A third phytoene synthase is devoted to abiotic stress-induced abscisic acid formation in rice and defines functional diversification of phytoene synthase genes

Carotenoid biosynthesis in Arabidopsis: a colorful pathway

Plant carotenoids are a family of pigments that participate in light harvesting and are essential for photoprotection against excess light. Furthermore, they act as precursors for the production of apocarotenoid hormones such as abscisic acid and strigolactones. In this review, we summarize the current knowledge on the genes and enzymes of the carotenoid biosynthetic pathway (which is now almost completely elucidated) and on the regulation of carotenoid biosynthesis at both transcriptional and post-transcriptional levels. We also discuss the relevance of Arabidopsis as a model system for the study of carotenogenesis and how metabolic engineering approaches in this plant have taught important lessons for carotenoid biotechnology.

Maize provitamin a carotenoids, current resources, and future metabolic engineering challenges

Vitamin A deficiency is a serious global health problem that can be alleviated by improved nutrition. Development of cereal crops with increased provitamin A carotenoids can provide a sustainable solution to eliminating vitamin A deficiency worldwide. Maize is a model for cereals and a major staple carbohydrate source. Here, we discuss maize carotenogenesis with regard to pathway regulation, available resources, and current knowledge for improving carotenoid content and levels of provitamin A carotenoids in edible maize endosperm. This knowledge will be applied to improve the nutritional composition of related Poaceae crops. We discuss opportunities and challenges for optimizing provitamin A carotenoid biofortification of cereal food crops.

Carotenoids in nature: insights from plants and beyond

Carotenoids are natural isoprenoid pigments that provide leaves, fruits, vegetables and flowers with distinctive yellow, orange and some reddish colours as well as several aromas in plants. Their bright colours serve as attractants for pollination and seed dispersal. Carotenoids comprise a large family of C40 polyenes and are synthesised by all photosynthetic organisms, aphids, some bacteria and fungi alike. In animals carotenoid derivatives promote health, improve sexual behaviour and are essential for reproduction. As such, carotenoids are commercially important in agriculture, food, health and the cosmetic industries. In plants, carotenoids are essential components required for photosynthesis, photoprotection and the production of carotenoid-derived phytohormones, including ABA and strigolactone. The carotenoid biosynthetic pathway has been extensively studied in a range of organisms providing an almost complete pathway for carotenogenesis. A new wave in carotenoid biology has revealed implications for epigenetic and metabolic feedback control of carotenogenesis. Developmental and environmental signals can regulate carotenoid gene expression thereby affecting carotenoid accumulation. This review highlights mechanisms controlling (1) the first committed step in phytoene biosynthesis, (2) flux through the branch to synthesis of α- and β-carotenes and (3) metabolic feedback signalling within and between the carotenoid, MEP and ABA pathways.

Expression, subcellular localization, and cis-regulatory structure of duplicated phytoene synthase genes in melon (Cucumis melo L.)

Carotenoids perform many critical functions in plants, animals, and humans. It is therefore important to understand carotenoid biosynthesis and its regulation in plants. Phytoene synthase (PSY) catalyzes the first committed and rate-limiting step in carotenoid biosynthesis. While PSY is present as a single copy gene in Arabidopsis, duplicated PSY genes have been identified in many economically important monocot and dicot crops. CmPSY1 was previously identified from melon (Cucumis melo L.), but was not functionally characterized. We isolated a second PSY gene, CmPSY2, from melon in this work. CmPSY2 possesses a unique intron/exon structure that has not been observed in other plant PSYs. Both CmPSY1 and CmPSY2 are functional in vitro, but exhibit distinct expression patterns in different melon tissues and during fruit development, suggesting differential regulation of the duplicated melon PSY genes. In vitro chloroplast import assays verified the plastidic localization of CmPSY1 and CmPSY2 despite the lack of an obvious plastid target peptide in CmPSY2. Promoter motif analysis of the duplicated melon and tomato PSY genes and the Arabidopsis PSY revealed distinctive cis-regulatory structures of melon PSYs and identified gibberellin-responsive motifs in all PSYs except for SlPSY1, which has not been reported previously. Overall, these data provide new insights into the evolutionary history of plant PSY genes and the regulation of PSY expression by developmental and environmental signals that may involve different regulatory networks.

