Why is golden rice golden (yellow) instead of red?

Comprehensive genetic diversity and genome-wide association studies revealed the genetic basis of avocado fruit quality traits

Introduction

Avocado (Persea americana) is a highly nutritious fruit gaining worldwide popularity. However, its cultivation is currently reliant on a limited number of cultivars with restricted genetic diversity. This study aims to investigate the genetic diversity and population structure of avocado germplasm and identify genetic loci associated with key fruit quality traits that influence customer preference.

Methods

A diversity panel of 110 avocado accessions was analyzed using 4,706 high-quality single nucleotide polymorphisms (SNPs). Genetic diversity and population structure were analyzed using pairwise FST, AMOVA, admixture analysis, and phylogenetic analysis. Genome-wide association studies (GWAS) were conducted targeting nine fruit quality traits using two models: General Linear Model (GLM) with Principal Component Analysis (PCA) and Mixed Linear Model (MLM) with PCA and kinship (PCA + K).

Results

The analysis revealed three distinct populations corresponding to the three avocado ecotypes: Guatemalan, West Indian, and Mexican. Phylogenetic analysis indicated a closer relationship between the Guatemalan and West Indian races compared to the Mexican race in our Florida germplasm collection. GWAS led to identification of 12 markers within 11 genomic regions significantly associated with fruit quality traits such as fruit color, shape, taste, and skin texture. These markers explained between 14.84% to 43.96% of the phenotypic variance, with an average of 24.63%. Annotation of these genomic regions unveiled candidate genes potentially responsible for controlling these traits.

Discussion

The findings enhance our understanding of genetic diversity and population structure in avocado germplasm. The identified genetic loci provide valuable insights into the genetic basis of fruit quality traits, aiding breeding programs in developing improved avocado cultivars. Marker-assisted selection can accelerate the development of new varieties, promoting a more diverse and resilient avocado market.

Improving Rice Grain Quality Through Ecotype Breeding for Enhancing Food and Nutritional Security in Asia-Pacific Region

Rice grain is widely consumed as a staple food, providing essential nutrition for households, particularly marginalized families. It plays a crucial role in ensuring food security, promoting human nutrition, supporting good health, and contributing to global food and nutritional security. Addressing the diverse quality demands of emerging diverse and climate-risked population dietary needs requires the development of a single variety of rice grain that can meet the various dietary and nutritional requirements. However, there is a lack of concrete definition for rice grain quality, making it challenging to cater to the different demands. The lack of sufficient genetic study and development in improving rice grain quality has resulted in widespread malnutrition, hidden hunger, and micronutrient deficiencies affecting a significant portion of the global population. Therefore, it is crucial to identify genetically evolved varieties with marked qualities that can help address these issues. Various factors account for the declining quality of rice grain and requires further study to improve their quality for healthier diets. We characterized rice grain quality using Lancastrians descriptor and a multitude of intrinsic and extrinsic quality traits. Next, we examined various components of rice grain quality favored in the Asia-Pacific region. This includes preferences by different communities, rice industry stakeholders, and value chain actors. We also explored the biological aspects of rice grain quality in the region, as well as specific genetic improvements that have been made in these traits. Additionally, we evaluated the factors that can influence rice grain quality and discussed the future directions for ensuring food and nutritional security and meeting consumer demands for grain quality. We explored the diverse consumer bases and their varied preferences in Asian-Pacific countries including India, China, Nepal, Bhutan, Vietnam, Sri Lanka, Pakistan, Thailand, Cambodia, Philippines, Bangladesh, Indonesia, Korea, Myanmar and Japan. The quality preferences encompassed a range of factors, including rice head recovery, grain shape, uniform size before cooking, gelatinization, chalkiness, texture, amylose content, aroma, red-coloration of grain, soft and shine when cooked, unbroken when cooked, gelatinization, less water required for cooking, gelatinization temperature (less cooking time), aged rice, firm and dry when cooked (gel consistency), extreme white, soft when chewed, easy-to-cook rice (parboiled rice), vitamins, and minerals. These preferences were evaluated across high, low, and medium categories. A comprehensive analysis is provided on the enhancement of grain quality traits, including brown rice recovery, recovery rate of milled rice, head rice recovery, as well as morphological traits such as grain length, grain width, grain length-width ratio, and grain chalkiness. We also explored the characteristics of amylose, gel consistency, gelatinization temperature, viscosity, as well as the nutritional qualities of rice grains such as starch, protein, lipids, vitamins, minerals, phytochemicals, and bio-fortification potential. The various factors that impact the quality of rice grains, including pre-harvest, post-harvest, and genotype considerations were explored. Additionally, we discussed the future direction and genetic strategies to effectively tackle these challenges. These qualitative characteristics represent the fundamental focus of regional and national breeding strategies employed by different countries to meet consumer preference. Given the significance of rice as a staple food in Asia-Pacific countries, it is primarily consumed domestically, with only a small portion being exported internationally. All the important attributes must be clearly defined within specific parameters. It is crucial for geneticists and breeders to develop a rice variety that can meet the diverse demands of consumers worldwide by incorporating multiple desirable traits. Thus, the goal of addressing global food and nutritional security, and human healthy can be achieved.

