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Plant Cell. Jan 2004; 16(1): 144–156.
PMCID: PMC301401

Distinct Light-Mediated Pathways Regulate the Biosynthesis and Exchange of Isoprenoid Precursors during Arabidopsis Seedling Development

Abstract

Plants synthesize an astonishing diversity of isoprenoids, some of which play essential roles in photosynthesis, respiration, and the regulation of growth and development. Two independent pathways for the biosynthesis of isoprenoid precursors coexist within the plant cell: the cytosolic mevalonic acid (MVA) pathway and the plastidial methylerythritol phosphate (MEP) pathway. In at least some plants (including Arabidopsis), common precursors are exchanged between the cytosol and the plastid. However, little is known about the signals that coordinate their biosynthesis and exchange. To identify such signals, we arrested seedling development by specifically blocking the MVA pathway with mevinolin (MEV) or the MEP pathway with fosmidomycin (FSM) and searched for MEV-resistant Arabidopsis mutants that also could survive in the presence of FSM. Here, we show that one such mutant, rim1, is a new phyB allele (phyB-m1). Although the MEV-resistant phenotype of mutant seedlings is caused by the upregulation of MVA synthesis, its resistance to FSM most likely is the result of an enhanced intake of MVA-derived isoprenoid precursors by the plastid. The analysis of other light-hyposensitive mutants showed that distinct light perception and signal transduction pathways regulate these two differential mechanisms for resistance, providing evidence for a coordinated regulation of the activity of the MVA pathway and the crosstalk between cell compartments for isoprenoid biosynthesis during the first stages of seedling development.

INTRODUCTION

Metabolic plasticity is key for sessile organisms such as plants to survive in their changing environments. A good example of such plasticity is the astonishing diversity of isoprenoid products that plants can produce from the five-carbon building units isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). Unlike animal cells, which produce sterols as their bulk isoprenoid end products, plants synthesize tens of thousands of isoprenoid compounds and derivatives (Chappell, 1995; McGarvey and Croteau, 1995; Croteau et al., 2000). Although many of these compounds are secondary metabolites that protect plants against herbivores and pathogens or attract pollinators and seed-dispersing animals, other isoprenoids are synthesized in all plants and play essential (primary) roles in photosynthesis, respiration, and the regulation of growth and development. Plants synthesize IPP and DMAPP by two independent pathways: the mevalonic acid (MVA) pathway, which produces cytosolic IPP, and the methylerythritol phosphate (MEP) pathway, which is localized in the plastids (Lichtenthaler, 1999; Eisenreich et al., 2001; Rodríguez-Concepción and Boronat, 2002). Upon the synthesis of IPP and DMAPP, condensation of these units leads to the synthesis of prenyl diphosphates of increasing size, which are the starting points for multiple branches leading to the final isoprenoid products (Figure 1).

Figure 1.
Isoprenoid Biosynthesis in Plant Cells.

Although the MEP pathway was elucidated only recently (reviewed by Rodríguez-Concepción and Boronat, 2002), the MVA pathway has been studied extensively in animals and yeast since the 1950s (reviewed by Goldstein and Brown, 1990). The main MVA-derived isoprenoid end products in plants are sterols (modulators of membrane architecture and plant growth and developmental processes), brassinosteroids (steroid hormones), dolichol (involved in protein glycosylation), and the prenyl groups used for protein prenylation and cytokinin biosynthesis. The side chain of ubiquinones also is formed from IPP synthesized in the cytosol and imported into the mitochondria. The plastidial MEP pathway produces IPP and DMAPP for the biosynthesis of photosynthesis-related isoprenoids (carotenoids and the side chains of chlorophylls, plastoquinones, and phylloquinones) and hormones (gibberellins and abscisic acid). Despite the compartmentalization of these two pathways, MVA-derived precursors can be used for the synthesis of isoprenoids in the plastid, and MEP-derived precursors can be exported to the cytosol in at least some plants, tissues, or/and developmental stages (reviewed by Lichtenthaler, 1999; Eisenreich et al., 2001). An active uptake of IPP into isolated plastids was reported in several plants (Kreuz and Kleinig, 1984; Soler et al., 1993; Milborrow and Lee, 1998), whereas the transport of IPP in the plastid-to-cytosol direction was shown recently to be mediated by a plastidial proton symport system (Bick and Lange, 2003). Other prenyl diphosphates, such as geranyl diphosphate, farnesyl diphosphate, and geranylgeranyl diphosphate (Figure 1), also might be exchanged between the cytosol and the plastids (Adam et al., 1999; Steliopoulos et al., 2002; Bick and Lange, 2003; Hemmerlin et al., 2003). Very little is known, however, about the regulation of the exchange mechanism and the crosstalk between the MVA and MEP pathways.

