PSY3, a new member of the phytoene synthase gene family conserved in the Poaceae and regulator of abiotic stress-induced root carotenogenesis

From in planta Function to Vitamin-Rich Food Crops: The ACE of Biofortification

Humans are highly dependent on plants to reach their dietary requirements, as plant products contribute both to energy and essential nutrients. For many decades, plant breeders have been able to gradually increase yields of several staple crops, thereby alleviating nutritional needs with varying degrees of success. However, many staple crops such as rice, wheat and corn, although delivering sufficient calories, fail to satisfy micronutrient demands, causing the so called 'hidden hunger.' Biofortification, the process of augmenting nutritional quality of food through the use of agricultural methodologies, is a pivotal asset in the fight against micronutrient malnutrition, mainly due to vitamin and mineral deficiencies. Several technical advances have led to recent breakthroughs. Nutritional genomics has come to fruition based on marker-assisted breeding enabling rapid identification of micronutrient related quantitative trait loci (QTL) in the germplasm of interest. As a complement to these breeding techniques, metabolic engineering approaches, relying on a continuously growing fundamental knowledge of plant metabolism, are able to overcome some of the inevitable pitfalls of breeding. Alteration of micronutrient levels does also require fundamental knowledge about their role and influence on plant growth and development. This review focuses on our knowledge about provitamin A (beta-carotene), vitamin C (ascorbate) and the vitamin E group (tocochromanols). We begin by providing an overview of the functions of these vitamins in planta, followed by highlighting some of the achievements in the nutritional enhancement of food crops via conventional breeding and genetic modification, concluding with an evaluation of the need for such biofortification interventions. The review further elaborates on the vast potential of creating nutritionally enhanced crops through multi-pathway engineering and the synergistic potential of conventional breeding in combination with genetic engineering, including the impact of novel genome editing technologies.

Early molecular events associated with nitrogen deficiency in rice seedling roots

Nitrogen (N) deficiency is one of the most common problems in rice. The symptoms of N deficiency are well documented, but the underlying molecular mechanisms are largely unknown in rice. Here, we studied the early molecular events associated with N starvation (−N, 1 h), focusing on amino acid analysis and identification of −N-regulated genes in rice roots. Interestingly, levels of glutamine rapidly decreased within 15 min of −N treatment, indicating that part of the N-deficient signals could be mediated by glutamine. Transcriptome analysis revealed that genes involved in metabolism, plant hormone signal transduction (e.g. abscisic acid, auxin, and jasmonate), transporter activity, and oxidative stress responses were rapidly regulated by −N. Some of the −N-regulated genes encode transcription factors, protein kinases and protein phosphatases, which may be involved in the regulation of early −N responses in rice roots. Previously, we used similar approaches to identify glutamine-, glutamate-, and ammonium nitrate-responsive genes. Comparisons of the genes induced by different forms of N with the −N-regulated genes identified here have provided a catalog of potential N regulatory genes for further dissection of the N signaling pathwys in rice.

Unraveling the induction of phytoene synthase 2 expression by salt stress and abscisic acid in Daucus carota

Phytoene synthase (PSY) is the first committed enzyme of the carotenoid biosynthesis pathway and the most important point of regulation. Carotenoids are precursors of abscisic acid (ABA), which mediates abiotic stress tolerance responses in plants. ABA activates the synthesis of its own precursors through induction of PSY expression. Carrot, a species that accumulates very high amounts of carotenoids in its reserve root, has two PSY paralog genes that are expressed differentially in the root. Here, we determined that DcPSY2 expression is induced by salt stress and ABA. A DcPSY2 promoter fragment was obtained and characterized. Bioinformatic analysis showed the presence of three ABA responsive elements (ABREs). Through overexpressing pPSY2:GFP in Nicotiana tabacum we determined that all three ABREs are necessary for the ABA response. In the carrot transcriptome, we identified three ABRE binding protein (DcAREB) transcription factor candidates that localized in the nucleus, but only one, DcAREB3, was induced under ABA treatment in carrot roots. We found that AREB transcription factors bind to the carrot DcPSY2 promoter and transactivate the expression of reporter genes. We conclude that DcPSY2 is involved in ABA-mediated salt stress tolerance in carrot through the binding of AREB transcription factors to its promoter.

cis-carotene biosynthesis, evolution and regulation in plants: The emergence of novel signaling metabolites

Carotenoids are isoprenoid pigments synthesised by plants, algae, photosynthetic bacteria as well as some non-photosynthetic bacteria, fungi and insects. Abundant carotenoids found in nature are synthesised via a linear route from phytoene to lycopene after which the pathway bifurcates into cyclised α- and β-carotenes. Plants evolved additional steps to generate a diversity of cis-carotene intermediates, which can accumulate in fruits or tissues exposed to an extended period of darkness. Enzymatic or oxidative cleavage, light-mediated photoisomerization and histone modifications can affect cis-carotene accumulation. cis-carotene accumulation has been linked to the production of signaling metabolites that feedback and forward to regulate nuclear gene expression. When cis-carotenes accumulate, plastid biogenesis and operational control can become impaired. Carotenoid derived metabolites and phytohormones such as abscisic acid and strigolactones can fine-tune cellular homeostasis. There is a hunt to identify a novel cis-carotene derived apocarotenoid signal and to elucidate the molecular mechanism by which it facilitates communication between the plastid and nucleus. In this review, we describe the biosynthesis and evolution of cis-carotenes and their links to regulatory switches, as well as highlight how cis-carotene derived apocarotenoid signals might control organelle communication, physiological and developmental processes in response to environmental change.

Elucidation and functional characterization of CsPSY and CsUGT promoters in Crocus sativus L

The dried stigmas of Crocus sativus constitute the saffron, which is considered to be the costliest spice of the world. Saffron is valuable for its constituents, which are mainly apocarotenoids. In order to enhance the production of apocarotenoids, it is imperative to understand the regulation of apocarotenoid biosynthetic pathway. In C. sativus, although the pathway has been elucidated, the information regarding the regulation of the pathwaygenes is scanty. During the present investigation, the characterization of promoters regulating the expression of two important genes i.e. CsPSY and CsUGT was performed. We successfully cloned the promoters of both the genes, which were functionally characterized in Crocus sativus and Nicotiana tabaccum. In silico analysis of the promoters demonstrated the presence of several important cis regulatory elements responding tolight, hormonesand interaction with transcription factors (TFs). Further analysis suggested the regulation of CsPSY promoter by Abscisic acid (ABA) and that of CsUGT by Gibberellic acid (GA). In addition, we also observed ABA and GA mediated modulation in the expression of significant TFs and CsPSY and CsUGT transcripts. Overall, the study addresses issues related to regulation of key genes of apocarotenoid pathway in C.sativus.

