Arabidopsis OR proteins are the major posttranscriptional regulators of phytoene synthase in controlling carotenoid biosynthesis

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.

Creating plant molecular factories for industrial and nutritional isoprenoid production

Chemical refining is a highly efficient process that has driven industrialisation and globalisation. However, dwindling fuel reserves and climatic fluctuation are now imposing key societal and economic challenges to health and welfare provision, agriculture, manufacturing outputs and energy. Plants are potentially exploitable 'green' chemical factories, with vast chemical diversity that can be used for the discovery and production of food, feed, medicines and biomaterials. Despite notable advances, plant based production under real-life scenarios remains, in most cases, economically uncompetitive when compared to inherently non-sustainable petrochemical based processes. In the present review the strategies available and those emerging will be described. Furthermore, how can the new evolving molecular tools such as genome editing be utilised to create a new paradigm of plant-based production? To illustrate the present status quo, we have chosen the isoprenoids as the class of natural products. These compounds display vast chemical diversity and have been used across multiple industrial sectors as medicines, supplements in food and feedstuffs, colourants and fragrances.

Overexpression of DnaJ-Like Chaperone Enhances Carotenoid Synthesis in Chlamydomonas reinhardtii

Production of functional carotenoids using microalgae may facilitate the commercialization of anti-aging nutritional supplements. The green alga Chlamydomonas reinhardtii uses a non-mevalonate (MEP) pathway for isopentenyl diphosphate (IPP) synthesis. Two enzymes thought to play important roles in this MEP pathway to IPP synthesis are 1-deoxy-D-xylulose 5-phosphate synthase (DXS) and reductase (DXR). DnaJ-like chaperone (Orange protein) is thought to support phytoene synthase, a key enzyme in plant carotenoid synthesis. Genes for Orange (OR), DXS, and DXR were overexpressed via nuclear transformation into C. reinhardtii. CDS of OR, DXS, and DXR were amplified and connected with dual promoters of heat-shock protein 70A and ribulose bisphosphate carboxylase small chain 2. Compared with the parental strain, transformant CrOR#2 produced increased lutein and β-carotene (1.9-fold and 1.7-fold per cell, respectively). Transformant CrDXS#1 produced lutein and β-carotene at lower per-cell abundances than those for the parental strain. CrDXR#2 transformant produced lutein and β-carotene at higher per-cell abundances than their parental counterpart; however, these transformants produced lutein and β-carotene at lower per-medium abundances than their parental counterparts. These results suggest that OR protein supports phytoene synthase in C. reinhardtii and that the phytoene synthesis step is rate-limiting in carotenoid synthesis.

The Arabidopsis ORANGE (AtOR) gene promotes carotenoid accumulation in transgenic corn hybrids derived from parental lines with limited carotenoid pools

The AtOR gene enhances carotenoid levels in corn by promoting the formation of plastoglobuli when the carotenoid pool is limited, but has no further effect when carotenoids are already abundant. The cauliflower orange (or) gene mutation influences carotenoid accumulation in plants by promoting the transition of proplastids into chromoplasts, thus creating intracellular storage compartments that act as metabolic sink. We overexpressed the Arabidopsis OR gene under the control of the endosperm-specific wheat LMW glutenin promoter in a white corn variety that normally accumulates only trace amounts of carotenoids. The total endosperm carotenoid content in the best-performing AtOR transgenic corn line was 32-fold higher than wild-type controls (~25 µg/g DW at 30 days after pollination) but the principal carotenoids remained the same, suggesting that AtOR increases the abundance of existing carotenoids without changing the metabolic composition. We analyzed the expression of endogenous genes representing the carotenoid biosynthesis and MEP pathways, as well as the plastid fusion/translocation factor required for chromoplast formation, but only the DXS1 gene was upregulated in the transgenic corn plants. The line expressing AtOR at the highest level was crossed with four transgenic corn lines expressing different carotenogenic genes and accumulating different carotenoids. The introgression of AtOR increased the carotenoid content of the hybrids when there was a limited carotenoid pool in the parental line, but had no effect when carotenoids were already abundant in the parent. The AtOR gene therefore appears to enhance carotenoid levels by promoting the formation of carotenoid-sequestering plastoglobuli when the carotenoid pool is limited, but has no further effect when carotenoids are already abundant because high levels of carotenoids can induce the formation of carotenoid-sequestering plastoglobuli even in the absence of AtOR.