Plastid terminal oxidase (PTOX) protects photosystem I and not photosystem II against photoinhibition in Arabidopsis thaliana and Marchantia polymorpha

The plastid terminal oxidase PTOX controls the oxidation level of the plastoquinone pool in the thylakoid membrane and acts as a safety valve upon abiotic stress, but detailed characterization of its role in protecting the photosynthetic apparatus is limited. Here we used PTOX mutants in two model plants Arabidopsis thaliana and Marchantia polymorpha. In Arabidopsis, lack of PTOX leads to a severe defect in pigmentation, a so-called variegated phenotype, when plants are grown at standard light intensities. We created a green Arabidopsis PTOX mutant expressing the bacterial carotenoid desaturase CRTI and a double mutant in Marchantia lacking both PTOX isoforms, the plant-type and the alga-type PTOX. In both species, lack of PTOX affected the redox state of the plastoquinone pool. Exposure of plants to high light intensity showed in the absence of PTOX higher susceptibility of photosystem I to light-induced damage while photosystem II was more stable compared with the wild type demonstrating that PTOX plays both, a pro-oxidant and an anti-oxidant role in vivo. Our results shed new light on the function of PTOX in the protection of photosystem I and II.

Rice carotenoid biofortification and yield improvement conferred by endosperm-specific overexpression of OsGLK1

Carotenoids, indispensable isoprenoid phytonutrients, are synthesized in plastids and are known to be deficient in rice endosperm. Many studies, involving transgenic manipulations of carotenoid biosynthetic genes, have been performed to obtain carotenoid-enriched rice grains. Nuclear-encoded GOLDEN2-LIKE (GLK) transcription factors play important roles in the regulation of plastid and thylakoid grana development. Here, we show that endosperm-specific overexpression of rice GLK1 gene (OsGLK1) leads to enhanced carotenoid production, increased grain yield, but deteriorated grain quality in rice. Subsequently, we performed the bioengineering of carotenoids biosynthesis in rice endosperm by introducing other three carotenogenic genes, tHMG1, ZmPSY1, and PaCrtI, which encode the enzymes truncated 3-hydroxy-3-methylglutaryl-CoA reductase, phytoene synthase, and phytoene desaturase, respectively. Transgenic overexpression of all four genes (OsGLK1, tHMG1, ZmPSY1, and PaCrtI) driven by rice endosperm-specific promoter GluB-1 established a mini carotenoid biosynthetic pathway in the endosperm and exerted a roughly multiplicative effect on the carotenoid accumulation as compared with the overexpression of only three genes (tHMG1, ZmPSY1, and PaCrtI). In addition, the yield enhancement and quality reduction traits were also present in the transgenic rice overexpressing the selected four genes. Our results revealed that OsGLK1 confers favorable characters in rice endosperm and could help to refine strategies for the carotenoid and other plastid-synthesized micronutrient fortification in bioengineered plants.

Does Plant Breeding for Antioxidant-Rich Foods Have an Impact on Human Health?

Given the general beneficial effects of antioxidants-rich foods on human health and disease prevention, there is a continuous interest in plant secondary metabolites conferring attractive colors to fruits and grains and responsible, together with others, for nutraceutical properties. Cereals and Solanaceae are important components of the human diet, thus, they are the main targets for functional food development by exploitation of genetic resources and metabolic engineering. In this review, we focus on the impact of antioxidants-rich cereal and Solanaceae derived foods on human health by analyzing natural biodiversity and biotechnological strategies aiming at increasing the antioxidant level of grains and fruits, the impact of agronomic practices and food processing on antioxidant properties combined with a focus on the current state of pre-clinical and clinical studies. Despite the strong evidence in in vitro and animal studies supporting the beneficial effects of antioxidants-rich diets in preventing diseases, clinical studies are still not sufficient to prove the impact of antioxidant rich cereal and Solanaceae derived foods on human

A novel panel of yeast assays for the assessment of thiamin and its biosynthetic intermediates in plant tissues