Strigolactone Levels in Dicot Roots Are Determined by an Ancestral Symbiosis-Regulated Clade of the PHYTOENE SYNTHASE Gene Family

Strigolactones (SLs) are apocarotenoid phytohormones synthesized from carotenoid precursors. They are produced most abundantly in roots for exudation into the rhizosphere to cope with mineral nutrient starvation through support of root symbionts. Abscisic acid (ABA) is another apocarotenoid phytohormone synthesized in roots, which is involved in responses to abiotic stress. Typically low carotenoid levels in roots raise the issue of precursor supply for the biosynthesis of these two apocarotenoids in this organ. Increased ABA levels upon abiotic stress in Poaceae roots are known to be supported by a particular isoform of phytoene synthase (PSY), catalyzing the rate-limiting step in carotenogenesis. Here we report on novel PSY3 isogenes from Medicago truncatula (MtPSY3) and Solanum lycopersicum (SlPSY3) strongly expressed exclusively upon root interaction with symbiotic arbuscular mycorrhizal (AM) fungi and moderately in response to phosphate starvation. They belong to a widespread clade of conserved PSYs restricted to dicots (dPSY3) distinct from the Poaceae-PSY3s involved in ABA formation. An ancient origin of dPSY3s and a potential co-evolution with the AM symbiosis is discussed in the context of PSY evolution. Knockdown of MtPSY3 in hairy roots of M. truncatula strongly reduced SL and AM-induced C₁₃ α-ionol/C₁₄ mycorradicin apocarotenoids. Inhibition of the reaction subsequent to phytoene synthesis revealed strongly elevated levels of phytoene indicating induced flux through the carotenoid pathway in roots upon mycorrhization. dPSY3 isogenes are coregulated with upstream isogenes and downstream carotenoid cleavage steps toward SLs (D27, CCD7, CCD8) suggesting a combined carotenoid/apocarotenoid pathway, which provides "just in time"-delivery of precursors for apocarotenoid formation.

Differential accumulation of β-carotene and tissue specific expression of phytoene synthase (MaPsy) gene in banana (Musa sp) cultivars

An experiment was conducted with twelve major Indian banana cultivars to investigate the molecular relationship between the differential accumulation of β-carotene in peel and pulp of the banana fruit and carotenoid biosynthetic pathway genes. The high performance liquid chromatography showed that all banana cultivars accumulated two-three fold more β-carotene in non-edible portion of the banana fruit. However, Nendran, a famous orange fleshed cultivar of South India, had high β-carotene content (1362 µg/100 g) in edible pulp. The gene encoding Musa accuminata phytoene synthase (MaPsy) was successfully amplified using a pair of degenerate primers designed from Oncidium orchid. The deduced amino acid sequences shared a high level of identity to phytoene synthase gene from other plants. Gene expression analysis confirmed the presence of two isoforms (MaPsy1 and MaPsy2) of MaPsy gene in banana fruits. Presence of two isoforms of MaPsy gene in peel and one in pulp confirmed the differential accumulation of β-carotene in banana fruits. However, Nendran accumulated more β-carotene in edible pulp due to presence of both the isoforms of MaPsy gene. Thus, carotenoid accumulation is a tissue specific process strongly dependent on differential expression pattern of two isoforms of MaPsy gene in banana.

Perturbations in the Photosynthetic Pigment Status Result in Photooxidation-Induced Crosstalk between Carotenoid and Porphyrin Biosynthetic Pathways

Possible crosstalk between the carotenoid and porphyrin biosynthetic pathways under photooxidative conditions was investigated by using their biosynthetic inhibitors, norflurazon (NF) and oxyfluorfen (OF). High levels of protoporphyrin IX (Proto IX) accumulated in rice plants treated with OF, whereas Proto IX decreased in plants treated with NF. Both NF and OF treatments resulted in greater decreases in MgProto IX, MgProto IX methyl ester, and protochlorophyllide. Activities and transcript levels of most porphyrin biosynthetic enzymes, particularly in the Mg-porphyrin branch, were greatly down-regulated in NF and OF plants. In contrast, the transcript levels of GSA, PPO1, and CHLD as well as FC2 and HO2 were up-regulated in NF-treated plants, while only moderate increases in FC2 and HO2 were observed in the early stage of OF treatment. Phytoene, antheraxanthin, and zeaxanthin showed high accumulation in NF-treated plants, whereas other carotenoid intermediates greatly decreased. Transcript levels of carotenoid biosynthetic genes, PSY1 and PDS, decreased in response to NF and OF, whereas plants in the later stage of NF treatment exhibited up-regulation of BCH and VDE as well as recovery of PDS. However, perturbed porphyrin biosynthesis by OF did not noticeably influence levels of carotenoid metabolites, regardless of the strong down-regulation of carotenoid biosynthetic genes. Both NF and OF plants appeared to provide enhanced protection against photooxidative damage, not only by scavenging of Mg-porphyrins, but also by up-regulating FC2, HO2, and Fe-chelatase, particularly with increased levels of zeaxanthin via up-regulation of BCH and VDE in NF plants. On the other hand, the up-regulation of GSA, PPO1, and CHLD under inhibition of carotenogenic flux may be derived from the necessity to recover impaired chloroplast biogenesis during photooxidative stress. Our study demonstrates that perturbations in carotenoid and porphyrin biosynthesis coordinate the expression of their biosynthetic genes to sustain plastid function at optimal levels by regulating their metabolic flux in plants under adverse stress conditions.

An R2R3-MYB transcription factor represses the transformation of α- and β-branch carotenoids by negatively regulating expression of CrBCH2 and CrNCED5 in flavedo of Citrus reticulate

Although the functions of carotenogenic genes are well documented, little is known about the mechanisms that regulate their expression, especially those genes involved in α - and β-branch carotenoid metabolism. In this study, an R2R3-MYB transcriptional factor (CrMYB68) that directly regulates the transformation of α- and β-branch carotenoids was identified using Green Ougan (MT), a stay-green mutant of Citrus reticulata cv Suavissima. A comprehensive analysis of developing and harvested fruits indicated that reduced expression of β-carotene hydroxylases 2 (CrBCH2) and 9-cis-epoxycarotenoid dioxygenase 5 (CrNCED5) was responsible for the delay in the transformation of α- and β-carotene and the biosynthesis of ABA. Additionally, the expression of these genes was negatively correlated with the expression of CrMYB68 in MT. Further, electrophoretic mobility shift assays (EMSAs) and dual luciferase assays indicated that CrMYB68 can directly and negatively regulate CrBCH2 and CrNCED5. Moreover, transient overexpression experiments using leaves of Nicotiana benthamiana indicated that CrMYB68 can also negatively regulate NbBCH2 and NbNCED5. To overcome the difficulty of transgenic validation, we quantified the concentrations of carotenoids and ABA, and gene expression in a revertant of MT. The results of these experiments provide more evidence that CrMYB68 is an important regulator of carotenoid metabolism.

Allelic diversity of phytoene synthase gene influences the transcription level in citrus fruit among a citrus F1 hybrid population

Phytoene synthase (PSY) is one of the key regulatory enzyme on the biosynthesis and accumulation of carotenoid in citrus fruits. The transcriptional diversity of PSY is mainly attributed to the structural variation in promoter region among PSY alleles. In aim to clarify how this transcriptional diversity is regulated among them, PSY alleles responsible for carotenoid biosynthesis in the fruits are characterized and their promoter sequences were compared. Based on gene structure and expression pattern of PSY homologues on the clementine mandarin genome sequence, PSY alleles responsible for carotenoid biosynthesis are derived from a single locus in the scaffold 6. AG mapping population possessed four PSY alleles derived from parent lines of A255 and G434, and their F₁ individuals with PSY-g2 allele tended to have low transcription level. From sequence comparison of their promoter regions, the cis-motif alternation from MYBPZM to RAV1AAT might be a candidate to influence the transcription level. Among the ancestral pedigree varieties of AG mapping population, the transcription level of PSY correlated with genotypes of MYBPZM and RAV1AAT motifs in the promoter region of PSY alleles, so that homozygous genotype of MYBPZM showed higher transcription level while heterozygous genotype of MYBPZM and RAV1AAT showed lower transcription level.