Illuminating colors: regulation of carotenoid biosynthesis and accumulation by light

Light stimulates the biosynthesis of carotenoids and regulates the development of plastid structures to accommodate these photoprotective pigments. Work with Arabidopsis revealed molecular factors coordinating carotenoid biosynthesis and storage with photosynthetic development during deetiolation, when underground seedlings emerge to the light. Some of these factors also adjust carotenoid biosynthesis in response to plant proximity (i.e., shade), a mechanism that was readapted in tomato to monitor fruit ripening progression. While light positively impacts carotenoid production and accumulation in most cases, total carotenoid levels decrease in roots of colored carrot cultivars when illuminated. The recent discovery that such cultivars might be photomorphogenic mutants provides an explanation for this striking phenotype.

SNOWY COTYLEDON 2 Promotes Chloroplast Development and Has a Role in Leaf Variegation in Both Lotus japonicus and Arabidopsis thaliana

Plants contain various factors that transiently interact with subunits or intermediates of the thylakoid multiprotein complexes, promoting their stable association and integration. Hence, assembly factors are essential for chloroplast development and the transition from heterotrophic to phototrophic growth. Snowy cotyledon 2 (SCO2) is a DNAJ-like protein involved in thylakoid membrane biogenesis and interacts with the light-harvesting chlorophyll-binding protein LHCB1. In Arabidopsis thaliana, SCO2 function was previously reported to be restricted to cotyledons. Here we show that disruption of SCO2 in Lotus japonicus results not only in paler cotyledons but also in variegated true leaves. Furthermore, smaller and pale-green true leaves can also be observed in A. thaliana sco2 (atsco2) mutants under short-day conditions. In both species, SCO2 is required for proper accumulation of PSII-LHCII complexes. In contrast to other variegated mutants, inhibition of chloroplastic translation strongly affects L. japonicus sco2 mutant development and fails to suppress their variegated phenotype. Moreover, inactivation of the suppressor of variegation AtClpR1 in the atsco2 background results in an additive double-mutant phenotype with variegated true leaves. Taken together, our results indicate that SCO2 plays a distinct role in PSII assembly or repair and constitutes a novel factor involved in leaf variegation.

Cytological and molecular characterization of carotenoid accumulation in normal and high-lycopene mutant oranges

Ripe Cara Cara sweet orange contains 25 times as much carotenoids in flesh as Newhall sweet orange, due to high accumulation of carotenes, mainly phytoene, lycopene and phytofluene. Only yellow globular chromoplasts were observed in Newhall flesh. Distinct yellow globular and red elongated crystalline chromoplasts were found in Cara Cara but only one type of chromoplast was present in each cell. The red crystalline chromoplasts contained lycopene as a dominant carotenoid and were associated with characteristic carotenoid sequestering structures. The increased accumulation of linear carotenes in Cara Cara is not explained by differences in expression of all 18 carotenogenic genes or gene family members examined, or sequence or abundance of mRNAs from phytoene synthase (PSY) and chromoplast-specific lycopene β-cyclase (CYCB) alleles. 2-(4-Chlorophenylthio)-triethylamine hydrochloride (CPTA) enhanced lycopene accumulation and induced occurrence of red crystalline chromoplasts in cultured Newhall juice vesicles, indicating that carotenoid synthesis and accumulation can directly affect chromoplast differentiation and structure. Norflurazon (NFZ) treatment resulted in high accumulation of phytoene and phytofluene in both oranges, and the biosynthetic activity upstream of phytoene desaturase was similar in Newhall and Cara Cara. Possible mechanisms for high carotene accumulation and unique development of red crystalline chromoplasts in Cara Cara are discussed.