Thiamin (or thiamine), known as vitamin B1, represents an indispensable component of human diets, being pivotal in energy metabolism. Thiamin research depends on adequate vitamin quantification in plant tissues. A recently developed quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) method is able to assess the level of thiamin, its phosphorylated entities and its biosynthetic intermediates in the model plant Arabidopsis thaliana, as well as in rice. However, their implementation requires expensive equipment and substantial technical expertise. Microbiological assays can be useful in deter-mining metabolite levels in plant material and provide an affordable alternative to MS-based analysis. Here, we evaluate, by comparison to the LC-MS/MS reference method, the potential of a carefully chosen panel of yeast assays to estimate levels of total vitamin B1, as well as its biosynthetic intermediates pyrimidine and thiazole in Arabidopsis samples. The examined panel of Saccharomyces cerevisiae mutants was, when implemented in microbiological assays, capable of correctly assigning a series of wild-type and thiamin biofortified Arabidopsis plant samples. The assays provide a readily applicable method allowing rapid screening of vitamin B1 (and its biosynthetic intermediates) content in plant material, which is particularly useful in metabolic engineering approaches and in germplasm screening across or within species.

A multifocal approach towards understanding the complexities of carotenoid biosynthesis and accumulation in rice grains

Carotenoids are mostly C40 terpenoids that participate in several important functions in plants including photosynthesis, responses to various forms of stress, signal transduction and photoprotection. While the antioxidant potential of carotenoids is of particular importance for human health, equally important is the role of β-carotene as the precursor for vitamin A in the human diet. Rice, which contributes upto 40% of dietary energy for mankind, contains very low level of β-carotene, thereby making it an important crop for enhancing β-carotene accumulation in its grains and consequently targeting vitamin A deficiency. Biosynthesis of carotenoids in the endosperm of white rice is blocked at the first enzymatic step wherein geranylgeranyl diphosphate is converted to phytoene by the action of phytoene synthase (PSY). Strategies aimed at enhancing β-carotene levels in the endosperm of white rice identified Narcissus pseudonarcissus (npPSY) and bacterial CRT1 as the regulators of the carotenoid biosynthetic pathway in rice. Besides transcriptional regulation of PSY, posttranscriptional regulation of PSY expression by OR gene, molecular synergism between ε-LCY and β-LCY and epigenetic control of CRITSO through SET DOMAIN containing protein appear to be the other regulatory nodes which regulate carotenoid biosynthesis and accumulation in rice grains. In this review, we elucidate a comprehensive and deeper understanding of the regulatory mechanisms of carotenoid metabolism in crops that will enable us to identify an effective tool to alleviate carotenoid content in rice grains.

Genetic Manipulation for Improved Nutritional Quality in Rice

Food with higher nutritional value is always desired for human health. Rice is the prime staple food in more than thirty developing countries, providing at least 20% of dietary protein, 3% of dietary fat and other essential nutrients. Several factors influence the nutrient content of rice which includes agricultural practices, post-harvest processing, cultivar type as well as manipulations followed by selection through breeding and genetic means. In addition to mutation breeding, genetic engineering approach also contributed significantly for the generation of nutrition added varieties of rice in the last decade or so. In the present review, we summarize the research update on improving the nutritional characteristics of rice by using genetic engineering and mutation breeding approach. We also compare the conventional breeding techniques of rice with modern molecular breeding techniques toward the generation of nutritionally improved rice variety as compared to other cereals in areas of micronutrients and availability of essential nutrients such as folate and iron. In addition to biofortification, our focus will be on the efforts to generate low phytate in seeds, increase in essential fatty acids or addition of vitamins (as in golden rice) all leading to the achievements in rice nutrition science. The superiority of biotechnology over conventional breeding being already established, it is essential to ascertain that there are no serious negative agronomic consequences for consumers with any difference in grain size or color or texture, when a nutritionally improved variety of rice is generated through genetic engineering technology.

Carotenoid biofortification in crop plants: citius, altius, fortius

Carotenoids are indispensable for human health, required as precursors of vitamin A and efficient antioxidants. However, these plant pigments that play a vital role in photosynthesis are represented at insufficient levels in edible parts of several crops, which creates a need for increasing their content or optimizing their composition through biofortification. In particular, vitamin A deficiency, a severe health problem affecting the lives of millions in developing countries, has triggered the development of a series of high-provitamin A crops, including Golden Rice as the best-known example. Further carotenoid-biofortified crops have been generated by using genetic engineering approaches or through classical breeding. In this review, we depict carotenoid metabolism in plants and provide an update on the development of carotenoid-biofortified plants and their potential to meet needs and expectations. Furthermore, we discuss the possibility of using natural variation for carotenoid biofortification and the potential of gene editing tools. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Elucidating Carotenoid Biosynthetic Enzyme Localization and Interactions Using Fluorescent Microscopy