Characterization and developmental expression of genes encoding the early carotenoid biosynthetic enzymes in Citrus paradisi Macf

In the present study, the full-length cDNA sequences of PSY, PDS, and ZDS, encoding the early carotenoid biosynthetic enzymes in the carotenoid pathway of grapefruit (Citrus paradisi), were isolated and characterized for the first time. CpPSY contained a 1311-bp open reading frame (ORF) encoding a polypeptide of 436 amino acids, CpPDS contained a 1659-bp ORF encoding a polypeptide of 552 amino acids, and CpZDS contained a 1713-bp ORF encoding a polypeptide of 570 amino acids. Phylogenetic analysis indicated that CpPSY shares homology with PSYs from Citrus, tomato, pepper, Arabidopsis, and the monocot PSY1 group, while CpPDS and CpZDS are most closely related to orthologs from Citrus and tomato. Expression analysis revealed fluctuations in CpPSY, CpPDS, and CpZDS transcript abundance and a non-coordinated regulation between the former and the two latter genes during fruit development in albedo and juice vesicles of white ('Duncan') and red ('Flame') grapefruits. A 3× higher upregulation of CpPSY expression in juice vesicles of red-fleshed 'Flame' as compared to white-fruited 'Duncan' was observed in the middle stages of fruit development, which correlates with the well documented accumulation pattern of lycopene in red grapefruit. Together with previous data, our results suggest that the primary mechanism controlling lycopene accumulation in red grapefruit involves the transcriptional upregulation of CpPSY, which controls the flux into the carotenoid pathway, and the downregulated expression of CpLCYB2, which controls the step of cyclization of lycopene in chromoplasts during fruit ripening. A correlation between CpPSY expression and fruit color evolution in red grapefruit is demonstrated.

A transcriptional analysis of carotenoid, chlorophyll and plastidial isoprenoid biosynthesis genes during development and osmotic stress responses in Arabidopsis thaliana

Background

The carotenoids are pure isoprenoids that are essential components of the photosynthetic apparatus and are coordinately synthesized with chlorophylls in chloroplasts. However, little is known about the mechanisms that regulate carotenoid biosynthesis or the mechanisms that coordinate this synthesis with that of chlorophylls and other plastidial synthesized isoprenoid-derived compounds, including quinones, gibberellic acid and abscisic acid. Here, a comprehensive transcriptional analysis of individual carotenoid and isoprenoid-related biosynthesis pathway genes was performed in order to elucidate the role of transcriptional regulation in the coordinated synthesis of these compounds and to identify regulatory components that may mediate this process in Arabidopsis thaliana.

Results

A global microarray expression correlation analysis revealed that the phytoene synthase gene, which encodes the first dedicated and rate-limiting enzyme of carotenogenesis, is highly co-expressed with many photosynthesis-related genes including many isoprenoid-related biosynthesis pathway genes. Chemical and mutant analysis revealed that induction of the co-expressed genes following germination was dependent on gibberellic acid and brassinosteroids (BR) but was inhibited by abscisic acid (ABA). Mutant analyses further revealed that expression of many of the genes is suppressed in dark grown plants by Phytochrome Interacting transcription Factors (PIFs) and activated by photoactivated phytochromes, which in turn degrade PIFs and mediate a coordinated induction of the genes. The promoters of PSY and the co-expressed genes were found to contain an enrichment in putative BR-auxin response elements and G-boxes, which bind PIFs, further supporting a role for BRs and PIFs in regulating expression of the genes. In osmotically stressed root tissue, transcription of Calvin cycle, methylerythritol 4-phosphate pathway and carotenoid biosynthesis genes is induced and uncoupled from that of chlorophyll biosynthesis genes in a manner that is consistent with the increased synthesis of carotenoid precursors for ABA biosynthesis. In all tissues examined, induction of β-carotene hydroxylase transcript levels are linked to an increased demand for ABA.

