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Carotenoids are plastidial isoprenoid pigments essential for plant life. High carotenoid levels are found in chloroplasts and chromoplasts, but they are also produced in the etioplasts of seedlings that germinate in the dark. Our recent work has shown that an enhanced production of carotenoids in plastids of dark-grown Arabidopsis thaliana seedlings results in an improved transition to photosynthetic development (greening) upon illumination, illustrating the relevance of regulating etioplast carotenoid biosynthesis for plant fitness. We showed that the biosynthesis of etioplast carotenoids is controlled at the level of phytoene synthase (PSY), the enzyme catalyzing the first committed step of the pathway. Upregulation of PSY is necessary and sufficient to increase the production of carotenoids in dark-grown seedlings, in part because it triggers a feedback mechanism leading to the post-transcriptional accumulation of flux-controlling enzymes of the methylerythritol 4-phosphate (MEP) pathway, which synthesizes the substrates for PSY activity. Based on these and other recent data on the molecular mechanisms controlling deetiolation, we propose a model for the regulation of carotenoid biosynthesis in etioplasts.
After germination, plant development can follow two different pathways depending on light conditions: skotomorphogenesis in darkness and photomorphogenesis in the light.1 Skotomorphogenic development results in etiolated seedlings with long hypocotyls and closed unexpanded cotyledons with etioplasts containing chlorophyll precursors and relatively low levels of carotenoids associated to the pro-lamellar body (PLB). Etioplast carotenoids (lutein, violaxanthin and much smaller amounts of other carotenoids) have been proposed to participate in the assembly of the PLB, a lattice of tubular membranes that facilitates greening when underground seedlings emerge into the light.2,3 Upon light perception, photomorphogenic development is derepressed, resulting in decreased hypocotyl elongation, cotyledon expansion and differentiation of etioplasts into chloroplasts.1 This light-induced deetiolation process involves the production of high levels of chlorophylls and carotenoids in chloroplasts to support photosynthetic development. Chloroplast carotenoids act as membrane stabilizers and accessory light-harvesting pigments, but their essential role is to channel excess energy away from chlorophylls for protection against photooxidative damage.4 Carotenoids (but not chlorophylls) also accumulate in chromoplasts of flowers and fruits (providing distinctive yellow to red colors) and, to a much lower level, in other non-photosynthetic plastids of adult plants such as amyloplasts (starch-storing plastids), elaioplasts (lipid-storing plastids) and leucoplasts.5
Deetiolation can be derepressed in the absence of actual light by manipulating the levels of proteins and hormones involved in its control. For example, reduced activity of the E3 ubiquitin ligase COP1 (which interacts with HY5 and other transcription factors required for photomorphogenesis and promotes their proteasome-mediated degradation) or of the members of the PIF subfamily of bHLH transcription factors (PIF1, PIF3, PIF4 and PIF5) results in a partially deetiolated phenotype in the dark.6,7 Similary, a block in the production or signaling of hormones controlling deetiolation such as brassinosteroids (BRs) and gibberellins (GAs) also leads to a photomorphogenetic phenotype in the absence of illumination.8–10 In particular, GAs appear to promote skotomorphogenic (etiolated) growth after germination by maintaining low levels of DELLA, GA signalling proteins that are negative regulators of HY5 and PIF function.11,12 Derepression of deetiolation in mutants defective in COP1 (cop1–4) or BRs (det2-1) or in wild-type seedlings treated with paclobutrazol (PAC, an inhibitor of GA biosynthesis) results in a ca. Two-fold accumulation of carotenoids in the dark.13 Interestingly, PAC-treated seedlings became greener faster when transferred to light in the absence of the inhibitor, indicating that carotenoids contribute to a proper adaptation of soil-emerging seedlings to sunlight. Photomorphogenic dark-grown seedlings showed an increased activity of phytoene synthase (PSY), the enzyme catalyzing the first committed step in the biosynthesis of carotenoids.14 Such increase resulted, at least in part, from an enhanced promoter activity and transcript accumulation of the only Arabidopsis gene encoding PSY in cotyledons, the carotenoid-accumulating organs of etiolated seedlings. Furthermore, it was demonstrated that the upregulation of PSY activity was sufficient to activate carotenoid synthesis in etioplasts.13 Analysis of gene expression and treatment with specific inhibitors demonstrated that the methylerythritol 4-phosphate (MEP) pathway supplied most of the prenyl diphosphate precursors required for carotenoid biosynthesis under these conditions. Although expression of genes encoding flux-controlling enzymes of the MEP pathway was unaltered in photomorphogenic dark-grown seedlings compared to skotomorphogenic controls, protein levels were higher in PAC-treated seedlings. Work with one of these enzymes, deoxyxylulose 5-phosphate synthase (DXS), showed that the observed post-transcriptional upregulation of MEP pathway enzyme levels was specifically triggered by the induction of PSY activity, most likely to ensure an appropriate supply of metabolic precursors.13 These results indicate that the regulation of PSY expression is the main driving force controlling carotenoid biosynthesis and precursor supply in etioplasts.