Carotenoids are essential for survival of all plants, where these colorful pigments and derivatives are biosynthesized, as well as for humans and other species that obtain plant-derived carotenoids in their diets and rely upon them for vitamin biosynthesis or antioxidant actions. The plant carotenoid biosynthetic pathway consists of nuclear encoded enzymes that are imported into chloroplasts and other plastids. The pathway structural genes are known and have been targeted for metabolic engineering to improve carotenoid profiles or content. However, results are not always as expected because there remain fundamental gaps in understanding how the pathway is physically organized. Many of the enzymes have been found in high molecular weight complexes which are poorly described. Elucidation of enzyme localization as well as enzyme interactions in vivo are needed for advancing the carotenoid field and facilitating our understanding of the three-dimensional organization of this important pathway. Fluorescent protein fusions with carotenoid enzymes can provide in vivo information when these fusions are introduced and transiently expressed in plant cells. Current advances in fluorescent microscopy, especially confocal microscopy, provide the resolution needed to localize fluorescently tagged carotenoid enzymes within suborganellar locations of plastids. Interactions between carotenoid biosynthetic enzymes can be determined using bimolecular fluorescence complementation (BiFC), a method whereby genes of interest are fused with sequences encoding nonfluorescent N- and C-terminal halves of YFP (yellow fluorescent protein), and then introduced into plant protoplasts to allow expression and visualization by fluorescence microscopy. The YFP fluorescence is restored only if the N and C-terminal regions are brought together by interacting fusion partners. Here we describe the methodology, with extensive tips and notes, for determining in vivo carotenoid enzyme localization and enzyme interactions by transient expression of enzyme-fluorescent protein fusions.

Enzyme Fusion Removes Competition for Geranylgeranyl Diphosphate in Carotenogenesis

Geranylgeranyl diphosphate (GGPP), a prenyl diphosphate synthesized by GGPP synthase (GGPS), represents a metabolic hub for the synthesis of key isoprenoids, such as chlorophylls, tocopherols, phylloquinone, gibberellins, and carotenoids. Protein-protein interactions and the amphipathic nature of GGPP suggest metabolite channeling and/or competition for GGPP among enzymes that function in independent branches of the isoprenoid pathway. To investigate substrate conversion efficiency between the plastid-localized GGPS isoform GGPS11 and phytoene synthase (PSY), the first enzyme of the carotenoid pathway, we used recombinant enzymes and determined their in vitro properties. Efficient phytoene biosynthesis via PSY strictly depended on simultaneous GGPP supply via GGPS11. In contrast, PSY could not access freely diffusible GGPP or time-displaced GGPP supply via GGPS11, presumably due to liposomal sequestration. To optimize phytoene biosynthesis, we applied a synthetic biology approach and constructed a chimeric GGPS11-PSY metabolon (PYGG). PYGG converted GGPP to phytoene almost quantitatively in vitro and did not show the GGPP leakage typical of the individual enzymes. PYGG expression in Arabidopsis resulted in orange-colored cotyledons, which are not observed if PSY or GGPS11 are overexpressed individually. This suggests insufficient GGPP substrate availability for chlorophyll biosynthesis achieved through GGPP flux redirection to carotenogenesis. Similarly, carotenoid levels in PYGG-expressing callus exceeded that in PSY- or GGPS11-overexpression lines. The PYGG chimeric protein may assist in provitamin A biofortification of edible plant parts. Moreover, other GGPS fusions may be used to redirect metabolic flux into the synthesis of other isoprenoids of nutritional and industrial interest.

Changing Form and Function through Carotenoids and Synthetic Biology

The diverse structures and multifaceted roles of carotenoids make these colorful pigments attractive targets for synthetic biology.

Comparative transcriptomics approach in elucidation of carotenoid biosynthesis regulation in grains of rice (Oryza sativa L.)