Conclusions

This analysis provides compelling evidence to suggest that coordinated transcriptional regulation of isoprenoid-related biosynthesis pathway genes plays a major role in coordinating the synthesis of functionally related chloroplast localized isoprenoid-derived compounds.

Allelic variation, alternative splicing and expression analysis of Psy1 gene in Hordeum chilense Roem. et Schult

Background

The wild barley Hordeum chilense Roem. et Schult. is a valuable source of genes for increasing carotenoid content in wheat. Tritordeums, the amphiploids derived from durum or common wheat and H. chilense, systematically show higher values of yellow pigment colour and carotenoid content than durum wheat. Phytoene synthase 1 gene (Psy1) is considered a key step limiting the carotenoid biosynthesis, and the correlation of Psy1 transcripts accumulation and endosperm carotenoid content has been demonstrated in the main grass species.

Methodology/Principal findings

We analyze the variability of Psy1 alleles in three lines of H. chilense (H1, H7 and H16) representing the three ecotypes described in this species. Moreover, we analyze Psy1 expression in leaves and in two seed developing stages of H1 and H7, showing mRNA accumulation patterns similar to those of wheat. Finally, we identify thirty-six different transcripts forms originated by alternative splicing of the 5′ UTR and/or exons 1 to 5 of Psy1 gene. Transcripts function is tested in a heterologous complementation assay, revealing that from the sixteen different predicted proteins only four types (those of 432, 370, 364 and 271 amino acids), are functional in the bacterial system.

Conclusions/Significance

The large number of transcripts originated by alternative splicing of Psy1, and the coexistence of functional and non functional forms, suggest a fine regulation of PSY activity in H. chilense. This work is the first analysis of H. chilense Psy1 gene and the results reported here are the bases for its potential use in carotenoid enhancement in durum wheat.

Carotenoids and their cleavage products: biosynthesis and functions

This review focuses on plant carotenoids, but it also includes progress made on microbial and animal carotenoid metabolism to better understand the functions and the evolution of these structurally diverse compounds with a common backbone. Plants have evolved isogenes for specific key steps of carotenoid biosynthesis with differential expression profiles, whose characteristic features will be compared. Perhaps the most exciting progress has been made in studies of carotenoid cleavage products (apocarotenoids) with an ever-expanding variety of novel functions being discovered. This review therefore covers structural, molecular genetic and functional aspects of carotenoids and apocarotenoids alike. Apocarotenoids are specifically tailored from carotenoids by a family of oxidative cleavage enzymes, but whether there are contributions to their generation from chemical oxidation, photooxidation or other mechanisms is largely unknown. Control of carotenoid homeostasis is discussed in the context of biosynthetic and degradative reactions but also in the context of subcellular environments for deposition and sequestration within and outside of plastids. Other aspects of carotenoid research, including metabolic engineering and synthetic biology approaches, will only be covered briefly.