Carotenoids biosynthesis and cleavage related genes from bacteria to plants

Carotenoids are essential for photosynthesis and photoprotection in photosynthetic organisms and beneficial for human health. Apocarotenoids derived from carotenoid degradation can serve critical functions including hormones, volatiles, and signals. They have been used commercially as food colorants, animal feed supplements, and nutraceuticals for cosmetic and pharmaceutical purposes. This review focuses on the molecular evolution of carotenogenic enzymes and carotenoid cleavage oxygenases (CCOs) from bacteria, fungi, cyanobacteria, algae, and plants. The diversity of carotenoids and apocarotenoids as well as their complicated biosynthetic pathway in different species can shed light on the history of early molecular evolution. Some carotenogenic genes (such as phytoene synthases) have high protein sequence similarity from bacteria to land plants, but some (such as phytoene desaturases, lycopene cyclases, carotenoid hydroxylases, and CCOs) have low similarity. The broad diversity of apocarotenoid volatile compounds can be attributed to large numbers of carotenoid precursors and the various cleavage sites catalyzed by CCOs enzymes. A variety of carotenogenic enzymes and CCOs indicate the functional diversification of carotenoids and apocrotenoids in different species. New carotenoids, new apocarotenoids, new carotenogenic enzymes, new CCOs, and new pathways still need to be explored.

Molecular analysis of the expression of a crtB transgene and the endogenous psy2-y 1 and psy2-y 2 genes of cassava and their effect on root carotenoid content

A conventional breeding program was established to transfer the bacterial phytoene synthase transgene-crtB-from a transgenic, white-rooted cassava to yellow-rooted cassava plants carrying the endogenous phytoene synthase alleles named psy2-y ₁ and/or psy2-y ₂. Combining endogenous phytoene synthase enzymes (PSYs) with CRTB in a single cassava plant would allow the molecular dissection of individual allele contributions to carotenoid synthesis and/or accumulation in cassava roots. The simultaneous expression of the crtB transgene and psy2-y ₂ in individuals planted in the field coincided with higher total, HPLC-quantified carotenoid content in roots, although the variability among replications (plants) precluded the detection of statistically significant differences. Nevertheless, the highest total carotenoid content in roots within a family coincided with one individual of the F1 progeny carrying both psy2-y ₂ and crtB genes. The results also indicated the presence of at least one more key gene-different from psy or crtB-which too is necessary for the synthesis and/or accumulation of Pro-Vitamin A carotenoids in cassava roots.

Genome-wide identification and characterization of genes involved in carotenoid metabolic in three stages of grapevine fruit development

Carotenoids not only play indispensable roles in plant growth and development but also enhance nutritional value and health benefits for humans. In this study, total carotenoids progressively decreased during fruit ripening. Fifty-four genes involving in mevalonate (MVA), 2-C-methyl-D-erythritol 4-phosphate (MEP), carotenoid biosynthesis and catabolism pathway were identified. The expression levels of most of the carotenoid metabolism related genes kept changing during fruit ripening generating a metabolic flux toward carotenoid synthesis. Down regulation of VvDXS, VvDXR, VvGGPPS and VvPSY and a dramatic increase in the transcription levels of VvCCD might be responsible for the reduction of carotenoids content. The visible correlation between carotenoid content and gene expression profiles suggested that transcriptional regulation of carotenoid biosynthesis pathway genes is a key mechanism of carotenoid accumulation. In addition, the decline of carotenoids was also accompanied with the reduction of chlorophyll content. The reduction of chlorophyll content might be due to the obstruction in chlorophyll synthesis and acceleration of chlorophyll degradation. These results will be helpful for better understanding of carotenoid biosynthesis in grapevine fruit and contribute to the development of conventional and transgenic grapevine cultivars for further enrichment of carotenoid content.

Regulation of Banana Phytoene Synthase (MaPSY) Expression, Characterization and Their Modulation under Various Abiotic Stress Conditions

Phytoene synthase (PSY) is a key regulatory enzyme of carotenoid biosynthesis pathway in plants. The present study examines the role of PSY in carotenogenesis and stress management in banana. Germplasm screening of 10 Indian cultivars showed that Nendran (3011.94 μg/100 g dry weight) and Rasthali (105.35 μg/100 g dry weight) contained the highest and lowest amounts of β-carotene, respectively in ripe fruit-pulp. Nendran ripe pulp also showed significantly higher antioxidant activity as compared to Rasthali. Meta-analysis of three banana PSY genes (MaPSY1, MaPSY2, and MaPSY3) was performed to identify their structural features, subcellular, and chromosomal localization in banana genome. The distinct expression patterns of MaPSY1, MaPSY2, and MaPSY3 genes were observed in various tissues, and fruit developmental stages of these two contrasting cultivars, suggesting differential regulation of the banana PSY genes. A positive correlation was observed between the expression of MaPSY1 and β-carotene accumulation in the ripe fruit-peel and pulp of Nendran. The presence of stress responsive cis-regulatory motifs in promoter region of MaPSY genes were correlated with the expression pattern during various stress (abscisic acid, methyl jasmonate, salicylic acid and dark) treatments. The positive modulation of MaPSY1 noticed under abiotic stresses suggested its role in plant physiological functions and defense response. The amino acid sequence analysis of the PSY proteins in contrasting cultivars revealed that all PSY comprises conserved domains related to enzyme activity. Bacterial complementation assay has validated the functional activity of six PSY proteins and among them PSY1 of Nendran (Nen-PSY1) gave the highest activity. These data provide new insights into the regulation of PSY expression in banana by developmental and stress related signals that can be explored in the banana improvement programs.

Carotenoid gene expression explains the difference of carotenoid accumulation in carrot root tissues

Main conclusion Variations in gene expression can partially explain the difference of carotenoid accumulation in secondary phloem and xylem of fleshy carrot roots. The carrot root is well divided into two different tissues separated by vascular cambium: the secondary phloem and xylem. The equilibrium between these two tissues represents an important issue for carrot quality, but the knowledge about the respective carotenoid accumulation is sparse. The aim of this work was (i) to investigate if variation in carotenoid biosynthesis gene expression could explain differences in carotenoid content in phloem and xylem tissues and (ii) to investigate if this regulation is differentially modulated in the respective tissues by water-restricted growing conditions. In this work, five carrot genotypes contrasting by their root color were studied in control and water-restricted conditions. Carotenoid content and the relative expression of 13 genes along the carotenoid biosynthesis pathway were measured in the respective tissues. Results showed that in orange genotypes and the purple one, carotenoid content was higher in phloem compared to xylem. For the red one, no differences were observed. Moreover, in control condition, variations in gene expression explained the different carotenoid accumulations in both tissues, while in water-restricted condition, no clear association between gene expression pattern and variations in carotenoid content could be detected except in orange-rooted genotypes. This work shows that the structural aspect of carrot root is more important for carotenoid accumulation in relation with gene expression levels than the consequences of expression changes upon water restriction.

Comparative transcriptome analysis of two contrasting watermelon genotypes during fruit development and ripening

BACKGROUND

Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is an economically important crop with an attractive ripe fruit that has colorful flesh. Fruit ripening is a complex, genetically programmed process.

RESULTS

In this study, a comparative transcriptome analysis was performed to identify the regulators and pathways that are involved in the fruit ripening of pale-yellow-flesh cultivated watermelon (COS) and red-flesh cultivated watermelon (LSW177). We first identified 797 novel genes to extend the available reference gene set. Second, 3958 genes in COS and 3503 genes in LSW177 showed at least two-fold variation in expression, and a large number of these differentially expressed genes (DEGs) during fruit ripening were related to carotenoid biosynthesis, plant hormone pathways, and sugar and cell wall metabolism. Third, we noted a correlation between ripening-associated transcripts and metabolites and the key function of these metabolic pathways during fruit ripening.