The activity of the enzymes that catalyze the first committed steps of the MVA and MEP pathways can be blocked with the specific inhibitors mevinolin (MEV) and fosmidomycin (FSM), respectively (Figure 1). MEV (Alberts et al., 1980) blocks the conversion of 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) into MVA, which is catalyzed by HMG-CoA reductase (HMGR; EC 1.1.1.34). FSM (Kuzuyama et al., 1998; Steinbacher et al., 2003) inhibits the production of MEP from 1-deoxy-d-xylulose 5-phosphate (DXP), which is catalyzed by DXP reductoisomerase (DXR; EC 1.1.1.267). We reasoned that the upregulation of the inhibitor-targeted enzymes or the activation of the exchange of isoprenoid precursors between the cytosol and the plastid could be sufficient to release the blockage of the MVA pathway with MEV or that of the MEP pathway with FSM. Taking advantage of the potential of the T-DNA activation-tagging approach to generate both gain-of-function and loss-of-function mutants (Weigel et al., 2000), we screened the ~23,000 Weigel activation-tagging lines for Arabidopsis mutants capable of overcoming the blockage of the MVA pathway with MEV. To identify the mutants that potentially alter the crosstalk between pathways, we selected MEV-resistant mutants that also could survive in the presence of FSM. Here, we report that one such mutant is allelic to phyB. The MEV-resistant phenotype of mutant seedlings is the result of the upregulation of HMGR gene expression and enzyme activity, whereas resistance to FSM likely results from an activated intake of MVA-derived precursors by the plastid. We also provide genetic evidence that these two differential mechanisms for resistance are regulated by distinct light perception and signal transduction pathways.

RESULTS

Inhibition of Isoprenoid Biosynthesis with MEV or FSM Affects Different Aspects of Arabidopsis Seedling Development

It has been reported that the blockage of the MVA pathway with MEV results in root growth inhibition and an arrest of seedling development in Arabidopsis (Re et al., 1995; Kasahara et al., 2002). However, we observed a variety of additional concentration-dependent developmental abnormalities (Figure 2). At 0.1 μM MEV, shoots developed normally but root growth was inhibited strongly (cf. Figures 2A and 2C). After several days, a proliferation of adventitious roots was observed on the hypocotyls of some seedlings (Figure 2C). Growth on 0.5 μM MEV was associated with a decrease in the production of adventitious roots and increasing defects in the development of true leaves (Figures 2E and 2I). At 1 μM MEV, seedling establishment (defined as the production of true leaves that can support further plant development) was heavily compromised (Figure 2F). In some plants, a highly disorganized proliferation of cells in the shoot apical meristem (SAM) also was observed (Figure 2J). Seedlings grown on 5 μM MEV showed an almost complete blockage of seedling establishment (Figure 2G). SAM activity was arrested, and only undeveloped leaf primordia were observed (Figures 2K and 2L).

Figure 2.
Developmental Defects Caused by a Blockage of Isoprenoid Biosynthesis in Arabidopsis Seedlings.

To test whether the described developmental defects were caused specifically by the inhibition of HMGR, we used transgenic plants engineered to produce increased HMGR activity levels. H1S plants carrying the 35S:HMG1 construct to constitutively overexpress the Arabidopsis HMG1 gene showed up to fourfold greater HMGR activity compared with untransformed plants (González, 2002). Higher activity levels were obtained when only the catalytic domain of the enzyme was overproduced in 35S:HMG1cd plants (González, 2002). The H1cd/3 line, which showed a 10.7-fold increase in HMGR activity, was selected for MEV resistance studies. These plants did not show the described alterations in root development when grown on low concentrations of MEV (cf. Figures 2B and 2D). In addition, the production of adventitious roots, the proliferation of undifferentiated cells in the SAM, and the impairment of seedling establishment required higher concentrations of MEV in H1cd/3 transgenic plants compared with untransformed plants (Figure 3), indicating that the described phenotypes resulted solely from the inhibition of HMGR activity.

Figure 3.
Quantification of MEV and FSM Resistance in the rim1 Mutant.

FSM is a strong inhibitor of DXR activity (Schwender et al., 1999; Mueller et al., 2000) that specifically blocks the biosynthesis of plastidial isoprenoids (Zeidler et al., 1998; Rodríguez-Concepción et al., 2001; Sharkey et al., 2001; Okada et al., 2002; Laule et al., 2003), causing a bleached phenotype (Figure 2H). FSM also led to a concentration-dependent inhibition of seedling establishment (Figure 3). Seedling establishment rates correlated with the level of photosynthetic pigments in the presence of sublethal concentrations of FSM. However, not every yellowish-green seedling was capable of eventually developing, suggesting that a threshold level of plastidial isoprenoid biosynthesis is required for seedling development. From 50 μM FSM, all of the seedlings showed an albino phenotype and complete developmental arrest. Together, our results highlight the essential nature of both the MVA pathway and the MEP pathway for seedling development in Arabidopsis. They also show that under normal growth conditions, blockage in one of the pathways is not rescued by the activity of the other pathway, arguing against a constitutively active exchange of prenyl diphosphate precursors between the cytosol and the plastid.