Estimation of phytoene, lycopene, β-carotene, lutein, and zeaxanthin in grains of white, brown and purple cultivars of rice revealed marked differences in the levels of these carotenoid intermediates amongst the cultivars. Grains of white rice did not show any significant accumulation of carotenoid intermediates at any stage of development. On the other hand, grains of the purple cultivar accumulated 49.16 ± 5 µg of β-carotene, 28.89 ± 3.2 µg of lutein and 34.65 ± 4.6 µg of zeaxanthin per gm of grain fresh weight. In addition to PSY1, higher expression of βLCY than εLCY appears to be an important factor in determining the flux of pathway towards synthesis of β-β branch carotenoids in purple rice. This cultivar showed a higher fold change in carotenoid precursors during transition from milky to doughing stages and an enhanced flux of lycopene towards β-carotene during grain maturation. Our results indicate that higher level of carotenoids in purple rice is a consequence of higher expression of genes involved in pyruvate metabolism as well as those involved in carotenoid biosynthesis such as PSY1, PDS and β-LCY. Co-expression networking revealed a strong positive relationship between the expression profiles of genes involved in carotenoid biosynthesis and genes coding for geranylgeranyl transferase type II, glutathione S-transferase, DnaJ and SET domain containing proteins as well as MADS26 and R2R3MYB family of transcription factors.

Characterization of Cauliflower OR Mutant Variants

Cauliflower Orange (Or) mutant is characterized by high level of β-carotene in its curd. Or mutation affects the OR protein that was shown to be involved in the posttranslational control of phytoene synthase (PSY), a major rate-limiting enzyme of carotenoid biosynthesis, and in maintaining PSY proteostasis with the plastid Clp protease system. A transposon integration into the cauliflower wild-type Or gene (BoOR-wt) results in the formation of three differently spliced transcripts. One of them is characterized by insertion (BoOR-Ins), while the other two have exon-skipping deletions (BoOR-Del and BoOR-LD). We investigated the properties of individual BoOR variants and examined their effects on carotenoid accumulation. Using the yeast split-ubiquitin system, we showed that all variants were able to form OR dimers except BoOR-LD. The deletion in BoOR-LD eliminated the first of two adjacent transmembrane domains and was predicted to result in a misplacement of the C-terminal zinc finger domain to the opposite side of membrane, thus preventing OR dimerization. As interaction with PSY is mediated by the N-terminus of BoOR, which remains unaffected after splicing, all BoOR variants including BoOR-LD maintained interactions with PSY. Expression of individual BoOR mutant variants in Arabidopsis revealed that their protein stability varied greatly. While expression of BoOR-Del and BoOR-Ins resulted in increased BoOR protein levels as BoOR-wt, minimal amounts of BoOR-LD protein accumulated. Carotenoid accumulation showed correlated changes in calli of Arabidopsis expressing these variants. Furthermore, we found that OR also functions in E. coli to increase the proportion of native, enzymatically active PSY from plants upon co-expression, but not of bacterial phytoene synthase CrtB. Taken together, these results suggest that OR dimerization is required for OR stability in planta and that the simultaneous presence of PSY interaction-domains in both OR and PSY proteins is required for the holdase function of OR. The more pronounced effect of simultaneous expression of all BoOR variants in cauliflower Or mutant compared with individual overexpression on carotenoid accumulation suggests an enhanced activity with possible formation of various BoOR heterodimers.

Stepwise pathway engineering to the biosynthesis of zeaxanthin, astaxanthin and capsanthin in rice endosperm

Carotenoid pigments are valuable components of the human diet. A notable example is β-carotene, or provitamin A, which is converted into the derivatives astaxanthin and capsanthin, via the common intermediate zeaxanthin. To generate rice varieties producing diverse carotenoids beyond β-carotene, we specifically used a Capsicum β-carotene hydroxylase gene, B (CaBch) and a codon optimized version of the same gene, stB (stBch) to increase zeaxanthin synthesis. We also used a recombinant BAK gene (CaBch-2A-HpBkt), consisting of the CaBch sequence and a Haematococcus β-carotene ketolase gene (HpBkt) linked by a bicistronic 2 A sequence, as well as a codon optimized recombinant stBAK gene (stBch-2A-stBkt) to create astaxanthin synthesis. The four cassettes to seed-specifically express the B, stB, BAK and stBAK genes were individually combined with a PAC gene (CaPsy-2A-PaCrtI) cassette to previously impart β-carotene-enriched trait in rice endosperm. The single T-DNA vectors of B-PAC, stB-PAC, BAK-PAC and stBAK-PAC resulted in the accumulation of zeaxanthin and astaxanthin in the endosperm of the transgenic rice seeds. In addition, an extended version on the carotenoid pathway was introduced into rice to allow the production of capsanthin, by intercrossing a B-PAC rice line with a Ccs rice line, which harbors a Capsicum capsanthin-capsorubin synthase gene. Ultimately, we developed three functional rice varieties: B-PAC (0.8 μg/g zeaxanthin, deep yellow), stBAK-PAC (1.4 μg/g ketocarotenoids, including astaxanthin, pinkish red) and B-PAC x Ccs (0.4 μg/g of ketoxanthophylls, including capsanthin, orange-red) with the similar levels of total carotenoids to PAC rice, suggesting the capacity was dependent on β-carotene levels. Collectively, a combination of genetic engineering and conventional breeding is effective for multi-step metabolic engineering and biochemical pathway extension.