CONCLUSION

The results revealed several ripening-associated actions and provide novel insights into the molecular mechanisms underlying the regulation of watermelon fruit ripening.

A carotenogenic mini-pathway introduced into white corn does not affect development or agronomic performance

High-carotenoid corn (Carolight®) has been developed as a vehicle to deliver pro-vitamin A in the diet and thus address vitamin A deficiency in at-risk populations in developing countries. Like any other novel crop, the performance of Carolight® must be tested in different environments to ensure that optimal yields and productivity are maintained, particularly in this case to ensure that the engineered metabolic pathway does not attract a yield penalty. Here we compared the performance of Carolight® with its near isogenic white corn inbred parental line under greenhouse and field conditions, and monitored the stability of the introduced trait. We found that Carolight® was indistinguishable from its near isogenic line in terms of agronomic performance, particularly grain yield and its main components. We also established experimentally that the functionality of the introduced trait was indistinguishable when plants were grown in a controlled environment or in the field. Such thorough characterization under different agronomic conditions is rarely performed even for first-generation traits such as herbicide tolerance and pest resistance, and certainly not for complex second-generation traits such as the metabolic remodeling in the Carolight® variety. Our results therefore indicate that Carolight® can now be incorporated into breeding lines to generate hybrids with locally adapted varieties for further product development and assessment.

Carotenogenesis Is Regulated by 5'UTR-Mediated Translation of Phytoene Synthase Splice Variants

Phytoene synthase (PSY) catalyzes the highly regulated, frequently rate-limiting synthesis of the first biosynthetically formed carotene. While PSY constitutes a small gene family in most plant taxa, the Brassicaceae, including Arabidopsis (Arabidopsis thaliana), predominantly possess a single PSY gene. This monogenic situation is compensated by the differential expression of two alternative splice variants (ASV), which differ in length and in the exon/intron retention of their 5'UTRs. ASV1 contains a long 5'UTR (untranslated region) and is involved in developmentally regulated carotenoid formation, such as during deetiolation. ASV2 contains a short 5'UTR and is preferentially induced when an immediate increase in the carotenoid pathway flux is required, such as under salt stress or upon sudden light intensity changes. We show that the long 5'UTR of ASV1 is capable of attenuating the translational activity in response to high carotenoid pathway fluxes. This function resides in a defined 5'UTR stretch with two predicted interconvertible RNA conformations, as known from riboswitches, which might act as a flux sensor. The translation-inhibitory structure is absent from the short 5'UTR of ASV2 allowing to bypass translational inhibition under conditions requiring rapidly increased pathway fluxes. The mechanism is not found in the rice (Oryza sativa) PSY1 5'UTR, consistent with the prevalence of transcriptional control mechanisms in taxa with multiple PSY genes. The translational control mechanism identified is interpreted in terms of flux adjustments needed in response to retrograde signals stemming from intermediates of the plastid-localized carotenoid biosynthesis pathway.

ABA accumulation in water-stressed Citrus roots does not rely on carotenoid content in this organ

Sustained abscisic acid (ABA) accumulation in dehydrated citrus roots depends on the transport from aerial organs. Under this condition, the role of the β,β-carotenoids (ABA precursors) to the de novo synthesis of ABA in roots needs to be clarified since their low availability in this organ restricts its accumulation. To accomplish that, detached citrus roots were exposed to light (to increase their carotenoid content) and subsequently dehydrated (to trigger ABA accumulation). Stress imposition sharply decreased the pool of β,β-carotenoids but, unexpectedly, no concomitant rise in ABA content was observed. Contrastingly, roots of intact plants (with low levels of carotenoids) showed a similar decrease of ABA precursor together with a significant ABA accumulation. Furthermore, upon dehydration both types of roots showed similar upregulation of the key genes involved in biosynthesis of carotenoids and ABA (CsPSY3a; CsβCHX1; CsβCHX2; CsNCED1; CsNCED2), demonstrating a conserved transcriptional response triggered by water stress. Thus, the sharp decrease in root carotenoid levels in response to dehydration should be related to other stress-related signals instead of contributing to ABA biosynthesis. In summary, ABA accumulation in dehydrated-citrus roots largely relies on the presence of the aerial organs and it is independent of the amount of available root β,β-carotenoids.

Genetic analysis of phytoene synthase 1 (Psy1) gene function and regulation in common wheat

BACKGROUND

Phytoene synthase 1 (PSY1) is the most important regulatory enzyme in carotenoid biosynthesis, whereas its function is hardly known in common wheat. The aims of the present study were to investigate Psy1 function and genetic regulation using reverse genetics approaches.

RESULTS

Transcript levels of Psy1 in RNAi transgenic lines were decreased by 54-76 % and yellow pigment content (YPC) was reduced by 26-35 % compared with controls, confirming the impact of Psy1 on carotenoid accumulation. A series of candidate genes involved in secondary metabolic pathways and core metabolic processes responded to Psy1 down-regulation. The aspartate rich domain (DXXXD) was important for PSY1 function, and conserved nucleotides adjacent to the domain influenced YPC by regulating gene expression, enzyme activity or alternative splicing. Compensatory responses analysis indicated that three Psy1 homoeologs may be coordinately regulated under normal conditions, but separately regulated under stress. The period 14 days post anthesis (DPA) was found to be a key regulation node during grain development.

CONCLUSION

The findings define key aspects of flour color regulation in wheat and facilitate the genetic improvement of wheat quality targeting color/nutritional specifications required for specific end products.

Characterization and Expression Analysis of Phytoene Synthase from Bread Wheat (Triticum aestivum L.)

Phytoene synthase (PSY) regulates the first committed step of the carotenoid biosynthetic pathway in plants. The present work reports identification and characterization of the three PSY genes (TaPSY1, TaPSY2 and TaPSY3) in wheat (Triticum aestivum L.). The TaPSY1, TaPSY2, and TaPSY3 genes consisted of three homoeologs on the long arm of group 7 chromosome (7L), short arm of group 5 chromosome (5S), and long arm of group 5 chromosome (5L), respectively in each subgenomes (A, B, and D) with a similarity range from 89% to 97%. The protein sequence analysis demonstrated that TaPSY1 and TaPSY3 retain most of conserved motifs for enzyme activity. Phylogenetic analysis of all TaPSY revealed an evolutionary relationship among PSY proteins of various monocot species. TaPSY derived from A and D subgenomes shared proximity to the PSY of Triticum urartu and Aegilops tauschii, respectively. The differential expression of TaPSY1, TaPSY2, and TaPSY3 in the various tissues, seed development stages, and stress treatments suggested their role in plant development, and stress condition. TaPSY3 showed higher expression in all tissues, followed by TaPSY1. The presence of multiple stress responsive cis-regulatory elements in promoter region of TaPSY3 correlated with the higher expression during drought and heat stresses has suggested their role in these conditions. The expression pattern of TaPSY3 was correlated with the accumulation of β-carotene in the seed developmental stages. Bacterial complementation assay has validated the functional activity of each TaPSY protein. Hence, TaPSY can be explored in developing genetically improved wheat crop.