Developmental Defects Caused by both MEV and FSM Are Partially Rescued in rim1 Seedlings

To gain new insights into the regulation of isoprenoid biosynthesis in plant cells, we searched the ~23,000 Weigel activation-tagging T-DNA lines (Weigel et al., 2000) for MEV-resistant mutants. After germination on 5 μM MEV, M1 seedlings that produced at least two sets of true leaves were transferred to soil and allowed to develop and set seed. The M2 seeds were germinated on MEV to confirm their resistance. From the 142 M1 plants isolated, only 14 M2 lines were true rim (resistant to inhibition with mevinolin) mutants. To identify those mutants that potentially alter the exchange mechanism, we searched for MEV-resistant mutants that also could survive in the presence of FSM. Only one of them, rim1, was able to produce green leaves in the presence of FSM. Therefore, rim1 plants, which were homozygous for the single Mendelian locus conferring resistance to both inhibitors (see below), were selected for further analysis.

Figure 3 shows a quantification of the rim1 phenotype of resistance to MEV and FSM compared with the wild-type line and HMGR-overproducing plants. In the case of MEV, three phenotypes were measured: development of adventitious roots, undifferentiated proliferation of the SAM, and seedling establishment rates. Both rim1 and H1cd/3 transgenic plants showed a shift in the production of adventitious roots at higher concentrations of MEV (Figure 3A) and a decreased rate of undifferentiated SAM growth (Figure 3B). Seedling establishment rates were increased in rim1 and H1cd/3 plants compared with wild-type plants (Figure 3C). This phenotype was the easiest to detect visually and quantify accurately; therefore, it was selected to monitor MEV resistance in subsequent experiments. The rate of seedling establishment also was used to quantify FSM resistance. No significant differences were found between wild-type and H1cd/3 plants, whereas the percentage of seedling establishment in the rim1 mutant shifted at higher concentrations of FSM (Figure 3D). These results show that the activation of the MVA pathway by increasing HMGR activity is not sufficient to rescue the blockage of the MEP pathway.

The rim1 Mutation Results in an Upregulation of HMGR Gene Expression and Enzyme Activity but Does Not Activate Genes of the MEP Pathway

The resistance of rim1 to both inhibitors might result from either the upregulation of the mechanism for the interchange of prenyl diphosphates between the cytosol and the plastid or the increased activity of both the MVA and MEP pathways. We first determined whether higher levels of HMGR-encoding mRNAs were present in rim1 (Figure 4). Transcript levels of HMG1 and HMG2, the two Arabidopsis genes that encode HMGR (Enjuto et al., 1994, 1995; Learned and Connolly, 1997), were increased significantly in rim1 seedlings compared with the wild type (Figures 4A and 4B), resulting in 45% higher HMGR activity (Figure 4C). To determine whether this increase in enzyme activity could explain the MEV resistance of rim1, we quantified this phenotype in transgenic H1S and H1cd/3 plants showing different HMGR activity levels (González, 2002). Figure 4D shows that MEV resistance increased with HMGR activity in a hyperbolic manner, with an initial nearly linear portion of the curve showing the maximum slope. As a consequence, moderate increases in HMGR activity resulted in a dramatic increase in MEV resistance. In the case of rim1, the 1.45-fold increment in HMGR activity was sufficient to explain the observed seedling establishment rate of ~35% on 5 μM MEV (Figure 3C), as deduced from representing the two values graphically (Figure 4D). Therefore, we conclude that the MEV-resistant phenotype of rim1 plants is caused by higher HMGR activity derived from the upregulation of HMG1 and HMG2 gene expression.

Figure 4.
Analysis of HMGR Gene Expression and Activity in rim1 Seedlings.

An increase in DXR activity and in DXP levels could result in resistance to FSM (Zeidler et al., 1998). Therefore, we reasoned that the FSM-resistant phenotype of rim1 might be caused by an upregulation of the genes that encode DXR and/or DXP synthase (DXS; Figure 1). Figure 5A shows that DXS transcripts levels were increased slightly in the mutant, whereas those of DXR were unchanged. Immunoblot analysis, however, showed that the observed increase in DXS transcripts was not correlated with higher protein levels (Figure 5B). In the absence of efficient methods to determine DXS or DXR activity in plant extracts, we measured the amounts of chlorophylls and carotenoids as an indirect estimate of the activity of the MEP pathway. Mutant seedlings produced only 62% of the chlorophylls and 69% of the carotenoids found in wild-type seedlings (Figure 5C). Together, our results indicate that the FSM-resistant phenotype of rim1 seedlings was not derived from increased DXS and DXR levels.