The regulation of carotenoid formation in tomato fruit

Carotenoid biosynthesis in plants includes a complex series of desaturation/isomerisation reactions, catalyzed by four independent enzymes. In bacteria and fungi one desaturase/isomerase enzyme completes the same series of reactions. In the present study, a bacterial desaturase (crtI) from Pantoea ananatis has been overexpressed in the tangerine mutant of tomato (Solanum lycopersicon) which accumulates cis-carotene isomers in the fruit due to a defective isomerase (CRTISO) and the old gold crimson (og^c ) tomato mutant, which is defective in the fruit-enhanced lycopene β-cyclase (CYCB). Comprehensive molecular and biochemical characterization of the resulting lines expressing crtI has revealed negative feedback mechanisms, acting predominantly at the level of phytoene synthase-1 (PSY1), and feed-forward mechanisms inducing cyclisation. In both cases, altered transcription appears to be the progenitor, with subsequent post-transcriptional modulation highlighting the complexity of the processes involved in modulating carotenoid homeostasis in plant tissues.

StMYB44 negatively regulates phosphate transport by suppressing expression of PHOSPHATE1 in potato

Phosphorus is an important macronutrient for plant growth, but often deficient in soil. To understand the molecular basis of the complex responses of potato (Solanum tuberosum L.) to phosphate (Pi) deficiency stress, the RNA-Seq approach was taken to identify genes responding to Pi starvation in potato roots. A total of 359 differentially expressed genes were identified, among which the Solanum tuberosum transcription factor gene MYB44 (StMYB44) was found to be down-regulated by Pi starvation. StMYB44 was ubiquitously expressed in potato tissues and organs, and StMYB44 protein was exclusively localized in the nucleus. Overexpression of StMYB44 in potato resulted in lower accumulation of Pi in shoots. Transcriptomic analysis indicated that the abundance of S. tuberosum PHOSPHATE1 (StPHO1), a Pi transport-related gene, was reduced in StMYB44 overexpression lines. In contrast, knock-out of StMYB44 by a CRISPR/Cas9 system failed to increase transcription of StPHO1. Moreover, StMYB44 was found to interact in the nucleus with AtWRKY6, a known Arabidopsis transcription factor directly regulating PHO1 expression, and StWRKY6, indicating that StMYB44 could be a member of the regulatory complex controlling transcription of StPHO1. Taken together, our study demonstrates that StMYB44 negatively regulates Pi transport in potato by suppressing StPHO1 expression.

Distinct Mechanisms of the ORANGE Protein in Controlling Carotenoid Flux

β-Carotene adds nutritious value and determines the color of many fruits, including melon (Cucumis melo). In melon mesocarp, β-carotene accumulation is governed by the Orange gene (CmOr) golden single-nucleotide polymorphism (SNP) through a yet to be discovered mechanism. In Arabidopsis (Arabidopsis thaliana), OR increases carotenoid levels by posttranscriptionally regulating phytoene synthase (PSY). Here, we identified a CmOr nonsense mutation (Cmor-lowβ) that lowered fruit β-carotene levels with impaired chromoplast biogenesis. Cmor-lowβ exerted a minimal effect on PSY transcripts but dramatically decreased PSY protein levels and enzymatic activity, leading to reduced carotenoid metabolic flux and accumulation. However, the golden SNP was discovered to not affect PSY protein levels and carotenoid metabolic flux in melon fruit, as shown by carotenoid and immunoblot analyses of selected melon genotypes and by using chemical pathway inhibitors. The high β-carotene accumulation in golden SNP melons was found to be due to a reduced further metabolism of β-carotene. This was revealed by genetic studies with double mutants including carotenoid isomerase (yofi), a carotenoid-isomerase nonsense mutant, which arrests the turnover of prolycopene. The yofi F2 segregants accumulated prolycopene independently of the golden SNP Moreover, Cmor-lowβ was found to inhibit chromoplast formation and chloroplast disintegration in fruits from 30 d after anthesis until ripening, suggesting that CmOr regulates the chloroplast-to-chromoplast transition. Taken together, our results demonstrate that CmOr is required to achieve PSY protein levels to maintain carotenoid biosynthesis metabolic flux but that the mechanism of the CmOr golden SNP involves an inhibited metabolism downstream of β-carotene to dramatically affect both carotenoid content and plastid fate.