ZEBRA2, encoding a carotenoid isomerase, is involved in photoprotection in rice

"zebra" mutants have alternating green and chlorotic crossbands on leaf blades and are widely distributed in monocotyledonous crops. Most recently, we cloned the first responsible gene from rice, ZEBRA2, which also leads to the phenotype of rice preharvest sprouting. ZEBRA2, a single-copy gene in the rice genome, encodes a carotenoid isomerase (CRTISO), the key enzyme catalyzing the conversion of cis-lycopene to all-trans lycopene. ZEBRA2 shares high identity with known CRTISOs from other species. Expression analysis via both RT-PCR and ZEBRA2-promoter-β-glucuronidase (GUS) transgenic rice indicates that ZEBRA2 is predominantly expressed in mesophyll cells of mature leaves where active photosynthesis occurs. Consistent with the alteration in agronomic traits, the zebra2 mutant exhibits decreased photosynthetic rate and chlorophyll content. Mutation of the ZEBRA2 gene results in the accumulation of all-trans-lycopene precursor, prolycopene (7Z,9Z,7'Z,9'Z tetra cis-lycopene), in dark-grown zebra2 tissues. Light-grown zebra2 mutant exhibits the characteristic "zebra" phenotype and decreased level of lutein, the xanthophyll that is essential for efficient chl triplet quenching. More severe phenotype of the zebra2 mutant under high light intensity indicates that "zebra" phenotype might be caused by photooxidative damages. We conclude that ZEBRA2 is involved in photoprotection in rice.

Sequence Variation and Expression Analysis of Seed Dormancy- and Germination-Associated ABA- and GA-Related Genes in Rice Cultivars

Abscisic acid (ABA) and Gibberellic acid (GA) play key roles in regulating seed dormancy and germination. First, when examining germination of different rice cultivars, we found that their germination timing and dormancy status are rather distinct, coupled with different GA/ABA ratio. Second, we studied genomic sequences of ABA and GA dormancy- and germination-associated genes in rice and discovered single nucleotide polymorphisms and insertions/deletions (Indels) in both coding and regulatory sequences. We aligned all these variations to the genome assemblies of 9311 and PA64s and demonstrated their relevance to seed dormancy both quantitatively and qualitatively based on gene expression data. Third, we surveyed and compared differentially expressed genes in dry seeds between 9311 and PA64s to show that these differentially expressed genes may play roles in seed dormancy and germination.

Monitoring the expression of maize genes in developing kernels under drought stress using oligo-microarray

Preharvest aflatoxin contamination of grain grown on the US southeastern Coast Plain is provoked and aggravated by abiotic stress. The primary abiotic stress is drought along with high temperatures. The objectives of the present study were to monitor gene expression in developing kernels in response to drought stress and to identify drought-responsive genes for possible use in germplasm assessment. The maize breeding line Tex6 was used, and gene expression profiles were analyzed in developing kernels under drought stress verses well-watered conditions at the stages of 25, 30, 35, 40, 45 d after pollination (DAP) using the 70 mer maize oligo-arrays. A total of 9 573 positive array spots were detected with unique gene IDs, and 7 988 were common in both stressed and well-watered samples. Expression patterns of some genes in several stress response-associated pathways, including abscisic acid, jasmonic acid and phenylalanine ammonia-lyase, were examined, and these specific genes were responsive to drought stress positively. Real-time quantitative polymerase chain reaction validated microarray expression data. The comparison between Tex6 and B73 revealed that there were significant differences in specific gene expression, patterns and levels. Several defense-related genes had been downregulated, even though some defense-related or drought responsive genes were upregulated at the later stages.

Characterization of phytoene synthases from cassava and their involvement in abiotic stress-mediated responses

Abiotic stress stimuli induce the increased synthesis of abscisic acid (ABA), which is generated through the cleavage of xanthophyll precursors. To cope with the increased xanthophyll demand, maize and rice contain a third stress-induced gene copy, coding for phytoene synthase (PSY), which catalyzes the first carotenoid-specific reaction in the pathway. To investigate whether this specific response extends beyond the Poaceae, cassava was analyzed, an important tropical crop known for its drought tolerance. We also found three PSY genes in cassava, one of which (MePSY3) forms a separate branch with the stress-specific Poaceae homologs. However, MePSY3 transcripts were virtually absent in all tissues investigated and did not change upon abiotic stress treatment. In contrast, the two remaining PSY genes contributed differentially to carotenoid biosynthesis in leaves, roots, and flower organs and responded towards drought and salt-stress conditions. Detailed analyses of PSY and 9-cis-epoxycarotenoid cleavage dioxygenase (MeNCED) expression and resulting ABA levels revealed MePSY1 as the main stress-responsive paralog. In the presence of high carotenoid levels in leaves, MePSY1 appeared to support, but not to be rate-limiting for ABA formation; MeNCED represented the main driver. The inverse situation was found in roots where carotenoid levels are low. Moreover, ABA formation and the relative induction kinetics showed discrimination between drought and salt stress. Compared to rice as a drought-intolerant species, the drought response in cassava followed a different kinetic regime. The difference is thought to represent a component contributing to the large differences in the adaptation towards water supply.