Figure 5.
Analysis of the Molecular Basis of the rim1 FSM-Resistant Phenotype.

To test whether the FSM resistance of rim1 could result from an increased uptake of cytosolic precursors for the synthesis of plastidial isoprenoids, we introduced the 35S:HMG1cd construct into the rim1 mutant by crossing with H1cd/3 transgenic plants. Seeds from F2 rim1 plants harboring the 35S:HMG1cd construct were germinated on 5 μM MEV (to confirm the presence of higher HMGR activity levels) or 40 μM FSM. As a control, we also germinated seeds with a sublethal concentration of norflurazon, a specific inhibitor of carotenoid biosynthesis that acts downstream of prenyl diphosphate synthesis (Fraser et al., 2000) but generates a phenotype very similar to that observed with FSM. Unlike the blockage caused by FSM, however, the developmental blockage caused by this inhibitor cannot be rescued by an import of precursors from the cytosol (Figure 1). Figure 5D shows that seedling establishment rates in the presence of norflurazon were independent of HMGR activity levels. Unlike that described for the wild type (Figure 3D), however, the upregulation of HMGR activity in rim1 seedlings expressing the 35S:HMG1cd construct led to significantly greater FSM resistance (Figure 5D). These data provide genetic evidence that the increased production of MVA-derived precursors in rim1 35S:HMG1cd seedlings can be channeled to the biosynthesis of plastidial isoprenoids, likely because the exchange mechanism is activated in the mutant.

rim1 Is a New Loss-of-Function phyB Allele

To identify the gene responsible for the rim1 phenotype, we first studied whether the mutation was linked to the presence of the T-DNA used to generate the activation-tagging lines (Weigel et al., 2000). Backcrossing of rim1 plants with the wild type followed by selfing resulted in the generation of an F2 population in which the T-DNA resistance marker (BASTA) segregated in a monogenic manner. Selfing of plants either azygous or homozygous for the T-DNA generated F3 seeds, which were evaluated for MEV and FSM resistance. Although the resistance to MEV and FSM was linked, we found both BASTA-sensitive/inhibitor-resistant and BASTA-resistant/inhibitor-sensitive plants, indicating that the MEV- and FSM-resistant phenotype of rim1 was not caused by the presence of the T-DNA. However, some other characteristic phenotypes of rim1 plants did cosegregate with resistance to both inhibitors. The hypocotyl of rim1 seedlings was significantly longer than that of the wild type under long-day conditions but not in the dark (Figure 6). In addition, mutant plants had elongated leaf petioles and inflorescences, flowered earlier under short-day conditions, and showed a distinctive pale green color (Figure 6). These phenotypes also have been described in mutants with an activated response to gibberellins (GAs) and in light-hyposensitive mutants. Mutants of the first class include Arabidopsis spy (Jacobsen and Olszewski, 1993). The second group comprises mutants defective in light perception and signal transduction, such as phyB (deficient in the photoreceptor phytochrome B), hy1, hy2, and hy6 (which affect the biosynthesis of the tetrapyrrole chromophore of phytochromes), hy4 (or cry1; deficient in the photoreceptor cryptochrome 1), and hy5 (a key component of phytochrome- and cryptochrome-mediated signaling pathways) (reviewed by Quail, 2002).

Figure 6.
Characteristic Phenotypes of the rim1 Mutant.

As a first approach to investigate whether the stimulation of GA biosynthesis or response resulted in MEV resistance, we germinated wild-type seeds on plates supplemented with MEV or with both gibberellic acid and MEV. The presence of GAs in the growth medium resulted in elongated hypocotyls (data not shown) but did not affect MEV resistance (Figure 7). Consistently, seedling establishment rates were not increased in the spy-5 mutant (Figure 7), confirming that GAs do not contribute significantly to the upregulation of HMGR activity and derived MEV resistance. We then tested whether inhibited light perception and signal transduction in wild-type plants resulted in MEV resistance. After stratification and a 1-h pulse of white light to stimulate germination, seedlings were kept for 2 days in the dark and then transferred to a long-day chamber. The elongated wild-type seedlings achieved seedling establishment rates similar to those of rim1 (Figure 7). Furthermore, the light-hyposensitive mutants phyB-5, hy1-1, hy4-1, and hy5-1 showed the MEV-resistant phenotype (Figure 7). These results suggest that reduced exposure or sensitivity to light increase MEV resistance.

Figure 7.
Effect of Environmental and Genetic Factors on MEV Resistance.