Repression of ARF10 by microRNA160 plays an important role in the mediation of leaf water loss

Solanum lycopersicum auxin response factor 10 (SlARF10) is post-transcriptionally regulated by Sl-miR160. Overexpression of a Sl-miR160-resistant SlARF10 (mSlARF10) resulted in narrower leaflet blades with larger stomata but lower densities. 35S:mSlARF10-6 plants with narrower excised leaves had greater water loss, which was in contrast to the wild type (WT). Further analysis revealed that the actual water loss was not consistent with the calculated stomatal water loss in 35S:mSlARF10-6 and the WT under the dehydration treatment, indicating that there is a difference in hydraulic conductance. Pretreatment with abscisic acid (ABA) and HgCl2 confirmed higher hydraulic conductance in 35S:mSlARF10, which is related to the larger stomatal size and higher activity of aquaporins (AQPs). Under ABA treatment, 35S:mSlARF10-6 showed greater sensitivity, and the stomata closed rapidly. Screening by RNA sequencing revealed that five AQP-related genes, fourteen ABA biosynthesis/signal genes and three stomatal development genes were significantly altered in 35S:mSlARF10-6 plants, and this result was verified by qRT-PCR. The promoter analysis showed that upregulated AQPs contain AuxRE and ABRE, implying that these elements may be responsible for the high expression levels of AQPs in 35S:mSlARF10-6. The three most upregulated AQPs (SlTIP1-1-like, SlPIP2;4 and SlNIP-type-like) were chosen to confirm AuxRE and ABRE function. Promoters transient expression demonstrated that the SlPIP2;4 and SlNIP-type-like AuxREs and SlPIP2;4 and SlTIP1-1-like ABREs could significantly enhance the expression of the GUS reporter in 35S:mSlARF10-6, confirming that AuxRE and ABRE may be the main factors inducing the expression of AQPs. Additionally, two upregulated transcription factors in 35S:mSlARF10-6, SlARF10 and SlABI5-like were shown to directly bind to those elements in an electromobility shift assay and a yeast one-hybrid assay. Furthermore, transient expression of down-regulated ARF10 or up-regulated ABI5 in tomato leaves demonstrated that ARF10 is the direct factor for inducing the water loss in 35S:mSlARF10-6. Here, we show that although SlARF10 increased the ABA synthesis/signal response by regulating stomatal aperture to mitigate water loss, SlARF10 also influenced stomatal development and AQP expression to affect water transport, and both act cooperatively to control the loss of leaf water in tomato. Therefore, this study uncovers a previously unrecognized leaf water loss regulatory factor and a network for coordinating auxin and ABA signalling in this important process. In an evolutionary context, miR160 regulates ARF10 to maintain the water balance in the leaf, thus ensuring normal plant development and environmental adaptation.

Transcript levels of phytoene desaturase gene in Dunaliella salina Teod. as affected by PbS nanoparticles and light intensity

Phytoene synthase (Psy) and Phytoene desaturase (Pds) are the first two regulatory enzymes in the carotenoids biosynthetic pathway. The genes Psy and Pds are under transcriptional control in many photosynthetic organisms. In the present study, using quantitative real time- PCR (qRT-PCR), the effects of uncoated and gum-Arabic coated PbS nanoparticles (GA-coated PbS NPs) and light intensity on the mRNA levels of Pds were investigated. Relative to mRNA level of Pds at 100 µmol photon m-2 s-1 light intensity (control culture), 2.2-fold increase in transcript levels occurred after 12 h of exposure to higher light intensity, which is significantly (P<0.05) different compared to control. After 48 h of exposure, the mRNA level of Pds was reduced to that in control. This indicates that light intensity regulates Pds at the mRNA level. In the presence of uncoated and GA-coated PbS NPs, the transcript levels of Pds were decreased over time, with uncoated PbS NPs having more inhibitory effects on mRNA levels compared to GA- coated PbS NPs. This shows that PbS NPs have adverse effects on transcription or post transcriptional processing and coating nanoparticles with biopolymers reduces their toxicity to organisms. Being under control, it seems that genetic manipulation of Pds may result in increased biotechnological production of carotenoids by D. salina.

Carotenoids in Staple Cereals: Metabolism, Regulation, and Genetic Manipulation

Carotenoids play a critical role in animal and human health. Animals and humans are unable to synthesize carotenoids de novo, and therefore rely upon diet as sources of these compounds. However, major staple cereals often contain only small amounts of carotenoids in their grains. Consequently, there is considerable interest in genetic manipulation of carotenoid content in cereal grain. In this review, we focus on carotenoid metabolism and regulation in non-green plant tissues, as well as genetic manipulation in staple cereals such as rice, maize, and wheat. Significant progress has been made in three aspects: (1) seven carotenogenes play vital roles in carotenoid regulation in non-green plant tissues, including 1-deoxyxylulose-5-phosphate synthase influencing isoprenoid precursor supply, phytoene synthase, β-cyclase, and ε-cyclase controlling biosynthesis, 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase and carotenoid cleavage dioxygenases responsible for degradation, and orange gene conditioning sequestration sink; (2) provitamin A-biofortified crops, such as rice and maize, were developed by either metabolic engineering or marker-assisted breeding; (3) quantitative trait loci for carotenoid content on chromosomes 3B, 7A, and 7B were consistently identified, eight carotenogenes including 23 loci were detected, and 10 gene-specific markers for carotenoid accumulation were developed and applied in wheat improvement. A comprehensive and deeper understanding of the regulatory mechanisms of carotenoid metabolism in crops will be beneficial in improving our precision in improving carotenoid contents. Genomic selection and gene editing are emerging as transformative technologies for provitamin A biofortification.

Increased Nicotiana tabacum fitness through positive regulation of carotenoid, gibberellin and chlorophyll pathways promoted by Daucus carota lycopene β-cyclase (Dclcyb1) expression

Carotenoids, chlorophylls and gibberellins are derived from the common precursor geranylgeranyl diphosphate (GGPP). One of the enzymes in carotenoid biosynthesis is lycopene β-cyclase (LCYB) that catalyzes the conversion of lycopene into β-carotene. In carrot, Dclcyb1 is essential for carotenoid synthesis in the whole plant. Here we show that when expressed in tobacco, increments in total carotenoids, β-carotene and chlorophyll levels occur. Furthermore, photosynthetic efficiency is enhanced in transgenic lines. Interestingly, and contrary to previous observations where overexpression of a carotenogenic gene resulted in the inhibition of the synthesis of gibberellins, we found raised levels of active GA4 and the concommitant increases in plant height, leaf size and whole plant biomass, as well as an early flowering phenotype. Moreover, a significant increase in the expression of the key carotenogenic genes, Ntpsy1, Ntpsy2 and Ntlcyb, as well as those involved in the synthesis of chlorophyll (Ntchl), gibberellin (Ntga20ox, Ntcps and Ntks) and isoprenoid precursors (Ntdxs2 and Ntggpps) was observed. These results indicate that the expression of Dclcyb1 induces a positive feedback affecting the expression of isoprenoid gene precursors and genes involved in carotenoid, gibberellin and chlorophyll pathways leading to an enhancement in fitness measured as biomass, photosynthetic efficiency and carotenoid/chlorophyll composition.

Plastids and Carotenoid Accumulation

Plastids are ubiquitously present in plants and are the organelles for carotenoid biosynthesis and storage. Based on their morphology and function, plastids are classified into various types, i.e. proplastids, etioplasts, chloroplasts, amyloplasts, and chromoplasts. All plastids, except proplastids, can synthesize carotenoids. However, plastid types have a profound effect on carotenoid accumulation and stability. In this chapter, we discuss carotenoid biosynthesis and regulation in various plastids with a focus on carotenoids in chromoplasts. Plastid transition related to carotenoid biosynthesis and the different capacity of various plastids to sequester carotenoids and the associated effect on carotenoid stability are described in light of carotenoid accumulation in plants.