Provitamin A accumulation in cassava (Manihot esculenta) roots driven by a single nucleotide polymorphism in a phytoene synthase gene

Cassava (Manihot esculenta) is an important staple crop, especially in the arid tropics. Because roots of commercial cassava cultivars contain a limited amount of provitamin A carotenoids, both conventional breeding and genetic modification are being applied to increase their production and accumulation to fight vitamin A deficiency disorders. We show here that an allelic polymorphism in one of the two expressed phytoene synthase (PSY) genes is capable of enhancing the flux of carbon through carotenogenesis, thus leading to the accumulation of colored provitamin A carotenoids in storage roots. A single nucleotide polymorphism present only in yellow-rooted cultivars cosegregates with colored roots in a breeding pedigree. The resulting amino acid exchange in a highly conserved region of PSY provides increased catalytic activity in vitro and is able to increase carotenoid production in recombinant yeast and Escherichia coli cells. Consequently, cassava plants overexpressing a PSY transgene produce yellow-fleshed, high-carotenoid roots. This newly characterized PSY allele provides means to improve cassava provitamin A content in cassava roots through both breeding and genetic modification.

Direct regulation of phytoene synthase gene expression and carotenoid biosynthesis by phytochrome-interacting factors

Carotenoids are key for plants to optimize carbon fixing using the energy of sunlight. They contribute to light harvesting but also channel energy away from chlorophylls to protect the photosynthetic apparatus from excess light. Phytochrome-mediated light signals are major cues regulating carotenoid biosynthesis in plants, but we still lack fundamental knowledge on the components of this signaling pathway. Here we show that phytochrome-interacting factor 1 (PIF1) and other transcription factors of the phytochrome-interacting factor (PIF) family down-regulate the accumulation of carotenoids by specifically repressing the gene encoding phytoene synthase (PSY), the main rate-determining enzyme of the pathway. Both in vitro and in vivo evidence demonstrate that PIF1 directly binds to the promoter of the PSY gene, and that this binding results in repression of PSY expression. Light-triggered degradation of PIFs after interaction with photoactivated phytochromes during deetiolation results in a rapid derepression of PSY gene expression and a burst in the production of carotenoids in coordination with chlorophyll biosynthesis and chloroplast development for an optimal transition to photosynthetic metabolism. Our results also suggest a role for PIF1 and other PIFs in transducing light signals to regulate PSY gene expression and carotenoid accumulation during daily cycles of light and dark in mature plants.

Transcriptional control of SET DOMAIN GROUP 8 and CAROTENOID ISOMERASE during Arabidopsis development