Insights into carotenoid accumulation using VIGS to block different steps of carotenoid biosynthesis in petals of California poppy

Viral-induced gene silencing of selected biosynthetic genes decreased overall carotenoid accumulation in California poppy. Regulation of carotenogenesis was linked with pigment sequestration, not changes in biosynthetic gene expression. Genes of carotenogenesis are well described, but understanding how they affect carotenoid accumulation has proven difficult because of plant lethality when the pigments are lacking. Here, we used a Tobacco Rattle Virus-based virus-induced-gene-silencing (VIGS) approach in California poppy (Eschscholzia californica) to investigate how silencing of the carotenoid biosynthetic pathway genes affects carotenoid metabolite accumulation and RNA transcript abundance of the carotenoid biosynthetic pathway genes. VIGS of upstream (PDS and ZDS) and downstream (βOH and ZEP) genes reduced transcript abundance of the targeted genes in the poppy petals while having no effect on abundance of the other carotenogenesis genes. Silencing of PDS, ZDS, βOH and ZEP genes reduced total pigment concentration by 75-90% and altered petal colour. HPLC and LC-MS measurements suggested that petal colour changes were caused by substantially altered pigment profiles and quantity. Carotenoid metabolites were different to those normally detected in wild-type petals accumulated but overall carotenoid concentration was less, suggesting the chemical form of carotenoid was important for whether it could be stored at high amounts. In poppy petals, eschscholtzxanthin and retro-carotene-triol were the predominant carotenoids, present mainly as esters. Specific esterification enzymes for specific carotenoids and/or fatty acids appear key for enabling petal carotenoids to accumulate to high amounts. Our findings argue against a direct role for carotenoid metabolites regulating carotenogenesis genes in the petals of California poppy as transcript abundance of carotenogenesis genes studied was unchanged, while the petal carotenoid metabolite profile changed substantially.

Establishment of an Arabidopsis callus system to study the interrelations of biosynthesis, degradation and accumulation of carotenoids

The net amounts of carotenoids accumulating in plant tissues are determined by the rates of biosynthesis and degradation. While biosynthesis is rate-limited by the activity of PHYTOENE SYNTHASE (PSY), carotenoid losses are caused by catabolic enzymatic and non-enzymatic degradation. We established a system based on non-green Arabidopsis callus which allowed investigating major determinants for high steady-state levels of β-carotene. Wild-type callus development was characterized by strong carotenoid degradation which was only marginally caused by the activity of carotenoid cleavage oxygenases. In contrast, carotenoid degradation occurred mostly non-enzymatically and selectively affected carotenoids in a molecule-dependent manner. Using carotenogenic pathway mutants, we found that linear carotenes such as phytoene, phytofluene and pro-lycopene resisted degradation and accumulated while β-carotene was highly susceptible towards degradation. Moderately increased pathway activity through PSY overexpression was compensated by degradation revealing no net increase in β-carotene. However, higher pathway activities outcompeted carotenoid degradation and efficiently increased steady-state β-carotene amounts to up to 500 μg g-1 dry mass. Furthermore, we identified oxidative β-carotene degradation products which correlated with pathway activities, yielding β-apocarotenals of different chain length and various apocarotene-dialdehydes. The latter included methylglyoxal and glyoxal as putative oxidative end products suggesting a potential recovery of carotenoid-derived carbon for primary metabolic pathways. Moreover, we investigated the site of β-carotene sequestration by co-localization experiments which revealed that β-carotene accumulated as intra-plastid crystals which was confirmed by electron microscopy with carotenoid-accumulating roots. The results are discussed in the context of using the non-green calli carotenoid assay system for approaches targeting high steady-state β-carotene levels prior to their application in crops.

Specific Upregulation of a Cotton Phytoene Synthase Gene Produces Golden Cottonseeds with Enhanced Provitamin A

Provitamin A (PVA) bio-fortification of crops offers a sustainable strategy to prevent the prevalence of vitamin A deficiency (VAD), one of the world’s major public health problems. The present work aimed to enhance PVA accumulation in cottonseed, the main by-product in the production of cotton fibers and the third largest source of edible plant oil in the world. On the basis of comprehensive identification of carotenoid synthase genes and their expression levels in various cotton tissues, we selected phytoene synthase as the target for manipulating carotenoid biosynthesis in the developing cottonseeds. After functional verification in transgenic tobacco, a cotton phytoene synthase gene (GhPSY2D) driven by a seed-specific promoter was transformed into cotton. The transgenic cottonseeds showed golden appearance and contained over 6-fold higher carotenoid contents in the extracted oil than the non-transgenic control. Thin layer chromatograph analysis indicated that the main PVA carotenoid β-carotene was predominant in the transgenic cottonseeds, but undetectable in the wild-type control. By simultaneously providing economically valuable fibers and edible oils, the transgenic cottons bio-fortified with β-carotene in seeds may be a new powerful tool against VAD in low-income regions.