Unraveling Massive Crocins Transport and Accumulation through Proteome and Microscopy Tools during the Development of Saffron Stigma

Crocins, the glucosides of crocetin, are present at high concentrations in saffron stigmas and accumulate in the vacuole. However, the biogenesis of the saffron chromoplast, the changes during the development of the stigma and the transport of crocins to the vacuole, are processes that remain poorly understood. We studied the process of chromoplast differentiation in saffron throughout stigma development by means of transmission electron microscopy. Our results provided an overview of a massive transport of crocins to the vacuole in the later developmental stages, when electron dense drops of a much greater size than plastoglobules (here defined "crocinoplast") were observed in the chromoplast, connected to the vacuole with a subsequent transfer of these large globules inside the vacuole. A proteome analysis of chromoplasts from saffron stigma allowed the identification of several well-known plastid proteins and new candidates involved in crocetin metabolism. Furthermore, expressions throughout five developmental stages of candidate genes responsible for carotenoid and apocarotenoid biogenesis, crocins transport to the vacuole and starch metabolism were analyzed. Correlation matrices and networks were exploited to identify a series of transcripts highly associated to crocetin (such as 1-Deoxy-d-xylulose 5-phosphate synthase (DXS), 1-Deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), carotenoid isomerase (CRTISO), Crocetin glucosyltransferase 2 (UGT2), etc.) and crocin (e.g., ζ-carotene desaturase (ZDS) and plastid-lipid-associated proteins (PLAP2)) accumulation; in addition, candidate aldehyde dehydrogenase (ADH) genes were highlighted.

Manipulation of Metabolic Pathways to Develop Vitamin-Enriched Crops for Human Health

Vitamin deficiencies are major forms of micronutrient deficiencies, and are associated with huge economic losses as well as severe physical and intellectual damages to humans. Much evidence has demonstrated that biofortification plays an important role in combating vitamin deficiencies due to its economical and effective delivery of nutrients to populations in need. Biofortification enables food plants to be enriched with vitamins through conventional breeding and/or biotechnology. Here, we focus on the progress in the manipulation of the vitamin metabolism, an essential part of biofortification, by the genetic modification or by the marker-assisted selection to understand mechanisms underlying metabolic improvement in food plants. We also propose to integrate new breeding technologies with metabolic pathway modification to facilitate biofortification in food plants and, thereby, to benefit human health.

Metabolic engineering of carotenoids in transgenic sweetpotato

Sweetpotato [Ipomoea batatas (L.) Lam], which contains high levels of antioxidants such as ascorbate and carotenoids in its storage root, is one of the healthiest foods, as well as one of the best starch crops for growth on marginal lands. In plants, carotenoid pigments are involved in light harvesting for photosynthesis and are also essential for photo-protection against excess light. As dietary antioxidants in humans, these compounds benefit health by alleviating aging-related diseases. The storage root of sweetpotato is a good source of both carotenoids and carbohydrates for human consumption. Therefore, metabolic engineering of sweetpotato to increase the content of useful carotenoids represents an important agricultural goal. This effort has been facilitated by cloning of most of the carotenoid biosynthetic genes, as well as the Orange gene involved in carotenoid accumulation. In this review, we describe our current understanding of the regulation of biosynthesis, accumulation and catabolism of carotenoids in sweetpotato. A deeper understanding of these topics should contribute to development of new sweetpotato cultivars with higher levels of nutritional carotenoids and abiotic stress tolerance.