Carotenoids are pigments required for photosynthesis, photoprotection and the production of carotenoid-derived hormones such as ABA and strigolactones. The carotenoid biosynthetic pathway bifurcates after lycopene to produce epsilon- and beta-carotenoids and this branch is critical for determining carotenoid composition. Here, we show how the branch point can be regulated by the chromatin-modifying histone methyltransferase, Set Domain Group 8 (SDG8) targeting the carotenoid isomerase (CRTISO). SDG8 is required to maintain permissive expression of CRTISO during seedling development, in leaves, shoot apex, and some floral organs. The CRTISO and SDG8 promoters show overlapping tissue-specific patterns of reporter gene activity. Interestingly, CRTISO showed atypical reporter gene expression in terms of greater variability between different lines compared to the Cauliflower Mosaic Virus 35S promoter (CaMV35s) and epsilonLCY promoters, potentially due to chromosomal position effects. Regulation of the CRTISO promoter was dependent in part upon the presence or absence of SDG8. Knockouts of SDG8 (carotenoid and chloroplast regulation (ccr1)) and CRTISO (ccr2) result in altered carotenoid composition and this could be restored in ccr2 using the CaMV35s or CRTISO promoters. In contrast, varying degrees of GUS expression and carotenoid complementation by CRTISO overexpression using CaMV35S or CRTISO promoters in the ccr1 background demonstrated that both the CRTISO promoter and open reading frame are necessary for SDG8-mediated expression of CRTISO.

Carotenoid biosynthesis genes in rice: structural analysis, genome-wide expression profiling and phylogenetic analysis

Carotenoids, important lipid-soluble antioxidants in photosynthetic tissues, are known to be completely absent in rice endosperm. Many studies, involving transgenic manipulations of carotenoid biosynthesis genes, have been performed to get carotenoid-enriched rice grain. Study of genes involved in their biosynthesis can provide further information regarding the abundance/absence of carotenoids in different tissues. We have identified 16 and 34 carotenoid biosynthesis genes in rice and Populus genomes, respectively. A detailed analysis of the domain structure of carotenoid biosynthesis enzymes in rice, Populus and Arabidopsis has shown that highly conserved catalytic domains, along with other domains, are present in these proteins. Phylogenetic analysis of rice genes with Arabidopsis and other characterized carotenoid biosynthesis genes has revealed that homologous genes exist in these plants, and the duplicated gene copies probably adopt new functions. Expression of rice and Populus genes has been analyzed by full-length cDNA- and EST-based expression profiling. In rice, this analysis was complemented by real-time PCR, microarray and signature-based expression profiling, which reveal that carotenoid biosynthesis genes are highly expressed in light-grown tissues, have differential expression pattern during vegetative/reproductive development and are responsive to stress.

Carotenoid crystal formation in Arabidopsis and carrot roots caused by increased phytoene synthase protein levels

BACKGROUND

As the first pathway-specific enzyme in carotenoid biosynthesis, phytoene synthase (PSY) is a prime regulatory target. This includes a number of biotechnological approaches that have successfully increased the carotenoid content in agronomically relevant non-green plant tissues through tissue-specific PSY overexpression. We investigated the differential effects of constitutive AtPSY overexpression in green and non-green cells of transgenic Arabidopsis lines. This revealed striking similarities to the situation found in orange carrot roots with respect to carotenoid amounts and sequestration mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

In Arabidopsis seedlings, carotenoid content remained unaffected by increased AtPSY levels although the protein was almost quantitatively imported into plastids, as shown by western blot analyses. In contrast, non-photosynthetic calli and roots overexpressing AtPSY accumulated carotenoids 10 and 100-fold above the corresponding wild-type tissues and contained 1800 and 500 microg carotenoids per g dry weight, respectively. This increase coincided with a change of the pattern of accumulated carotenoids, as xanthophylls decreased relative to beta-carotene and carotene intermediates accumulated. As shown by polarization microscopy, carotenoids were found deposited in crystals, similar to crystalline-type chromoplasts of non-green tissues present in several other taxa. In fact, orange-colored carrots showed a similar situation with increased PSY protein as well as carotenoid levels and accumulation patterns whereas wild white-rooted carrots were similar to Arabidopsis wild type roots in this respect. Initiation of carotenoid crystal formation by increased PSY protein amounts was further confirmed by overexpressing crtB, a bacterial PSY gene, in white carrots, resulting in increased carotenoid amounts deposited in crystals.