Carotenoid Metabolism in Plants: The Role of Plastids

Carotenoids are indispensable to plants and critical in human diets. Plastids are the organelles for carotenoid biosynthesis and storage in plant cells. They exist in various types, which include proplastids, etioplasts, chloroplasts, amyloplasts, and chromoplasts. These plastids have dramatic differences in their capacity to synthesize and sequester carotenoids. Clearly, plastids play a central role in governing carotenogenic activity, carotenoid stability, and pigment diversity. Understanding of carotenoid metabolism and accumulation in various plastids expands our view on the multifaceted regulation of carotenogenesis and facilitates our efforts toward developing nutrient-enriched food crops. In this review, we provide a comprehensive overview of the impact of various types of plastids on carotenoid biosynthesis and accumulation, and discuss recent advances in our understanding of the regulatory control of carotenogenesis and metabolic engineering of carotenoids in light of plastid types in plants.

Knocking out of carotenoid catabolic genes in rice fails to boost carotenoid accumulation, but reveals a mutation in strigolactone biosynthesis

Targeted mutations in five carotenoid catabolism genes failed to boost carotenoid accumulation in rice seeds, but produced dwarf and high tillering mutants when OsCCD7 gene was knocked out. Carotenoids play an important role in human diet as a source of vitamin A. Rice is a major staple food in Asia, but does not accumulate carotenoids in the endosperm because of the low carotenoid biosynthesis or the degradation in metabolism. In this study, the CRISPR/Cas9 system was investigated in the targeted knockout of five rice carotenoid catabolic genes (OsCYP97A4, OsDSM2, OsCCD4a, OsCCD4b and OsCCD7) and in an effort to increase β-carotene accumulation in rice endosperm. Transgenic plants that expressed OsNLSCas9 and sgRNAs were generated by Agrobacterium-mediated transformation. Various knockout mutations were identified at the T0 generation of the transgenic rice by TILLING and direct sequencing of the PCR products amplified from the target sites. Carotenoids were not accumulated in both mono-allelic and bi-allelic knockout mutations of the five genes. However, transgenic plants with homozygous or bi-allelic mutations to the OsCCD7 gene were extremely dwarfish with more tillers and lower seed setting than other transgenic or nontransgenic plants. This phenotype was similar to the previously reported ccd7 mutants, which are defective in the biosynthesis of strigolactone, a plant hormone that regulates branching in plants and tiller formation in rice.

Synthetic biology for production of natural and new-to-nature terpenoids in photosynthetic organisms

With tens of thousands of characterized members, terpenoids constitute the largest class of natural compounds that are synthesized by all living organisms. Several terpenoids play primary roles in the maintenance of cell membrane fluidity, as pigments or as phytohormones, but most of them function as specialized metabolites that are involved in plant resistance to herbivores or plant-environment interactions. Terpenoids are an essential component of human nutrition, and many are economically important pharmaceuticals, aromatics and potential next-generation biofuels. Because of the often low abundance in their natural source, as well as the demand for novel terpenoid structures with new or improved bioactivities, terpenoid biosynthesis has become a prime target for metabolic engineering and synthetic biology projects. In this review we focus on the creation of new-to-nature or tailor-made plant-derived terpenoids in photosynthetic organisms, in particular by means of combinatorial biosynthesis and the activation of silent metabolism. We reflect on the characteristics of different potential photosynthetic host organisms and recent advances in synthetic biology and discuss their utility for the (heterologous) production of (novel) terpenoids.

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.