Based on the coincidence of phenotypes between rim1 and the light-hyposensitive mutants, we speculated that the rim1 mutation might impair a known light perception or signal transduction pathway. To test this possibility, rim1 was crossed with some of these mutants to study their genetic interactions. The analysis of the F1 plants showed that rim1 was actually allelic with the phyB mutant. Sequencing of the PHYB gene from rim1 plants showed a point deletion that caused a shift in the reading frame after position 490 of the deduced protein and a stop codon at position 499 (Figure 8). Therefore, the protein encoded by the mutated phyB gene in rim1 lacks 682 residues of the C-terminal region. Because a missense mutation in the null phyB-5 allele (hy3-8-36) results in a protein lacking 620 residues of the C-terminal domain (Reed et al., 1993), it is likely that rim1 also is a null phyB allele (hereafter named phyB-m1). Together, these data reveal a role for phyB-mediated light perception and signaling in the regulation of isoprenoid metabolism in Arabidopsis.

Figure 8.
Map of the rim1 Mutation.

Light-Hyposensitive Mutants Show Differential Sensitivity to MEV and FSM

Figure 7 shows that impaired light perception and signaling in the hy4-1 (cry1) mutant results in MEV resistance, suggesting a role of the cryptochrome cry1 in the downregulation of HMGR activity. To dissect the contribution of phytochromes, we studied the MEV-resistant phenotype of phyA, phyB, and phyD mutants. As shown in Figure 9, phyA and phyD plants showed lower levels of resistance than did phyB plants. Double mutants showed mean seedling establishment rates that were higher than those in the single mutants, suggesting an additive contribution of phytochromes to the light-mediated repression of HMGR gene expression. Consistently, the phyA phyB phyD triple mutant showed the highest level of seedling establishment, ~70% (Figure 9). HMGR activity in the mutants ranged from 1.07-fold (phyD) to 1.77-fold (phyA phyB phyD) compared with that in wild-type seedlings, showing a good linear correlation (R2 = 0.97) with MEV resistance (Figure 9). Together, our data show that several photoreceptors participate in the light-mediated downregulation of HMGR activity in Arabidopsis seedlings. A role for the transcription factor HY5 also can be deduced from the MEV-resistant phenotype of hy5-1 seedlings and their increased HMGR activity (Figure 9).

Figure 9.
Correlation between MEV Resistance and HMGR Activity in Light-Hyposensitive Mutants.

To study whether the proteins involved in the perception and transduction of light signals that eventually result in HMGR upregulation and MEV resistance also were responsible for FSM resistance, seeds from light-hyposensitive mutants were germinated in the presence of three different concentrations of FSM. Unlike that shown for MEV, FSM resistance was similar in phyA and phyB plants (Figure 10), suggesting a prominent role for both phyA and phyB in the downregulation of the cellular mechanism responsible for FSM resistance upon illumination. Although a role for phyD in this process was not clearly supported by the data, the increasing levels of FSM resistance observed in the triple mutant phyA phyB phyD (Figure 10) supported an additive role for all of the phytochromes. By contrast, seedling establishment rates of hy4-1 and hy5-1 seedlings were similar to wild-type rates, arguing against a role for cry1 and HY5 in regulating FSM resistance (Figure 10). The differential sensitivity of the light-hyposensitive mutants to MEV or FSM reveals the existence of distinct genetic pathways that regulate resistance to the inhibitors.

Figure 10.
FSM Resistance of Light-Hyposensitive Mutants.

DISCUSSION

Molecular Basis of the Resistance of Mutant Seedlings to MEV and FSM

The identification of the phyB-m1 mutant provides genetic evidence that light-triggered signals coordinate isoprenoid biosynthesis in plant cells. The MEV-resistant phenotype of mutant plants is caused by an upregulation of HMGR gene expression and activity (Figure 4), but we found no evidence for an activated MEP pathway that could explain the FSM-resistant phenotype (Figure 5). Our results are consistent with an upregulated uptake of MVA-derived precursors for plastidial isoprenoid synthesis in phyB-m1 seedlings. Strong biochemical evidence for such an exchange of precursors between the cytosol and the plastid was reported recently (Kasahara et al., 2002; Nagata et al., 2002; Hemmerlin et al., 2003). Bidirectional cooperation of the two isoprenoid pathways for the biosynthesis of sterols and gibberellins in Arabidopsis seedlings (Kasahara et al., 2002), and sterols and plastoquinone in tobacco Bright Yellow-2 culture cells (Hemmerlin et al., 2003), was demonstrated in feeding experiments with labeled MVA or deoxyxylulose (DX; the dephosphorylated product of DXP). The authors also showed that DX partially rescued the inhibition of the MVA pathway, whereas exogenously provided MVA partially restored the production of plastidial isoprenoids when the MEP pathway was blocked (Kasahara et al., 2002; Nagata et al., 2002; Hemmerlin et al., 2003). In contrast with these results, increased MVA production by constitutive overexpression of HMGR in transgenic Arabidopsis plants rescued plastidial isoprenoid synthesis only in mutant phyB-m1 seedlings (Figure 5D). A nonspecific effect of increased HMGR activity on the activation of chloroplast development and/or plastidial isoprenoid biosynthesis was disproved by two lines of evidence. First, a similar increase of HMGR activity in transgenic plants that were not impaired in phyB function did not result in increased FSM resistance (Figure 3D). Second, pigment accumulation and the seedling establishment rate of phyB-m1 seedlings grown in the presence of norflurazon (Figure 1) were independent of HMGR activity levels (Figure 5D). Therefore, our results are best explained by a mechanism that allows the rescue of plastidial isoprenoid biosynthesis in FSM-grown mutant seedlings by activating the uptake of MVA-derived isoprenoid precursors from the cytosol.