Modern Breeding and Biotechnological Approaches to Enhance Carotenoid Accumulation in Seeds

There is an increasing demand for carotenoids, which are fundamental components of the human diet, for example as precursors of vitamin A. Carotenoids are also potent antioxidants and their health benefits are becoming increasingly evident. Protective effects against prostate cancer and age-related macular degeneration have been proposed for lycopene and lutein/zeaxanthin, respectively. Additionally, β-carotene, astaxanthin and canthaxanthin are high-value carotenoids used by the food industry as feed supplements and colorants. The production and consumption of these carotenoids from natural sources, especially from seeds, constitutes an important step towards fortifying the diet of malnourished people in developing nations. Therefore, attempts to metabolically manipulate β-carotene production in plants have received global attention, especially after the generation of Golden Rice (Oryza sativa). The endosperms of Golden Rice seeds synthesize and accumulate large quantities of β-carotene (provitamin A), yielding a characteristic yellow color in the polished grains. Classical breeding efforts have also focused in the development of cultivars with elevated seed carotenoid content, with maize and other cereals leading the way. In this communication we will summarize transgenic efforts and modern breeding strategies to fortify various crop seeds with nutraceutical carotenoids.

Carotenoid Biosynthesis in Daucus carota

Carrot (Daucus carota) is one of the most important vegetable cultivated worldwide and the main source of dietary provitamin A. Contrary to other plants, almost all carrot varieties accumulate massive amounts of carotenoids in the root, resulting in a wide variety of colors, including those with purple, yellow, white, red and orange roots. During the first weeks of development the root, grown in darkness, is thin and pale and devoid of carotenoids. At the second month, the thickening of the root and the accumulation of carotenoids begins, and it reaches its highest level at 3 months of development. This normal root thickening and carotenoid accumulation can be completely altered when roots are grown in light, in which chromoplasts differentiation is redirected to chloroplasts development in accordance with an altered carotenoid profile. Here we discuss the current evidence on the biosynthesis of carotenoid in carrot roots in response to environmental cues that has contributed to our understanding of the mechanism that regulates the accumulation of carotenoids, as well as the carotenogenic gene expression and root development in D. carota.

Arabidopsis GERANYLGERANYL DIPHOSPHATE SYNTHASE 11 is a hub isozyme required for the production of most photosynthesis-related isoprenoids

Most plastid isoprenoids, including photosynthesis-related metabolites such as carotenoids and the side chain of chlorophylls, tocopherols (vitamin E), phylloquinones (vitamin K), and plastoquinones, derive from geranylgeranyl diphosphate (GGPP) synthesized by GGPP synthase (GGPPS) enzymes. Seven out of 10 functional GGPPS isozymes in Arabidopsis thaliana reside in plastids. We aimed to address the function of different GGPPS paralogues for plastid isoprenoid biosynthesis. We constructed a gene co-expression network (GCN) using GGPPS paralogues as guide genes and genes from the upstream and downstream pathways as query genes. Furthermore, knock-out and/or knock-down ggpps mutants were generated and their growth and metabolic phenotypes were analyzed. Also, interacting protein partners of GGPPS11 were searched for. Our data showed that GGPPS11, encoding the only plastid isozyme essential for plant development, functions as a hub gene among GGPPS paralogues and is required for the production of all major groups of plastid isoprenoids. Furthermore, we showed that the GGPPS11 protein physically interacts with enzymes that use GGPP for the production of carotenoids, chlorophylls, tocopherols, phylloquinone, and plastoquinone. GGPPS11 is a hub isozyme required for the production of most photosynthesis-related isoprenoids. Both gene co-expression and protein-protein interaction likely contribute to the channeling of GGPP by GGPPS11.

Identification and Expression Analysis of Candidate Genes Involved in Carotenoid Biosynthesis in Chickpea Seeds

Plant carotenoids have a key role in preventing various diseases in human because of their antioxidant and provitamin A properties. Chickpea is a good source of carotenoid among legumes and its diverse germplasm and genome accessibility makes it a good model for carotenogenesis studies. The structure, location, and copy numbers of genes involved in carotenoid biosynthesis were retrieved from the chickpea genome. The majority of the single nucleotide polymorphism (SNPs) within these genes across five diverse chickpea cultivars was synonymous mutation. We examined the expression of the carotenogenesis genes and their association with carotenoid concentration at different seed development stages of five chickpea cultivars. Total carotenoid concentration ranged from 22 μg g^-1 in yellow cotyledon kabuli to 44 μg g^-1 in green cotyledon desi at 32 days post anthesis (DPA). The majority of carotenoids in chickpea seeds consists of lutein and zeaxanthin. The expression of the selected 19 genes involved in carotenoid biosynthesis pathway showed common pattern across five cultivars with higher expression at 8 and/or 16 DPA then dropped considerably at 24 and 32 DPA. Almost all genes were up-regulated in CDC Jade cultivar. Correlation analysis between gene expression and carotenoid concentration showed that the genes involved in the primary step of carotenoid biosynthesis pathway including carotenoid desaturase and isomerase positively correlated with various carotenoid components in chickpea seeds. A negative correlation was found between hydroxylation activity and provitamin A concentration in the seeds. The highest provitamin A concentration including β-carotene and β-cryptoxanthin were found in green cotyledon chickpea cultivars.

Isolation and Analysis of the Cppsy Gene and Promoter from Chlorella protothecoides CS-41

Phytoene synthase (PSY) catalyzes the condensation of two molecules of geranylgeranyl pyrophosphate to form phytoene, the first colorless carotene in the carotenoid biosynthesis pathway. So it is regarded as the crucial enzyme for carotenoid production, and has unsurprisingly been involved in genetic engineering studies of carotenoid production. In this study, the psy gene from Chlorella protothecoides CS-41, designated Cppsy, was cloned using rapid amplification of cDNA ends. The full-length DNA was 2488 bp, and the corresponding cDNA was 1143 bp, which encoded 380 amino acids. Computational analysis suggested that this protein belongs to the Isoprenoid_Biosyn_C1 superfamily. It contained the consensus sequence, including three predicted substrate-Mg²⁺ binding sites. The Cppsy gene promoter was also cloned and characterized. Analysis revealed several candidate motifs for the promoter, which exhibited light- and methyl jasmonate (MeJA)-responsive characteristics, as well as some typical domains universally discovered in promoter sequences, such as the TATA-box and CAAT-box. Light- and MeJA treatment showed that the Cppsy expression level was significantly enhanced by light and MeJA. These results provide a basis for genetically modifying the carotenoid biosynthesis pathway in C. protothecoides.

Metabolic Engineering of Wheat Provitamin A by Simultaneously Overexpressing CrtB and Silencing Carotenoid Hydroxylase (TaHYD)

Increasing the provitamin A content in staple crops via carotenoid metabolic engineering is one way to address vitamin A deficiency. In this work a combination of methods was applied to specifically increase β-carotene content in wheat by metabolic engineering. Endosperm-specific silencing of the carotenoid hydroxylase gene (TaHYD) increased β-carotene content 10.5-fold to 1.76 μg g(-1) in wheat endosperm. Overexpression of CrtB introduced an additional flux into wheat, accompanied by a β-carotene increase of 14.6-fold to 2.45 μg g(-1). When the "push strategy" (overexpressing CrtB) and "block strategy" (silencing TaHYD) were combined in wheat metabolic engineering, significant levels of β-carotene accumulation were obtained, corresponding to an increase of up to 31-fold to 5.06 μg g(-1). This is the first example of successful metabolic engineering to specifically improve β-carotene content in wheat endosperm through a combination of methods and demonstrates the potential of genetic engineering for specific nutritional enhancement of wheat.

