showed only a small proportion of nuclei with a DNA content of >4C (Figure 5F, right), whereas wild-type rosette leaves contained a substantial proportion of nuclei with DNA contents of 8C, 16C, and 32C (Figure 5F, left). Similar observations of reduced endoreduplication were made in a variety of mature tissues, including rosette leaves, bracts, and segments of the floral stalk (data not shown).

CYCD3;1 Overexpression Affects the Activity of Specific Genes in the CYCD-Rb Pathway

Three genes encoding CYCD3 are present in the Arabidopsis genome. To determine whether the abundance of CYCD3;2 or CYCD3;3 mRNA is affected by CYCD3;1 overexpression, we used semiquantitative reverse transcriptase–mediated PCR (RT-PCR) to detect transcript abundance in seedlings and mature rosette leaves. Low CYCD3;1 mRNA levels were detected in wild-type seedlings (Figure 6A) and were not detected with a gel-based assay in mature rosette leaves (Figure 6B), whereas CYCD3;1 OE seedlings contained very high levels of CYCD3;1 mRNA in both tissues. Little or no change in CYCD3;2 or CYCD3;3 mRNA levels was observed in CYCD3;1 OE seedlings. In addition, no change in CYCD2;1 transcript levels was observed, despite the morphological and cytological changes apparent in CYCD3;1 OE plants (Figure 6A). We conclude that the effects observed are the result of changes in CYCD3;1 levels rather than concomitant changes in the expression of other CYCD3 genes.

Figure 6.
Expression Analysis of G1/S Genes and Patterning Genes by Semiquantitative RT-PCR.

CYCD3 is proposed to operate in the CYCD-Rb pathway, controlling the entry into S-phase by phosphorylating Rb and hence resulting in E2F activation (Nakagami et al., 1999). Therefore, Rb functions as a negative regulator of CYCD3 kinase activity. Levels of Rb mRNA were increased in both seedlings (by 5.7-fold) and mature rosette leaves (by 3.5-fold) (Figures 6A and 6B), and a large increase in Rb protein level also was detected in CYCD3;1 OE seedlings (Figure 6D). All E2F genes were upregulated in CYCD3;1 OE plants (Figures 6A and 6B). These changes in levels of expression of other components of the CYCD-Rb pathway indicate that the overexpression of CYCD3 perturbs its normal functioning.

CYCD3;1 Acts Downstream of AINTEGUMENTA to Determine Leaf Cell Number

AINTEGUMENTA (ANT) is expressed early in the formation of incipient leaf primordia and regulates the number of cells incorporated into developing leaves. Its overexpression causes extra rounds of cell division during leaf development, which are associated with increased endogenous CYCD3;1 expression (Mizukami and Fischer, 2000). Using RT-PCR, we found that mRNA levels of ANT were unaffected in the transgenic plants (Figure 6C), despite their greatly increased number of leaf cells, suggesting that CYCD3;1 acts downstream of ANT to affect leaf cell number. The unchanged level of ANT expression also showed that CYCD3;1 OE leaf cells progressed beyond the differentiation stage at which ANT is expressed.

PHABULOSA is a gene involved in the early stages of leaf development (McConnell et al., 2001), whereas SHOOTMERISTEMLESS is involved in meristem identity and maintenance (Barton and Poethig, 1993). mRNA levels of both genes were unchanged despite the dramatic morphological differences (Figure 6C).

DISCUSSION

Within the plant apical region, spatially organized patterns of cell behavior are observed, representing a continuous process of ongoing differentiation that extends from undifferentiated stem cells within the central zone of the meristem to the fully differentiated and nondividing cells of mature tissues of the leaf and stem (Lyndon, 1990). Different stages along this developmental gradient are characterized not only by the expression of specific patterning and developmental regulatory genes but also by different rates of cell proliferation associated with cellular identity and differentiation states. However, the interaction between cell proliferation and differentiation is largely not understood in plants. In animals, the proper timing of cell cycle exit is important for normal differentiation, and this involves the Rb pathway (Skapek et al., 1995; Zacksenhaus et al., 1996; Guo and Walsh, 1997; Zhang et al., 1998).