Distinct Light Perception and Signal Transduction Pathways Regulate the Biosynthesis and Exchange of Isoprenoid Precursors within the Plant Cell

Allelism tests and sequencing confirmed a loss-of-function mutation of the gene that encodes phytochrome B in phyB-m1 plants (Figure 8), whereas analysis of other alleles, such as phyB-5, demonstrated that impairment of phyB function was responsible for the observed MEV-resistant phenotype of mutant seedlings (Figure 7). During seedling development, two main types of photoreceptors, phytochromes and cryptochromes, detect variations in the intensity, duration, and spectral quality of light signals. In Arabidopsis, there are two cryptochromes (cry1 and cry2) and five phytochromes (phyA to phyE). During deetiolation, cry1 has a prevalent role in the response to strong blue light, phyA plays a role in the response to continuous far-red light, and phyB plays a role in the response to continuous red light (Lin, 2000; Quail, 2002). Although the study of photoreceptor-mediated responses has been focused mostly on hypocotyl elongation and cotyledon expansion concomitant with deetiolation, limited attention has been given to the regulation of metabolic responses such as isoprenoid biosynthesis. A previous study (Learned, 1996) showed that the quantitative downregulation of HMG1 expression by light depends on fluence rate, time of illumination, and spectral quality involving multiple photoreceptors. In agreement with these results, we found that mutants for the corresponding main photoreceptors (cry1 and phyB, respectively) showed increased HMGR activity and derived MEV resistance (Figure 9). The additive phenotypes of the double and triple phytochrome mutants also revealed a role for phyA and phyD in the regulation of HMGR activity and suggested that light triggers independent signaling pathways that converge to downregulate HMGR gene expression and activity. It has been proposed that cryptochrome and phytochrome signaling pathways converge to upregulate the abundance of HY5, a transcription factor of the basic domain/Leu zipper protein family with a major role in regulating light-induced deetiolation (Oyama et al., 1997; Chattopadhyay et al., 1998; Quail, 2002). Our results (Figure 9) support the notion that such accumulation of HY5 results in the downregulation of HMGR activity. Therefore, HY5 might be involved in the regulation of HMGR activity by integrating different cryptochrome- and phytochrome-mediated signaling cascades. A distinct light-signaling pathway appears to regulate the exchange mechanism likely responsible (at least in part) for FSM resistance. The FSM resistance of phytochrome mutants indicate a main role for both phyA and phyB (Figure 10), whereas the sensitivity hy4-1 and hy5-1 seedlings suggests a cry1-independent, phytochrome-specific transduction pathway that does not involve HY5 function.

A Model for the Regulation of Arabidopsis Isoprenoid Biosynthesis during Seedling Development

Considering all of the available data, we propose a model for the regulation of the biosynthesis and exchange of isoprenoid precursors during Arabidopsis seedling development. When seeds germinate in the dark, a very early induction of HMGR-encoding genes leads to an active MVA pathway in etiolated seedlings (Enjuto et al., 1994, 1995; Learned and Connolly, 1997). Because etiolated growth involves high cell elongation rates, it can be expected that MVA-derived isoprenoids, such as sterols and brassinosteroids, are required at high levels. Brassinosteroids also have been shown to be key for the morphogenetic program in the dark (reviewed by Clouse, 2001). The activity of the MEP pathway appears to be reduced in etiolated seedlings, as deduced from the low level of expression of the genes that encode the proposed rate-limiting enzymes DXS and DXR (Mandel et al., 1996; Estévez et al., 2001; Carretero-Paulet et al., 2002). Therefore, it is likely that MVA-derived prenyl diphosphates can be transported into etioplasts for the synthesis of gibberellins (Kasahara et al., 2002) and carotenoids (which are required for plastid development) (Park et al., 2002). Upon illumination, cryptochrome and phytochrome photoreceptors trigger specific signaling pathways that converge in HY5 and possibly other factors, eventually repressing HMGR gene expression. A distinct phytochrome-specific and HY5-independent pathway would transduce the light signal to repress the uptake of cytosolic prenyl diphosphates by the developing chloroplasts. In addition, light upregulates the expression of MEP pathway genes (Mandel et al., 1996; Carretero-Paulet et al., 2002), resulting in an activated synthesis of plastidial isoprenoids, many of which are key for photosynthesis (Figure 1). In summary, our work shows that environmental light conditions are sensed and transduced by distinct signaling pathways that integrate this information to adjust the previously established genetic program of seedling development and coordinate the metabolism of isoprenoids. We are currently identifying and characterizing other mutants resistant to blockage in the MVA or MEP pathway, which should provide further insights into the molecular networks that regulate isoprenoid biosynthesis in plants.