More than meets the eye: from carotenoid biosynthesis, to new insights into apocarotenoid signaling

Carotenoids are a class of isoprenoids synthesized almost exclusively in plants involved in a myriad of roles including the provision of flower and fruit pigmentation for the attraction of pollinators and seed dispersing organisms. While carotenoids are essential throughout plant development, they are also extremely important in human diets providing necessary nutrition and aiding in the prevention of various cancers, age-related diseases and macular degeneration. Utilization of multiple plant models systems (i.e. Arabidopsis; maize; and tomato) has provided a comprehensive framework detailing the regulation of carotenogenesis throughout plant development covering all levels of genetic regulation from epigenetic to post-translational modifications. That said, the understanding of how carotenoids self-regulate remains fragmented. Recent reports demonstrate the potential influence of carotenoid-cleavage products (apocarotenoids) as signaling molecules regulating carotenoid biosynthesis in addition to various aspects of plants development (i.e. leaf and root development). This review highlights recent advances in carotenogenic regulation and insights into potential roles of novel apocarotenoids in plants.

A Sweetpotato Geranylgeranyl Pyrophosphate Synthase Gene, IbGGPS, Increases Carotenoid Content and Enhances Osmotic Stress Tolerance in Arabidopsis thaliana

Sweetpotato highly produces carotenoids in storage roots. In this study, a cDNA encoding geranylgeranyl phyrophosphate synthase (GGPS), named IbGGPS, was isolated from sweetpotato storage roots. Green fluorescent protein (GFP) was fused to the C-terminus of IbGGPS to obtain an IbGGPS-GFP fusion protein that was transiently expressed in both epidermal cells of onion and leaves of tobacco. Confocal microscopic analysis determined that the IbGGPS-GFP protein was localized to specific areas of the plasma membrane of onion and chloroplasts in tobacco leaves. The coding region of IbGGPS was cloned into a binary vector under the control of 35S promoter and then transformed into Arabidopsis thaliana to obtain transgenic plants. High performance liquid chromatography (HPLC) analysis showed a significant increase of total carotenoids in transgenic plants. The seeds of transgenic and wild-type plants were germinated on an agar medium supplemented with polyethylene glycol (PEG). Transgenic seedlings grew significantly longer roots than wild-type ones did. Further enzymatic analysis showed an increased activity of superoxide dismutase (SOD) in transgenic seedlings. In addition, the level of malondialdehyde (MDA) was reduced in transgenics. qRT-PCR analysis showed altered expressions of several genes involved in the carotenoid biosynthesis in transgenic plants. These data results indicate that IbGGPS is involved in the biosynthesis of carotenoids in sweetpotato storage roots and likely associated with tolerance to osmotic stress.

Carotenoid metabolism and regulation in horticultural crops

Carotenoids are a diverse group of pigments widely distributed in nature. The vivid yellow, orange, and red colors of many horticultural crops are attributed to the overaccumulation of carotenoids, which contribute to a critical agronomic trait for flowers and an important quality trait for fruits and vegetables. Not only do carotenoids give horticultural crops their visual appeal, they also enhance nutritional value and health benefits for humans. As a result, carotenoid research in horticultural crops has grown exponentially over the last decade. These investigations have advanced our fundamental understanding of carotenoid metabolism and regulation in plants. In this review, we provide an overview of carotenoid biosynthesis, degradation, and accumulation in horticultural crops and highlight recent achievements in our understanding of carotenoid metabolic regulation in vegetables, fruits, and flowers.

Novel Bread Wheat Lines Enriched in Carotenoids Carrying Hordeum chilense Chromosome Arms in the ph1b Background

The use of crop wild relative species to improve major crops performance is well established. Hordeum chilense has a high potential as a genetic donor to increase the carotenoid content of wheat. Crosses between the 7H^chH. chilense substitution lines in wheat and the wheat pairing homoeologous1b (ph1b) mutant allowed the development of wheat-H. chilense translocation lines for both 7H^chα and 7H^chβ chromosome arms in the wheat background. These translocation lines were characterized by in situ hybridization and using molecular markers. In addition, reverse phase chromatography (HPLC) analysis was carried out to evaluate the carotenoid content and both 7H^chα∙7AL and 7AS∙7H^chβ disomic translocation lines. The carotenoid content in 7H^chα∙7AL and 7AS∙7H^chβ disomic translocation lines was higher than the wheat-7H^ch addition line and double amount of carotenoids than the wheat itself. A proteomic analysis confirmed that the presence of chromosome 7H^ch introgressions in wheat scarcely altered the proteomic profile of the wheat flour. The Psy1 (Phytoene Synthase1) gene, which is the first committed step in the carotenoid biosynthetic pathway, was also cytogenetically mapped on the 7H^chα chromosome arm. These new wheat-H. chilense translocation lines can be used as a powerful tool in wheat breeding programs to enrich the diet in bioactive compounds.

Tissue-Specific Apocarotenoid Glycosylation Contributes to Carotenoid Homeostasis in Arabidopsis Leaves

Attaining defined steady-state carotenoid levels requires balancing of the rates governing their synthesis and metabolism. Phytoene formation mediated by phytoene synthase (PSY) is rate limiting in the biosynthesis of carotenoids, whereas carotenoid catabolism involves a multitude of nonenzymatic and enzymatic processes. We investigated carotenoid and apocarotenoid formation in Arabidopsis (Arabidopsis thaliana) in response to enhanced pathway flux upon PSY overexpression. This resulted in a dramatic accumulation of mainly β-carotene in roots and nongreen calli, whereas carotenoids remained unchanged in leaves. We show that, in chloroplasts, surplus PSY was partially soluble, localized in the stroma and, therefore, inactive, whereas the membrane-bound portion mediated a doubling of phytoene synthesis rates. Increased pathway flux was not compensated by enhanced generation of long-chain apocarotenals but resulted in higher levels of C13 apocarotenoid glycosides (AGs). Using mutant lines deficient in carotenoid cleavage dioxygenases (CCDs), we identified CCD4 as being mainly responsible for the majority of AGs formed. Moreover, changed AG patterns in the carotene hydroxylase mutants lutein deficient1 (lut1) and lut5 exhibiting altered leaf carotenoids allowed us to define specific xanthophyll species as precursors for the apocarotenoid aglycons detected. In contrast to leaves, carotenoid hyperaccumulating roots contained higher levels of β-carotene-derived apocarotenals, whereas AGs were absent. These contrasting responses are associated with tissue-specific capacities to synthesize xanthophylls, which thus determine the modes of carotenoid accumulation and apocarotenoid formation.

The Phytoene synthase gene family of apple (Malus x domestica) and its role in controlling fruit carotenoid content

BACKGROUND

Carotenoid compounds play essential roles in plants such as protecting the photosynthetic apparatus and in hormone signalling. Coloured carotenoids provide yellow, orange and red colour to plant tissues, as well as offering nutritional benefit to humans and animals. The enzyme phytoene synthase (PSY) catalyses the first committed step of the carotenoid biosynthetic pathway and has been associated with control of pathway flux. We characterised four PSY genes found in the apple genome to further understand their involvement in fruit carotenoid accumulation.

RESULTS

The apple PSY gene family, containing six members, was predicted to have three functional members, PSY1, PSY2, and PSY4, based on translation of the predicted gene sequences and/or corresponding cDNAs. However, only PSY1 and PSY2 showed activity in a complementation assay. Protein localisation experiments revealed differential localization of the PSY proteins in chloroplasts; PSY1 and PSY2 localized to the thylakoid membranes, while PSY4 localized to plastoglobuli. Transcript levels in 'Granny Smith' and 'Royal Gala' apple cultivars showed PSY2 was most highly expressed in fruit and other vegetative tissues. We tested the transient activation of the apple PSY1 and PSY2 promoters and identified potential and differential regulation by AP2/ERF transcription factors, which suggested that the PSY genes are controlled by different transcriptional mechanisms.