IbOr Regulates Photosynthesis under Heat Stress by Stabilizing IbPsbP in Sweetpotato

The Orange (Or) protein regulates carotenoid biosynthesis and environmental stress in plants. Previously, we reported that overexpression of the sweetpotato [Ipomoea batatas (L.) Lam] Or gene (IbOr) in transgenic Arabidopsis (referred to as IbOr-OX/At) increased the efficiency of photosystem II (PSII) and chlorophyll content after heat shock. However, little is known about the role of IbOr in PSII-mediated protection against abiotic stress. In this study, comparative proteomics revealed that expression of PsbP (an extrinsic subunit of PSII) is up-regulated in heat-treated IbOr-OX/At plants. We then identified and functionally characterized the PsbP-like gene (IbPsbP) from sweetpotato. IbPsbP is predominantly localized in chloroplast, and its transcripts are tissue-specifically expressed and up-regulated in response to abiotic stress. In addition, IbOr interacts with IbPsbP and protects it from heat-induced denaturation, consistent with the observation that transgenic sweetpotato overexpressing IbOr maintained higher PSII efficiency and chlorophyll content upon exposure to heat stress. These results indicate that IbOr can protect plants from environmental stress not only by controlling carotenoid biosynthesis but also by directly stabilizing PSII.

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.

The DnaJ-like zinc finger domain protein ORANGE localizes to the nucleus in etiolated cotyledons of Arabidopsis thaliana

Vitamin A deficiency (VAD) is a worldwide health problem. Overexpression of the DnaJ-like zinc finger domain protein ORANGE (OR) is a novel strategy for the biofortification of pro-vitamin A carotenoids in different staple crops to alleviate VAD. In plants, OR triggers the differentiation from non-pigmented plastids into carotenoid-accumulating plastids. There are different reports on the subcellular localization of this protein in either chloroplasts or the nucleus, both of which were supported by confocal observation and protein-protein interaction results. In this work, we studied the subcellular localization of OR in the cotyledons of germinating seedlings whose plastids were transitioning from non-pigmented proplastids into carotenoid-accumulating etioplasts in the dark, and then into chloroplasts upon illumination. Our Western blot analysis identified two bands of the Arabidopsis OR protein (AtOR) from the chloroplast fraction of the mature leaves (i.e., a 34-kDa form corresponding to the full-length peptide and a 30-kDa form suggesting the removal of the N-terminal chloroplast transit peptide). We found that the full-length AtOR was predominantly localized in the nucleus in etiolated cotyledons, although its abundance decreased upon illumination. Our bioinformatics analysis indicated a nuclear localization signal (NLS) after the N-terminal chloroplast transit peptide. When we substituted different N-terminal regions of AtOR with the green fluorescent protein, our confocal observations demonstrated that this NLS was sufficient to target AtOR to the nucleus. Our results demonstrate that AtOR is a dual-targeted protein that mainly localizes in the nucleus in etiolated cotyledons.

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.

Elevated vitamin E content improves all-trans β-carotene accumulation and stability in biofortified sorghum

Micronutrient deficiencies are common in locales where people must rely upon sorghum as their staple diet. Sorghum grain is seriously deficient in provitamin A (β-carotene) and in the bioavailability of iron and zinc. Biofortification is a process to improve crops for one or more micronutrient deficiencies. We have developed sorghum with increased β-carotene accumulation that will alleviate vitamin A deficiency among people who rely on sorghum as their dietary staple. However, subsequent β-carotene instability during storage negatively affects the full utilization of this essential micronutrient. We determined that oxidation is the main factor causing β-carotene degradation under ambient conditions. We further demonstrated that coexpression of homogentisate geranylgeranyl transferase (HGGT), stacked with carotenoid biosynthesis genes, can mitigate β-carotene oxidative degradation, resulting in increased β-carotene accumulation and stability. A kinetic study of β-carotene degradation showed that the half-life of β-carotene is extended from less than 4 wk to 10 wk on average with HGGT coexpression.