METHODS

Plant Genotypes and Growth Conditions

The Weigel activation-tagging collection (Weigel et al., 2000), generated in the Columbia-7 (Col) background of Arabidopsis thaliana, was purchased from the Nottingham Arabidopsis Stock Centre (NASC). Seeds from the mutants spy-5, phyB-5, hy1-1, hy4-1, and hy5-1, all of them in the Landsberg erecta background, also were obtained from the NASC. Monogenic, digenic, and trigenic phytochrome mutants in the Landsberg erecta ecotype (Devlin et al., 1999) were a gift from Robert Sharrock (Montana State University, Bozeman). Plasmid pBI221 (Clontech, Palo Alto, CA) was used to generate the constructs 35S:HMG1cd and 35S:HMG1 and to transform plants as described (González, 2002).

Seeds were surface-sterilized and germinated on Petri dishes with solid Murashige and Skoog (1962) medium as described (Carretero-Paulet et al., 2002). Unless stated otherwise, after stratification for at least 2 days at 4°C, plates were incubated at 22°C in a growth chamber under long-day conditions (16 h under fluorescent white light and 8 h in the dark). Where indicated, the medium was supplemented with different concentrations of mevinolin (MEV; Sigma, St. Louis, MO) or fosmidomycin (FSM; Gateway Chemical Technology, St. Louis, MO) prepared as described (Rodríguez-Concepción and Gruissem, 1999; Rodríguez-Concepción et al., 2001). Norflurazon (Zorial; Novartis, Basel, Switzerland) was prepared as a 0.64 mM stock solution in water and diluted to a final concentration of 50 nM in the growth medium. Gibberellic acid (Fluka, Milwaukee, WI) was used at a final concentration of 100 μM. If required, plants were transferred from the plates to a 1:1:1 (v/v) perlite:vermiculite:sphagnum soil mixture irrigated with mineral nutrients and grown in the greenhouse until seeds were produced. Flowering time was monitored in plants growing in a chamber under a short-day regime (cycles of 8 h of light and 16 h of dark).

Analysis of Inhibitor-Induced Phenotypes

Seedlings grown on Murashige and Skoog (1962) plates supplemented with MEV or FSM were inspected visually to monitor seedling establishment (defined as the percentage of seedlings producing green true leaves that are photosynthetically active and therefore able to support full plant development). Adventitious root formation and undifferentiated proliferation of the shoot apical meristem in MEV-grown seedlings was confirmed with a dissecting microscope. Samples prepared for scanning electron microscopy were fixed in 50% ethanol, 10% formaldehyde, 5% acetic acid, and 1% Triton X-100, dehydrated in ethanol, dried in a critical point dryer, sputter-coated with gold particles, and viewed on a JEOL JSM-840 scanning microscope.

Biochemical Characterization of the Plants

For the measurement of HMGR activity, ~250 mg of seedlings was frozen in liquid nitrogen, ground to a fine powder, and mixed with 500 μL of prechilled extraction buffer containing 100 mM sucrose, 40 mM sodium phosphate, pH 7.5, 30 mM EDTA, 50 mM NaCl, 10 mM DTT, 10 μg/mL aprotinin, 1 μg/mL E64, 0.5 μg/mL leupeptin, 1 μg/mL pepstatin, 0.5 mM phenylmethylsulfonyl fluoride, and 0.25% (w/v) Triton X-100. The slurry was centrifuged at 200g at 4°C for 10 min to remove cell debris. The HMGR activity of the supernatant was assayed as described (Dale et al., 1995).

Plastidial isoprenoid pigments were extracted and separated by HPLC on a Waters Alliance 2690 system (Milford, MA) using a reverse-phase YMC 5-μm (250 × 4.6 mm) C30 column as described (Fraser et al., 2000). Eluting compounds were monitored using a Waters 2996 photodiode array detector. Canthaxanthin (a kind gift from Merck) was used as an internal standard for quantification. Peak areas of chlorophylls (chlorophylls a and b) and major carotenoids (lutein and β-carotene) at 450 nm were determined using the Waters Millennium version 3.2 software supplied.