CONCLUSION

The first committed carotenoid pathway step in apple is controlled by MdPSY1 and MdPSY2, while MdPSY4 play little or no role in this respect. This has implications for apple breeding programmes where carotenoid enhancement is a target and would allow co-segregation with phenotypes to be tested during the development of new cultivars.

Control of carotenoid biosynthesis through a heme-based cis-trans isomerase

Plants synthesize carotenoids essential for plant development and survival. These metabolites also serve as essential nutrients for human health. The biosynthetic pathway leading to all plant carotenoids occurs in chloroplasts and other plastids and requires 15-cis-ζ-carotene isomerase (Z-ISO). It was not certain whether isomerization was achieved by Z-ISO alone or in combination with other enzymes. Here we show that Z-ISO is a bona fide enzyme and integral membrane protein. Z-ISO independently catalyzes the cis-to-trans isomerization of the 15–15′ C=C bond in 9,15,9′-cis-ζ-carotene to produce the substrate required by the following biosynthetic pathway enzyme. We discovered that isomerization depends upon a ferrous heme b cofactor that undergoes redox-regulated ligand-switching between the heme iron and alternate Z-ISO amino acid residues. Heme b-dependent isomerization of a large, hydrophobic compound in a membrane is unprecedented. As an isomerase, Z-ISO represents a new prototype for heme b proteins and potentially utilizes a novel chemical mechanism.

Evolution of root-specific carotenoid precursor pathways for apocarotenoid signal biogenesis

Various cleavage products of C40 carotenoid substrates are formed preferentially or exclusively in roots. Such apocarotenoid signaling or regulatory compounds differentially induced in roots during environmental stress responses including root colonization by arbuscular mycorrhizal fungi include ABA, strigolactones and C13 α-ionol/C14 mycorradicin derivatives. The low carotenoid levels in roots raise the question of whether there is a regulated precursor supply channeled into apocarotenoid formation distinct from default carotenoid pathways. This review describes root-specific isogene components of carotenoid pathways toward apocarotenoid formation, highlighting a new PSY3 class of phytoene synthase genes in dicots. It is clearly distinct from the monocot PSY3 class co-regulated with ABA formation. At least two members of the exclusive dicot PSY3s are regulated by nutrient stress and mycorrhization. This newly recognized dicot PSY3 (dPSY3 vs. mPSY3 from monocots) class probably represents an ancestral branch in the evolution of the plant phytoene synthase family. The evolutionary history of PSY genes is compared with the evolution of MEP pathway isogenes encoding 1-deoxy-d-xylulose 5-phosphate synthases (DXS), particularly DXS2, which is co-regulated with dPSY3s in mycorrhizal roots. Such stress-inducible isoforms for rate-limiting steps in root carotenogenesis might be components of multi-enzyme complexes committed to apocarotenoid rather than to carotenoid formation.

Carotenoid metabolism in plants

Carotenoids are mostly C40 terpenoids, a class of hydrocarbons that participate in various biological processes in plants, such as photosynthesis, photomorphogenesis, photoprotection, and development. Carotenoids also serve as precursors for two plant hormones and a diverse set of apocarotenoids. They are colorants and critical components of the human diet as antioxidants and provitamin A. In this review, we summarize current knowledge of the genes and enzymes involved in carotenoid metabolism and describe recent progress in understanding the regulatory mechanisms underlying carotenoid accumulation. The importance of the specific location of carotenoid enzyme metabolons and plastid types as well as of carotenoid-derived signals is discussed.

Regulation of carotenoid metabolism in tomato

Carotenoids serve diverse functions in vastly different organisms that both produce and consume them. Enhanced carotenoid accumulation is of great importance in the visual and functional properties of fruits and vegetables. Significant progress has been achieved in recent years in our understanding of carotenoid biosynthesis in tomato (Solanum lycopersicum) using biochemical and genetics approaches. The carotenoid metabolic network is temporally and spatially controlled, and plants have evolved strategic tactics to regulate carotenoid metabolism in response to various developmental and environmental factors. In this review, we summarize the current status of studies on transcription factors and phytohormones that regulate carotenoid biosynthesis, catabolism, and storage capacity in plastids, as well as the responses of carotenoid metabolism to environmental cues in tomato fruits. Transcription factors function either in cooperation with or independently of phytohormone signaling to regulate carotenoid metabolism, providing novel approaches for metabolic engineering of carotenoid composition and content in tomato.

Functional analysis of the Brassica napus L. phytoene synthase (PSY) gene family

Phytoene synthase (PSY) has been shown to catalyze the first committed and rate-limiting step of carotenogenesis in several crop species, including Brassica napus L. Due to its pivotal role, PSY has been a prime target for breeding and metabolic engineering the carotenoid content of seeds, tubers, fruits and flowers. In Arabidopsis thaliana, PSY is encoded by a single copy gene but small PSY gene families have been described in monocot and dicotyledonous species. We have recently shown that PSY genes have been retained in a triplicated state in the A- and C-Brassica genomes, with each paralogue mapping to syntenic locations in each of the three “Arabidopsis-like” subgenomes. Most importantly, we have shown that in B. napus all six members are expressed, exhibiting overlapping redundancy and signs of subfunctionalization among photosynthetic and non photosynthetic tissues. The question of whether this large PSY family actually encodes six functional enzymes remained to be answered. Therefore, the objectives of this study were to: (i) isolate, characterize and compare the complete protein coding sequences (CDS) of the six B. napus PSY genes; (ii) model their predicted tridimensional enzyme structures; (iii) test their phytoene synthase activity in a heterologous complementation system and (iv) evaluate their individual expression patterns during seed development. This study further confirmed that the six B. napus PSY genes encode proteins with high sequence identity, which have evolved under functional constraint. Structural modeling demonstrated that they share similar tridimensional protein structures with a putative PSY active site. Significantly, all six B. napus PSY enzymes were found to be functional. Taking into account the specific patterns of expression exhibited by these PSY genes during seed development and recent knowledge of PSY suborganellar localization, the selection of transgene candidates for metabolic engineering the carotenoid content of oilseeds is discussed.

Carotenoid metabolism is induced in rice bran during very high temperature stress

BACKGROUND

Postharvest yellowing (PhY) causes yellowing in rice endosperm during conditions of high temperature and moisture. Rice bran was investigated as a model tissue for studying PhY and for determining the possibility of carotenoids as the source of the yellow color.

RESULTS

A survey of different colored rice and wild (non-sativa) rice lines resulted in the identification of several purple bran lines having very low or no bran carotenoids. Transcription levels of phytoene synthase, the first committed step in carotenoid biosynthesis, were higher in carotenoid-containing bran, indicating that carotogenesis is an ongoing process in mature bran. Bran and endosperm subjected to PhY conditions had a similar temperature optimum and color responses, so bran was utilized to investigate carotenoid levels and transcription levels of genes for carotogenesis during PhY. During PhY, total carotenoid levels in bran increased while levels of the predominant xanthophyll carotenoid, lutein, decreased. This difference could be explained by carotenoids being metabolized into apocarotenoids, which have high antioxidant activities and can be highly colored. This mechanism is further supported by the long-term transcription during PhY of CCD1, which encodes an enzyme in apocarotenoid production.

CONCLUSION

We propose that PhY is an active, metabolic response to a very high temperature stress and that the increase in total carotenoids in bran during PhY is a result of the production of colored apocarotenoids.