CONCLUSIONS

The sequestration of carotenoids into crystals can be driven by the functional overexpression of one biosynthetic enzyme in non-green plastids not requiring a chromoplast developmental program as this does not exist in Arabidopsis. Thus, PSY expression plays a major, rate-limiting role in the transition from white to orange-colored carrots.

Timing and biosynthetic potential for carotenoid accumulation in genetically diverse germplasm of maize

Enhancement of the carotenoid biosynthetic pathway in food crops benefits human health and adds commercial value of natural food colorants. However, predictable metabolic engineering or breeding is limited by the incomplete understanding of endogenous pathway regulation, including rate-controlling steps and timing of expression in carotenogenic tissues. The grass family (Poaceae) contains major crop staples, including maize (Zea mays), wheat (Triticum aestivum), rice (Oryza sativa), sorghum (Sorghum bicolor), and millet (Pennisetum glaucum). Maize carotenogenesis was investigated using a novel approach to discover genes encoding limiting biosynthetic steps in the nutritionally targeted seed endosperm. A combination of bioinformatics and cloning were first used to identify and map gene families encoding enzymes in maize and other grasses. These enzymes represented upstream pathways for isopentenyl diphosphate and geranylgeranyl diphosphate synthesis and the downstream carotenoid biosynthetic pathway, including conversion to abscisic acid. A maize germplasm collection was used for statistical testing of the correlation between carotenoid content and candidate gene transcript levels. Multiple pathway bottlenecks for isoprenoid biosynthesis and carotenoid biosynthesis were discovered in specific temporal windows of endosperm development. Transcript levels of paralogs encoding isoprenoid isopentenyl diphosphate and geranylgeranyl diphosphate-producing enzymes, DXS3, DXR, HDR, and GGPPS1, were found to positively correlate with endosperm carotenoid content. For carotenoid pathway enzymes, transcript levels for CrtISO inversely correlated with seed carotenoid content, as compared with positive correlation of PSY1 transcripts. Since zeaxanthin epoxidase (ZEP) depletes the carotenoid pool in subsequent conversion to abscisic acid, ZEP transcripts were examined. Carotenoid accumulation was found to be inversely associated with ZEP1 and ZEP2 transcript levels. Extension of the maize results using phylogenetic analysis identified orthologs in other grass species that may serve as potential metabolic engineering targets.

Potential implications for epigenetic regulation of carotenoid biosynthesis during root and shoot development

Major regulators of carotenoid biosynthesis have remained rather elusive even though the flux through the branch in the carotenoid pathway can affect plant development in response to environmental stimuli, such as light. Our recent investigations demonstrated that the production of the most abundant carotenoid in plants, lutein, is regulated by carotenoid isomerase (CRTISO) activity at a rate-limiting step of this branch point in carotenoid biosynthesis. CRTISO is required to isomerase cis-carotenes, such as tetra-cis-lycopene to all-trans-lycopene. In order to maintain permissive transcriptional regulation of CRTISO, active marks of histone lysine methylation are targeted to the promoter region by the SET DOMAIN GROUP8 (SDG8) methyltransferase. Mutants of SDG8 (ccr1 ) and CRTISO (ccr2 ) show an increase in shoot branching, which may be partly explained by limiting synthesis of the carotenoid-derived branching hormone, strigolactone. In this addendum, we demonstrate new functions for SDG8 in mediating branching in Arabidopsis roots. The roles that carotenoids and SDG8 play in root and shoot development begins to open new doors for investigating the regulation of carotenoid composition in response to epigenetic events.