Based on the available data, we propose the following model (Fig. 1). When seedlings germinate and grow in the absence of light, high GA and COP1 levels result in low levels of DELLA and HY5 proteins, which together with high PIF levels result in skotomorphogenic (etiolated) development.6,12 Under these conditions, DXS and PSY gene expression and carotenoid synthesis are low (Fig. 1A). Derepression of photomorphogenesis (deetiolation) in dark-grown cop1–4 seedlings results from an increased accumulation of HY5 and other related transcription factors,7 some of which might regulate the expression of PSY but not DXS as deduced from the levels of the corresponding transcripts in mutant seedlings.13 Because COP1 has been reported to participate in the accumulation of PIF3 in the nucleus,15 it is possible that a decreased PIF activity in cop1–4 seedlings might also positively influence PSY expression and eventually lead to an enhanced accumulation of carotenoids. In PAC-treated seedlings, decreased GA levels would cause higher DELLA accumulation, leading to an enhanced accumulation of HY5 and a decreased PIF activity,11,16,17 as well as changes in carotenoid gene expression and accumulation similar to those described for cop1–4 seedlings.13 In both mutant and PAC-treated seedlings, upregulation of PSY gene expression would result in higher PSY protein and activity levels, which in turn would cause an enhanced accumulation of DXS enzymes and a concomitant increase in the supply of precursors by a feedback mechanism that remains to be characterized (Fig. 1B). Illumination quickly decreases the levels of bioactive GAs,18 COP1,7 and PIFs,19 which together could synergistically contribute to strongly upregulate PSY expression (Fig. 1C). The expression of DXS and other genes encoding MEP pathway enzymes is also upregulated by light, but the components of the signaling pathway involved remain unknown.20,21 Our data suggest that these components might be different from those regulating PSY expression in the dark. Post-transcriptional events appear to also regulate the level of the DXS protein during early development of illuminated seedlings,22 so it is possible that the feedback mechanism responsible for the enhanced accumulation of DXS protein in response to an induction of PSY activity in etioplasts might also be functional in chloroplasts. Because transgene-mediated upregulation of PSY levels in tomato fruit leads to an enhanced DXS activity without changes in gene expression,23 it is tempting to speculate that some of the features of the model proposed here for the regulation of carotenoid biosynthesis during the transition of etioplasts to chloroplasts might also be conserved when chloroplasts are transformed into chromoplasts.
Addendum to: Rodríguez-Villalón A, Gas E, Rodríguez-Concepción M. Phytoene synthase activity controls the biosynthesis of carotenoids and the supply of their metabolic precursors in dark-grown Arabidopsis seedlingsPlant J2009in press doi: 10.1111/j.1365313X.2009.03966.x..
Previously published online: www.landesbioscience.com/journals/psb/article/9672