In both animals and plants, D-type cyclins respond to extrinsic signals and form kinase complexes that themselves phosphorylate and inactivate the Rb protein (Meijer and Murray, 2000). In particular, CYCD3;1 expression and activity respond to both extracellular signals such as hormones and sugars (Riou-Khamlichi et al., 1999, 2000; Hu et al., 2000; Oakenfull et al., 2002) and developmental signals (Mizukami and Fischer, 2000). Moreover, constitutive CYCD3;1 expression replaces the requirement for cytokinin in leaf callus induction and proliferation (Riou-Khamlichi et al., 1999). This molecular evidence suggests that CYCD3;1 may play a key role as an interface between the cell cycle and physiological or morphological signaling, probably together with other D-type cyclins.

Gaudin et al. (2000) examined the expression of two CYCD3 genes in Antirrhinum. One of these genes (CYCD3b) appears to be expressed in all dividing cells, whereas transcripts of the other (CYCD3a) are restricted mainly to the early development of organ primordia of leaves, inflorescences, and floral organs. These results indicated potential specific developmental roles for different D-type cyclins. However, sequence alignments do not make clear the potential relationship between Antirrhinum and Arabidopsis CYCD3 genes (Oakenfull et al., 2002). The in situ analysis presented here shows that the highest levels of Arabidopsis CYCD3;1 mRNA are associated with proliferating tissues, particularly in primordia and the adaxial side of young leaves, and not with differentiated tissues. The pattern observed is consistent with a role for CYCD3;1 in determining cell proliferation in the apical region and developing primordia but does not concur completely with that of either Antirrhinum CYCD3 gene, although it shows stronger expression in primordia, similar to CYCD3a.

Increased CYCD3 Causes Premature Cell Cycle Progression and Ectopic Division and Perturbs the Rb Pathway

We analyzed transgenic Arabidopsis expressing CYCD3;1 under the control of the constitutive 35S promoter of CaMV. At the cellular level, overexpression of CYCD3;1 pushed cells from G1, causing ectopic cell divisions in both meristematic regions and regions in which cell division normally is absent or limited. This finding indicates that CYCD3;1 is able to accelerate progression through G1 and increase the number of cell divisions experienced by individual cells, thereby enlarging the population of proliferating cells. Associated with this promotion of mitotic cell cycles, CYCD3;1 prevents cell cycle exit and inhibits normal differentiation during leaf and other tissue development.

CYCD3;1 OE plants showed strong upregulation of specific components of the CYCD/Rb/E2F pathway, suggesting that CYCD3 action perturbs the pathway. Interestingly, activating components of the pathway were unaffected or affected only mildly (CYCD3;2, CYCD3;3, E2F1, and E2F3), whereas potential negative regulators were increased more strongly. Thus, CYCD3;1 OE plants contain very high levels of Rb mRNA and protein, and E2F2 showed the greatest increase among the E2F genes. Because E2F2 appears to have a weak or absent activation domain (de Jager et al., 2001; Mariconti et al., 2002) and was found not to interact with plant Rb in a yeast two-hybrid assay (de Jager et al., 2001), it is a potential competitive inhibitor of E2F-activated gene expression. The specific upregulation of Rb and E2F2 could be part of a feedback mechanism that normally regulates CYCD3 activity, because the CYCD3;1 promotor contains a consensus E2F binding site (de Jager et al., 2001).

CYCD3;1 Affects Cellular Differentiation and Arabidopsis Development

There are profound effects of the ectopic expression of CYCD3;1 on Arabidopsis development, observed as decreased cell size and altered cell cycle distribution in the shoot apex, increased cell number within developing and mature leaf tissues, and alterations in characteristics of some cells consistent with their inability to adopt fully differentiated characteristics. In particular, the increase of cell numbers in the epidermis and parenchyma in leaves of CYCD3;1 OE plants indicates that CYCD3;1 promotes ectopic divisions of the cells responsible for generating leaf mass. It is interesting that although many cells fail to differentiate properly, they seem to acquire the right identity, as shown by the presence of a distinguishable epidermis, vasculature, and eventually palisade mesophyll. This finding is in agreement with the previous observations of ongoing cell proliferation in maturing leaves (Donnelly et al., 1999; De Veylder et al., 2001) and indicates that the acquisition of cell and tissue identity is compatible with the maintenance of a proliferative capacity.