Molecular Characterization of the Mutants

Whole seedlings were homogenized in liquid nitrogen to a fine powder, which then was used for RNA extraction as described (Lois et al., 2000). Total RNA from Col seedlings was used as a template to amplify the 3′ untranslated region sequence of the HMG1 gene and the full-length DXS and DXR cDNAs with specific primers using the SuperScript One-Step reverse transcriptase–mediated PCR system (Gibco BRL). To confirm that the amplified PCR products corresponded to the target genes, direct sequencing of the PCR products with internal primers was performed using the Big Dye Terminator Cycle Sequencing version 2.0 kit of the ABI-PRISM system (Perkin-Elmer Biosystems). The 25S rDNA template was obtained as described (Lois et al., 2000). Probes for RNA gel blot experiments were made by labeling the corresponding cDNA molecules with 32P-dCTP using the Ready-to-Go kit (Pharmacia). Blotting, hybridization, and washes were performed as described (Lois et al., 2000). A Molecular Imager FX phosphorimager (Bio-Rad) was used for exposure and quantification of the radioactive signals with the supplied Quantity One software.

For reverse transcriptase–mediated PCR experiments, different amounts of cDNA generated with the cDNA Synthesis System (Gibco BRL) from the same RNA samples used for RNA gel blot analysis were distributed in separate tubes. Fragments of 0.3, 0.5, and 0.7 kb were amplified by PCR from the ACT2, HMG1, and HMG2 genes, respectively, with Taq-DNA polymerase (Gibco BRL) and the primers ACT2F (5′-GCAAGTCATCACGATTGGTG-3′) and ACT2R (5′-ACAGTGTCTGGATCGGTGGTTC-3′), H1-2F (5′-CAGCCTCGCACTTCCGATGAC-3′) and H1-3R (5′-GCAACGCCTCACGACGAATCG-3′), or H2-4F (5′-CTGTATCCGAGGTTTGCGTG-3′) and H2-3R (5′-GATCCTTTCACACCGAGTAG-3′). After adjusting the initial amount of cDNA and the number of amplification cycles for each gene, the relative abundance of HMG1 and HMG2 was normalized to the constitutive expression level of ATC2.

For immunoblot analysis, proteins were extracted from Arabidopsis seedlings, electrophoresed by 10% SDS-PAGE, and either stained with Coomassie Brilliant Blue R250 to monitor equal loading or electrotransferred to Hybond-P polyvinylidene difluoride membranes (Amersham) as described (Rodríguez-Concepción et al., 2001). The DXS (CLA1) protein was detected with a 1:500 dilution of the anti-GST:CLA1 antiserum (Estévez et al., 2000) as described (Carretero-Paulet et al., 2002).

Identification of the rim1 Mutation

Crossing of homozygous rim1 and phyB-5 plants resulted in an F1 population in which all of the individuals displayed the mutant phenotype, characterized by long hypocotyls and petioles and a distinctive pale green color. To sequence the PHYB gene from Col and rim1 plants, genomic DNA was extracted from seedlings as described (Carretero-Paulet et al., 2002) and used as a template for PCR amplification with Pfu DNA polymerase (Promega) and primers designed to amplify two overlapping fragments of 2.5 kb covering the complete gene sequence. Direct sequencing of the PCR products with internal primers was performed as described above. Upon confirmation that rim1 was a new phyB mutant, we changed its name to phyB-m1.

Upon request, materials integral to the findings presented in this publication will be made available in a timely manner to all investigators on similar terms for noncommercial research purposes. To obtain materials, please contact Manuel Rodríguez-Concepción, se.bu.qb.nus@ugirdorm.

Acknowledgments

We are very grateful to the NASC for providing valuable seed and information resources, to R. Sharrock (Montana State University) for providing the seeds of phytochrome mutants, and to P. León (Instituto de Biotecnología, Cuernavaca, Mexico) for the gift of the anti-DXS serum. The excellent technical support of A. Orozco and the staff of the Serveis Cientificotècnics and the Serveis de Camps Experimentals of the Universitat de Barcelona is greatly appreciated. We also thank S. Pelaz (Instituto de Biologia Molecular de Barcelona, Spain) and the members of our laboratories for stimulating discussions regarding the manuscript. This work was supported by Grants FEDER-BIO2002-01653, FEDER-BIO2002-00298, and BIO2000-0334 from the Spanish Ministerio de Ciencia y Tecnología to M.R.-C., J.F.M.-G., and A.F., respectively, and by Grant 2001SGR00109 from the Generalitat de Catalunya to A.B. O.F. and V.G. received a predoctoral Spanish Ministerio de Educación y Cultura fellowship.

Notes

Article, publication date, and citation information can be found at www.plantcell.org/cgi/doi/10.1105/tpc.016204.

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