Orange protein has a role in phytoene synthase stabilization in sweetpotato

Carotenoids have essential roles in light-harvesting processes and protecting the photosynthetic machinery from photo-oxidative damage. Phytoene synthase (PSY) and Orange (Or) are key plant proteins for carotenoid biosynthesis and accumulation. We previously isolated the sweetpotato (Ipomoea batatas) Or gene (IbOr), which is involved in carotenoid accumulation and salt stress tolerance. The molecular mechanism underlying IbOr regulation of carotenoid accumulation was unknown. Here, we show that IbOr has an essential role in regulating IbPSY stability via its holdase chaperone activity both in vitro and in vivo. This protection results in carotenoid accumulation and abiotic stress tolerance. IbOr transcript levels increase in sweetpotato stem, root, and calli after exposure to heat stress. IbOr is localized in the nucleus and chloroplasts, but interacts with IbPSY only in chloroplasts. After exposure to heat stress, IbOr predominantly localizes in chloroplasts. IbOr overexpression in transgenic sweetpotato and Arabidopsis conferred enhanced tolerance to heat and oxidative stress. These results indicate that IbOr holdase chaperone activity protects IbPSY stability, which leads to carotenoid accumulation, and confers enhanced heat and oxidative stress tolerance in plants. This study provides evidence that IbOr functions as a molecular chaperone, and suggests a novel mechanism regulating carotenoid accumulation and stress tolerance in plants.

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.

Learning the Languages of the Chloroplast: Retrograde Signaling and Beyond

The chloroplast can act as an environmental sensor, communicating with the cell during biogenesis and operation to change the expression of thousands of proteins. This process, termed retrograde signaling, regulates expression in response to developmental cues and stresses that affect photosynthesis and yield. Recent advances have identified many signals and pathways-including carotenoid derivatives, isoprenes, phosphoadenosines, tetrapyrroles, and heme, together with reactive oxygen species and proteins-that build a communication network to regulate gene expression, RNA turnover, and splicing. However, retrograde signaling pathways have been viewed largely as a means of bilateral communication between organelles and nuclei, ignoring their potential to interact with hormone signaling and the cell as a whole to regulate plant form and function. Here, we discuss new findings on the processes by which organelle communication is initiated, transmitted, and perceived, not only to regulate chloroplastic processes but also to intersect with cellular signaling and alter physiological responses.

The Quest for Golden Bananas: Investigating Carotenoid Regulation in a Fe'i Group Musa Cultivar

The regulation of carotenoid biosynthesis in a high-carotenoid-accumulating Fe'i group Musa cultivar, "Asupina", has been examined and compared to that of a low-carotenoid-accumulating cultivar, "Cavendish", to understand the molecular basis underlying carotenogenesis during banana fruit development. Comparisons in the accumulation of carotenoid species, expression of isoprenoid genes, and product sequestration are reported. Key differences between the cultivars include greater carotenoid cleavage dioxygenase 4 (CCD4) expression in "Cavendish" and the conversion of amyloplasts to chromoplasts during fruit ripening in "Asupina". Chromoplast development coincided with a reduction in dry matter content and fruit firmness. Chromoplasts were not observed in "Cavendish" fruits. Such information should provide important insights for future developments in the biofortification and breeding of banana.

Plastid ribosomal protein S5 is involved in photosynthesis, plant development, and cold stress tolerance in Arabidopsis

Plastid ribosomal proteins are essential components of protein synthesis machinery and have diverse roles in plant growth and development. Mutations in plastid ribosomal proteins lead to a range of developmental phenotypes in plants. However, how they regulate these processes is not fully understood, and the functions of some individual plastid ribosomal proteins remain unknown. To identify genes responsible for chloroplast development, we isolated and characterized a mutant that exhibited pale yellow inner leaves with a reduced growth rate in Arabidopsis. The mutant (rps5) contained a missense mutation of plastid ribosomal protein S5 (RPS5), which caused a dramatically reduced abundance of chloroplast 16S rRNA and seriously impaired 16S rRNA processing to affect ribosome function and plastid translation. Comparative proteomic analysis revealed that the rps5 mutation suppressed the expression of a large number of core components involved in photosystems I and II as well as many plastid ribosomal proteins. Unexpectedly, a number of proteins associated with cold stress responses were greatly decreased in rps5, and overexpression of the plastid RPS5 improved plant cold stress tolerance. Our results indicate that RPS5 is an important constituent of the plastid 30S subunit and affects proteins involved in photosynthesis and cold stress responses to mediate plant growth and development.