ANT is expressed in young leaf primordia and regulates the number of cells incorporated into developing leaves. Its overexpression causes extra rounds of cell division during leaf development, associated with increased CYCD3;1 expression (Mizukami and Fischer, 2000). We found that ANT expression was unchanged in CYCD3;1 OE plants, despite their greatly increased number of leaf cells (Figure 4E), confirming the notion that CYCD3 acts downstream of ANT to determine leaf cell number. Tight control of cell division is important in the proper development of organs, because the perturbation of local patterns of proliferation can grossly affect the development and shape of organs, as shown by the application of microbeads containing inducers or inhibitors of division to particular regions of developing leaf primordia (Wyrzykowska et al., 2002). Consistent with these results, the strong effects on cell division in CYCD3;1 OE plants resulted in a profound perturbation of leaf development. Superimposed on this effect were additional effects on the differentiation of many of the constituent cells of the leaf, because CYCD3;1 negatively affected their ability to adopt differentiated characteristics, including endoreduplication.

Thus, the results we report for CYCD3;1 indicate that proliferation and differentiation are linked during plant development and that CYCD3;1 is likely to play a key role in this process. Is the alteration of cellular differentiation observed a general feature of manipulation of the cell cycle genes, or do these results indicate a specific role for CYCD3? We believe the latter possibility is correct, because most reports of attempts to manipulate cell proliferation during development do not show a clear link to concomitant changes in cellular differentiation.

For example, overexpression of the CYCD2;1 gene in tobacco resulted in increased cell division and increased overall plant growth rate but no morphological alterations (Cockcroft et al., 2000). A dominant-negative allele of the Arabidopsis CDKA;1 gene inhibited cell division in transgenic tobacco, resulting in larger but normally differentiated leaf epidermal cells (Hemerly et al., 1995). Similarly, overexpression of the CDK inhibitor KRP2 strongly inhibits mitotic cell division, with profound effects on leaf morphology, but detailed analysis shows that it does not affect the timing of the onset of cell differentiation during leaf development (De Veylder et al., 2001).

As mentioned above, there are three CYCD3 genes in Arabidopsis, and loss of at least one (CYCD3;2) has no observable phenotype (Swaminathan et al., 2000). Therefore, it is likely that a loss-of-function genetic analysis could not have revealed the roles demonstrated by the overexpression of CYCD3;1. Indeed, a loss-of-function analysis cannot be used to demonstrate that CYCD3;1 downregulation is required for cell cycle exit and differentiation, which is the principal conclusion of the work presented here. However, we cannot conclude that the effects we report are specific for CYCD3;1, and indeed, it is probable that the three CYCD3 genes share overlapping roles, albeit perhaps with different importance in distinct tissues and/or developmental stages.

Therefore, we propose that developmental control of CYCD3 expression may be a key mechanism of cell number control within developing leaves and that its downregulation is required for the proper progression of certain differentiation pathways in shoot tissues. This hypothesis is strengthened by the recent findings of De Veylder et al. (2002) that a phenotype with some parallels to the CYCD3 phenotype described here can be obtained by the overexpression of a combination of E2Fa (E2F3 as named by de Jager et al. [2001]) and DPa genes. E2F and DP are the two components of active E2F complexes, which are bound and regulated by Rb. Because Rb itself is inactivated by CYCD/CDK kinase activity, E2F/DPs lie downstream of D-type cyclins in the Rb pathway. Overexpression of E2Fa-DPa activated S-phase–specific genes and also induced ectopic cell division during leaf development and inhibited some differentiated cell phenotypes.

In striking contrast to the results obtained with CYCD3, E2F-DPa overexpression caused a dramatic increase in endoreduplication levels. Therefore, the primary effect of E2Fa/DPa is to promote S-phase entry, driving mitotic cycles in cells in the mitotic phase of development and endoreduplication cycles in cells that have progressed to a more differentiated state and that therefore have mitosis-inhibiting factors (De Veylder et al., 2002). CYCD3 acts both to promote cell cycle progression from G1- to S-phase and simultaneously to reduce endoreduplication. This may be explained by a direct inhibition of endoreduplication by CYCD3 or by the fact that CYCD3;1 OE cells are denied the possibility of endoreduplication because most fail to reach the stage of differentiation at which endoreduplication is initiated.