The phytoene synthase gene family in the Grasses: subfunctionalization provides tissue-specific control of carotenogenesis

Carotenoids are a complex class of isoprenoid pigments playing diverse roles in plants and providing nutritional value. Metabolic engineering of the biosynthetic pathway has been of interest to specifically address global vitamin A deficiency by breeding cereal crop staples in the Poaceae (Grass family) for elevated levels of provitamin A carotenoids. However, there remain open questions about the rate-controlling steps that limit predictability of metabolic engineering in plants, whether by transgenic or nontransgenic means. We decided to focus on the first committed biosynthetic step which is mediated by phytoene synthase. Our studies revealed that in the Grasses, PSY is encoded by three genes. Maize transcript profiling, together with carotenoid and ABA analysis, revealed that the three PSY copies have subfunctionalized and provide the Grasses with a fine tine control of carotenogenesis in response to various developmental and external cues. Promoter analysis supports subfunctionalization; cis-element analysis of maize PSY1 alleles and comparison with Grass orthologs suggests that man's selection of yellow maize endosperm has occurred at the expense of a change of gene regulation in photosynthetic tissue as compared to the progenitor white endosperm PSY1 allele.

Abscisic acid and the pre-harvest sprouting in cereals

Pre-harvest sprouting (PHS) leads to loss of grain weight and a reduction in the end use quality of kernels in cereals, especially in wheat, and PHS in rice also becomes a more and more serious problem recent years. Many factors are involved in the controlling this complex trait. Only recently, we have reported the large scale screening and charactersation of the rice phs mutants, providing insight into the molecular mechanism of pre-harvest sprouting in rice. It has been shown that mutations of genes in synthesis of the carotenoid precursors of ABA resulted in the pre-harvest sprouting, which is consequence of ABA deficiency, and photobleaching is likewise due to the absence of photoprotective carotenoids. The further study of all different rice phs mutants will help us to elucidate the complex phenomena and finally capture the target for improving PHS in rice or other cereals.

Carotenoid metabolism: biosynthesis, regulation, and beyond

Carotenoids are indispensable to plants and play a critical role in human nutrition and health. Significant progress has been made in our understanding of carotenoid metabolism in plants. The biosynthetic pathway has been extensively studied. Nearly all the genes encoding the biosynthetic enzymes have been isolated and characterized from various organisms. In recent years, there is an increasing body of work on the signaling pathways and plastid development, which might provide global control of carotenoid biosynthesis and accumulation. Herein, we will highlight recent progress on the biosynthesis, regulation, and metabolic engineering of carotenoids in plants, as well as the future research towards elucidating the regulatory mechanisms and metabolic network that control carotenoid metabolism.

The maize phytoene synthase gene family: overlapping roles for carotenogenesis in endosperm, photomorphogenesis, and thermal stress tolerance

Carotenoids are essential for photosynthesis and photoprotection; they also serve as precursors to signaling molecules that influence plant development and biotic/abiotic stress responses. With potential to improve plant yield and nutritional quality, carotenoids are targets for metabolic breeding/engineering, particularly in the Poaceae (grass family), which includes the major food crops. Depending on genetic background, maize (Zea mays) endosperm carotenoid content varies, and therefore breeding-enhanced carotenoid levels have been of ongoing interest. The first committed step in the plastid-localized biosynthetic pathway is mediated by the nuclear-encoded phytoene synthase (PSY). The gene family in maize and other grasses contains three paralogs with specialized roles that are not well understood. Maize endosperm carotenoid accumulation requires PSY1 expression. A maize antibody was used to localize PSY1 to amyloplast envelope membranes and to determine PSY1 accumulation in relation to carotenoid accumulation in developing endosperm. To test when and if PSY transcript levels correlated with carotenoid content, advantage was taken of a maize germplasm diversity collection that exhibits genetic and chemical diversity. Total carotenoid content showed statistically significant correlation with endosperm transcript levels at 20 d after pollination for PSY1 but not PSY2 or PSY3. Timing of PSY1 transcript abundance, previously unknown, provides critical information for choosing breeding alleles or properly controlling introduced transgenes. PSY1 was unexpectedly found to have an additional role in photosynthetic tissue, where it was required for carotenogenesis in the dark and for heat stress tolerance. Leaf carotenogenesis was shown to require phytochrome-dependent and phytochrome-independent photoregulation of PSY2 plus nonphotoregulated PSY1 expression.