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.

The DnaJ-Like Zinc Finger Domain Protein PSA2 Affects Light Acclimation and Chloroplast Development in Arabidopsis thaliana

The biosynthesis of chlorophylls and carotenoids and the assembly of thylakoid membranes are critical for the photoautotrophic growth of plants. Different factors are involved in these two processes. In recent years, members of the DnaJ-like zinc finger domain proteins have been found to take part in the biogenesis and/or the maintenance of plastids. One member of this family of proteins, PSA2, was recently found to localize to the thylakoid lumen and regulate the accumulation of photosystem I. In this study, we report that the silencing of PSA2 in Arabidopsis thaliana resulted in variegated leaves and retarded growth. Although both chlorophylls and total carotenoids decreased in the psa2 mutant, violaxanthin, and zeaxanthin accumulated in the mutant seedlings grown under growth condition. Lower levels of non-photochemical quenching and electron transport rate were also found in the psa2 mutant seedlings under growth condition compared with those of the wild-type plants, indicating an impaired capability to acclimate to normal light irradiance when PSA2 was silenced. Moreover, we also observed an abnormal assembly of grana thylakoids and poorly developed stroma thylakoids in psa2 chloroplasts. Taken together, our results demonstrate that PSA2 is a member of the DnaJ-like zinc finger domain protein family that affects light acclimation and chloroplast development.

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.

A bulk segregant transcriptome analysis reveals metabolic and cellular processes associated with Orange allelic variation and fruit β-carotene accumulation in melon fruit

BACKGROUND

Melon fruit flesh color is primarily controlled by the "golden" single nucleotide polymorhism of the "Orange" gene, CmOr, which dominantly triggers the accumulation of the pro-vitamin A molecule, β-carotene, in the fruit mesocarp. The mechanism by which CmOr operates is not fully understood. To identify cellular and metabolic processes associated with CmOr allelic variation, we compared the transcriptome of bulks of developing fruit of homozygous orange and green fruited F3 families derived from a cross between orange and green fruited parental lines.

RESULTS

Pooling together F3 families that share same fruit flesh color and thus the same CmOr allelic variation, normalized traits unrelated to CmOr allelic variation. RNA sequencing analysis of these bulks enabled the identification of differentially expressed genes. These genes were clustered into functional groups. The relatively enriched functional groups were those involved in photosynthesis, RNA and protein regulation, and response to stress.

CONCLUSIONS

The differentially expressed genes and the enriched processes identified here by bulk segregant RNA sequencing analysis are likely part of the regulatory network of CmOr. Our study demonstrates the resolution power of bulk segregant RNA sequencing in identifying genes related to commercially important traits and provides a useful tool for better understanding the mode of action of CmOr gene in the mediation of carotenoid accumulation.

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.

Distinct Carotenoid and Flavonoid Accumulation in a Spontaneous Mutant of Ponkan (Citrus reticulata Blanco) Results in Yellowish Fruit and Enhanced Postharvest Resistance

As the most important fresh fruit worldwide, citrus is often subjected to huge postharvest losses caused by abiotic and biotic stresses. As a promising strategy to reduce postharvest losses, enhancing natural defense by potential metabolism reprogramming in citrus mutants has rarely been reported. The yellowish spontaneous mutant of Ponkan (Citrus reticulata Blanco) (YP) was used to investigate the influence of metabolism reprogramming on postharvest performance. Our results show that reduced xanthophyll accumulation is the cause of yellowish coloring of YP and might be attributed to the reduced carotenoid sequestration capacity and upregulated expression of carotenoid cleavage dioxygenase genes. Constantly higher levels of polymethoxylated flavones (PMFs) during the infection and the storage stage might make significant contribution to the more strongly induced resistance against Penicillium digitatum and lower rotting rate. The present study demonstrates the feasibility of applying bud mutants to improve the postharvest performance of citrus fruits.