Schnittger et al. (2002) recently examined the effects of the targeted ectopic expression of CYCD3;1 in trichomes under the control of the GL2 promoter. Trichomes normally contain a single endoreduplicated nucleus and do not express CYCD3;1. GL2::CYCD3;1 expression causes multicellular trichomes as a consequence of ectopic cell division as well as reduced endoreduplication of individual nuclei. These results demonstrate that cells programmed for endocycles normally lack CYCD3;1 expression and that ectopic CYCD3;1 expression in such cells inhibits endoreduplication. This finding supports a direct role for CYCD3;1 in inhibiting endoreduplication and/or promoting mitotic division cycles. We observed no trichome abnormalities in CYCD3;1 OE plants, despite the activity of the 35S promotor used in trichome cells (data not shown), suggesting that the effects observed using the GL2 promotor may be attributable to its very high expression.

The formation of multicellular trichomes as a result of ectopic CYCD3;1 expression raised the possibility that CYCD3;1 promotes mitosis as well as the progression of cells from G1- to S-phase. The results we present do not support this view, because analysis of cell cycle distribution in the shoot apices of these plants shows the predicted reduction in G1-phase cells, accompanied by an accumulation of cells in G2 of the cell cycle. A likely explanation for this observation is that, as in yeast and mammalian cells, the G1 control in plant cells also is the major decision point in the cell cycle, and cells that pass through this point are committed to complete a full cycle. Coupled with the inhibition of endocycles caused by CYCD3, this leads to mitotic cell divisions.

The CYCD Pathway Links Proliferation and Differentiation

We present a general model of the CYCD/Rb/E2F pathway linking developmental cues with the progression of cells along a pathway of progressive development, leading to a fully differentiated phenotype associated with mitotic cell cycle exit and/or endocycles (Figure 7). The results presented here show that CYCD3 promotes the proliferative phase of mitotic cycles and inhibits both endocycles and differentiation, either independently or through its regulation of Rb function. These possibilities cannot be distinguished at present.

Figure 7.
Model Illustrating CYCD3 Action on the Transition from Proliferation to Differentiation in Cells.

We believe that the advantage of this view of cellular development is that it integrates plant cell behavior with the understanding of differentiation in animals. Indeed, the effects of CYCD3;1 expression on cell cycle distribution in the shoot apex suggest that CYCD3;1 plays a primary role in promoting cell cycle progression from G1-phase. Although our results do not exclude the possibility of direct effects of CYCD3;1 in the inhibition of cellular differentiation independent of its effect on the cell cycle, we suggest that its effects on cell differentiation may be explained by continued cell proliferation or the failure to arrest in G1 in response to developmental cues. In mammals, the ability of cells to arrest in G1 and a cessation of cell division processes appear to be important in certain differentiation events (Zhu and Skoultchi, 2001), and overexpression of cyclin D can lead to an acceleration of G1-phase (Quelle et al., 1993). Moreover, loss of the cyclin D target Rb in the developing mouse embryo causes a failure in the final stages of differentiation of a spectrum of cell types, including those of the muscle, hematopoietic, and neural lineages (Skapek et al., 1995; Zacksenhaus et al., 1996; Guo and Walsh, 1997; Zhang et al., 1998). Thus, cell cycle exit mediated by the Rb pathway is of central importance in mammalian development, and the results we report demonstrate a similar link between cell proliferation and differentiation in plants and suggest that CYCD3;1 plays an important role in controlling both processes during leaf development. We propose that high levels of CYCD3;1 can inhibit aspects of the differentiation of cells of a variety of tissues and suggest that cell cycle exit and CYCD3;1 downregulation are required for normal plant development.

METHODS

Plant Material

35S::CYCD3;1 Arabidopsis thaliana ecotype Landsberg erecta plants were generated as described previously (Riou-Khamlichi et al., 1999). Line G54 was used for further analysis. Homozygote 35S::CYCD3;1 plants and the wild type were grown in soil for analysis (18 h of light/6 h of darkness at 23°C). In situ hybridization of CYCD3;1 mRNA was performed as described previously (Riou-Khamlichi et al., 1999).

For analysis of cell division patterns, plants homozygous for the CYCB1;1-uidA reporter (Colon-Carmona et al., 1999) were crossed to homozygous 35S::CYCD3;1 plants. Plants homozygous for the reporter gene in wild-type or homozygous 35S::CYCD3;1 backgrounds were used for analysis.

Protein Gel Blot Analysis and Kinase Assays

Protein gel blot analysis of CYCD3 and Rb protein levels in these transgenic plants was performed as described (Healy et al., 2001) with minor modifications. Rb protein was detected with mouse anti-Rb antibody (a kind gift from W. Gruissem, Swiss Federal Institute of Technology, Zurich) in conjunction with anti-mouse horseradish peroxidase conjugate (1:3000; Amersham, Buckinghamshire, UK). Histone H1 protein kinase activities were measured according to Riou-Khamlichi et al. (1999).

Histological Analysis

For microscopic examination, plant material was fixed with 0.25% glutaraldehyde and 3.7% paraformaldehyde in 100 mM K-phosphate buffer for 6 h at 4°C. After rinsing, samples were dehydrated in a progressive series of ethanol dilutions (70, 80, 90, 95, and 100% for 90 min each) and embedded in Technovit 7100 (Heraeus Kulzer, Wehrheim, Germany). Embedding was performed after gradual infiltration with basic resin (50% ethanol/50% basic resin for 4 h; 70% basic resin/30% ethanol overnight; and 100% basic resin for 1 day) and with a mixture of basic resin and activator (for 5 h) and washing with the complete resin mixture. Sections (3 μm thick) were cut with a rotary microtome (RM 2145; Leica, Nussloch, Germany) and stained with toluidine blue. For the analysis of stem structures, free-hand sections were observed by epifluorescence (Optiphot2; Nikon, Tokyo, Japan) using the autofluorescence of cell components as a marker.

Scanning electron micrographs were made according to Cockcroft et al. (2000). Analysis of cell numbers and shape in developing transgenic and wild-type leaves was performed according to De Veylder et al. (2001) with minor modifications. Differential interference contrast images of the adaxial epidermis were recorded with a MDS290 imaging system (Kodak, Rochester, NY). Cell outlines were traced in Adobe Photoshop (Mountain View, CA), and leaf area and shape factor were calculated using Openlab software (Improvision, Coventry, UK).

Analysis of DNA Content

The DNA content of transgenic and wild-type flowering plants was analyzed by means of the PASIII flow cytometer (Partec, Munster, Germany) using the high-resolution DNA kit type P (Partec).

Expression Analysis by Semiquantitative Reverse Transcriptase–Mediated PCR

For reverse transcriptase–mediated PCR analysis, RNA was isolated from the aerial part of 1-week-old wild-type and CYCD3;1-overexpressing seedlings (cotyledons and two visible leaves) and mature rosette leaves (flowering plants with four visible cauline leaves) with the TriPure isolation reagent (Roche Diagnostics, Mannheim, Germany) according to the manufacturer's instructions. After treatment with DNase, RNA was precipitated in 2 M LiCl, desalted by precipitation with 70% ethanol, and dissolved into 100 μL of water. cDNA synthesis was performed on 3 μL of RNA solution using the Ambion Retroscript kit (Austin, TX) according to the manufacturer's instructions. For the gel-based assay, the cycle number was optimized for each primer pair to give an end point in the linear amplification range of the reaction. Details of primers and cycle conditions are available on request. PCR products were separated on 1% agarose gels.

For the measurement of the relative levels of cDNA, relative quantification by real-time PCR was performed using actin as a reference gene transcript. For the detection by the real-time cycler (Rotorgene 2000; Corbett Research, Sydney, Australia) of newly formed PCR products during successive cycles, the Quantitect SYBR Green PCR kit (Qiagen, Hilden, Germany) was used. The critical threshold values were used to calculate the relative amounts of cDNA according to the Delta-delta method (Pfaffl, 2001).

Upon request, all novel materials described in this article will be made available in a timely manner for noncommercial research purposes.

Acknowledgments

We thank Gareth Davies for work on the FLP system used to generate CYCD3 transgenic plants, Peter Doerner for the CYCB-GUS line, Wilhelm Gruissem for the Rb antibody, and Bart den Boer for invaluable and stimulating discussions and additional analysis of the CYCD3 phenotype. Ann Oakenfull and Graham Armstrong are gratefully acknowledged for help with the preparation of the manuscript. This work was funded by the Biotechnology and Biological Science Research Council and Aventis CropScience. The work of A.J.'s group in the Département de Biologie Végétale, Université de Liège, was supported by the “Poles d'Attraction Interuniversitaires Belges” (Services du Premier Ministre, Services Fédéraux des Affaires Scientifiques, Techniques et Culturelles, P4/15).

Notes

Article, publication date, and citation information can be found at www.plantcell.org/cgi/doi/10.1105/tpc.004838.

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