A Single Amino Acid Substitution in an ORANGE Protein Promotes Carotenoid Overaccumulation in Arabidopsis

Carotenoids are crucial for plant growth and human health. The finding of ORANGE (OR) protein as a pivotal regulator of carotenogenesis offers a unique opportunity to comprehensively understand the regulatory mechanisms of carotenoid accumulation and develop crops with enhanced nutritional quality. Here, we demonstrated that alteration of a single amino acid in a wild-type OR greatly enhanced its ability to promote carotenoid accumulation. Whereas overexpression of OR from Arabidopsis (Arabidopsis thaliana; AtOR) or from the agronomically important crop sorghum (Sorghum bicolor; SbOR) increased carotenoid levels up to 2-fold, expression of AtOR(His) (R90H) or SbOR(His) (R104H) variants dramatically enhanced carotenoid accumulation by up to 7-fold in the Arabidopsis calli. Moreover, we found that AtOR(Ala) (R90A) functioned similarly to AtOR(His) to promote carotenoid overproduction. Neither AtOR nor AtOR(His) greatly affected carotenogenic gene expression. AtOR(His) exhibited similar interactions with phytoene synthase (PSY) as AtOR in posttranscriptionally regulating PSY protein abundance. AtOR(His) triggered biogenesis of membranous chromoplasts in the Arabidopsis calli, which shared structures similar to chromoplasts found in the curd of the orange cauliflower (Brassica oleracea) mutant. By contrast, AtOR did not cause plastid-type changes in comparison with the controls, but produced plastids containing larger and electron-dense plastoglobuli. The unique ability of AtOR(His) in mediating chromoplast biogenesis is responsible for its induced carotenoid overproduction. Our study demonstrates OR(His/Ala) as powerful tools for carotenoid enrichment in plants, and provides insights into the mechanisms underlying OR(His)-regulated carotenoid accumulation.

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.

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.

Structural characterization of highly glucosylated crocins and regulation of their biosynthesis during flower development in Crocus

Crocin biosynthesis in Crocus has been proposed to proceed through a zeaxanthin cleavage pathway catalyzed by carotenoid cleavage dioxygenase 2 (CCD2), and followed by glucosylation reactions catalyzed by CsGT2 (UGT74AD1). In Crocus ancyrensis flowers, crocins with eight (crocin-1), seven (crocin-2), and six glucose (crocin-3) moieties accumulated both in stigma and tepals. We have characterized the structure of these highly glucosylated crocins and follow up their accumulation by high-resolution liquid chromatography coupled with diode array detector along the development of both tissues, and coupled to the isolation and analysis of the expression of eighteen genes (PSY-I, PSY-II, PDS-(I-V), ISO-ZDS, ZDS, CtrISO, LYC-I and II, BCH, CaCCD2, UGT74AD2-5) related with the apocarotenoid metabolism in C. ancyrensis tepals and stigmas. Structure elucidation of crocin-1 and crocin-2 was done by the combined use of 1D and 2D [(1)H, (1)H] (gCOSY and TOCSY and ROESY) and [(1)H-(13)C] NMR experiments, revealing that for crocin-1 was all-trans-crocetin O-[β-D- Glucopyranosyl)-(1→4)-(β-D-glucopyranosyl)-(1→2)]-O-[β-D-glucopyranosyl-(1→6)]-β-D-glucopyranosyl diester, while crocin-2 showed an identical structure except for the absence of one glucose residue in one end of the molecule. Crocins accumulation was not synchronically regulated in stigma and tepals, although in both cases crocins accumulation parallels tissue development, decreasing at anthesis. The expression of the carotenogenic genes PSY, ZDS-V, BCH, and LCY-II was correlated with crocins accumulation. In addition, CaCCD2 and only one of the four glucosyltransferase encoding genes, UGT74AD2, were highly expressed, and the expression was correlated with high levels of crocins accumulation in stigma and tepals.