Bottlenecks in carotenoid biosynthesis and accumulation in rice endosperm are influenced by the precursor-product balance

The profile of secondary metabolites in plants reflects the balance of biosynthesis, degradation and storage, including the availability of precursors and products that affect the metabolic equilibrium. We investigated the impact of the precursor-product balance on the carotenoid pathway in the endosperm of intact rice plants because this tissue does not normally accumulate carotenoids, allowing us to control each component of the pathway. We generated transgenic plants expressing the maize phytoene synthase gene (ZmPSY1) and the bacterial phytoene desaturase gene (PaCRTI), which are sufficient to produce β-carotene in the presence of endogenous lycopene β-cyclase. We combined this mini-pathway with the Arabidopsis thaliana genes AtDXS (encoding 1-deoxy-D-xylulose 5-phosphate synthase, which supplies metabolic precursors) or AtOR (the ORANGE gene, which promotes the formation of a metabolic sink). Analysis of the resulting transgenic plants suggested that the supply of isoprenoid precursors from the MEP pathway is one of the key factors limiting carotenoid accumulation in the endosperm and that the overexpression of AtOR increased the accumulation of carotenoids in part by up-regulating a series of endogenous carotenogenic genes. The identification of metabolic bottlenecks in the pathway will help to refine strategies for the creation of engineered plants with specific carotenoid profiles.

Manipulation of Carotenoid Content in Plants to Improve Human Health

Carotenoids are essential components for human nutrition and health, mainly due to their antioxidant and pro-vitamin A activity. Foods with enhanced carotenoid content and composition are essential to ensure carotenoid feasibility in malnourished population of many countries around the world, which is critical to alleviate vitamin A deficiency and other health-related disorders. The pathway of carotenoid biosynthesis is currently well understood, key steps of the pathways in different plant species have been characterized and the corresponding genes identified, as well as other regulatory elements. This enables the manipulation and improvement of carotenoid content and composition in order to control the nutritional value of a number of agronomical important staple crops. Biotechnological and genetic engineering-based strategies to manipulate carotenoid metabolism have been successfully implemented in many crops, with Golden rice as the most relevant example of β-carotene improvement in one of the more widely consumed foods. Conventional breeding strategies have been also adopted in the bio-fortification of carotenoid in staple foods that are highly consumed in developing countries, including maize, cassava and sweet potatoes, to alleviate nutrition-related problems. The objective of the chapter is to summarize major breakthroughs and advances in the enhancement of carotenoid content and composition in agronomical and nutritional important crops, with special emphasis to their potential impact and benefits in human nutrition and health.

Metabolic Regulation of Carotenoid-Enriched Golden Rice Line

Vitamin A deficiency (VAD) is the leading cause of blindness among children and is associated with high risk of maternal mortality. In order to enhance the bioavailability of vitamin A, high carotenoid transgenic golden rice has been developed by manipulating enzymes, such as phytoene synthase (psy) and phytoene desaturase (crtI). In this study, proteome and metabolite analyses were carried out to comprehend metabolic regulation and adaptation of transgenic golden rice after the manipulation of endosperm specific carotenoid pathways. The main alteration was observed in carbohydrate metabolism pathways of the transgenic seeds. The 2D based proteomic studies demonstrated that carbohydrate metabolism-related enzymes, such as pullulanase, UDP-glucose pyrophosphorylase, and glucose-1-phosphate adenylyltransferase, were primarily up-regulated in transgenic rice seeds. In addition, the enzyme PPDK was also elevated in transgenic seeds thus enhancing pyruvate biosynthesis, which is the precursor in the carotenoids biosynthetic pathway. GC-MS based metabolite profiling demonstrated an increase in the levels of glyceric acid, fructo-furanose, and galactose, while decrease in galactonic acid and gentiobiose in the transgenic rice compared to WT. It is noteworthy to mention that the carotenoid content, especially β-carotene level in transgenic rice (4.3 μg/g) was significantly enhanced. The present study highlights the metabolic adaptation process of a transgenic golden rice line (homozygous T4 progeny of SKBR-244) after enhancing carotenoid biosynthesis. The presented information would be helpful in the development of crops enriched in carotenoids by expressing metabolic flux of pyruvate biosynthesis.

Insights from the cold transcriptome of Physcomitrella patens: global specialization pattern of conserved transcriptional regulators and identification of orphan genes involved in cold acclimation

The whole-genome transcriptomic cold stress response of the moss Physcomitrella patens was analyzed and correlated with phenotypic and metabolic changes. Based on time-series microarray experiments and quantitative real-time polymerase chain reaction, we characterized the transcriptomic changes related to early stress signaling and the initiation of cold acclimation. Transcription-associated protein (TAP)-encoding genes of P. patens and Arabidopsis thaliana were classified using generalized linear models. Physiological responses were monitored with pulse-amplitude-modulated fluorometry, high-performance liquid chromatography and targeted high-performance mass spectrometry. The transcript levels of 3220 genes were significantly affected by cold. Comparative classification revealed a global specialization of TAP families, a transcript accumulation of transcriptional regulators of the stimulus/stress response and a transcript decline of developmental regulators. Although transcripts of the intermediate to later response are from evolutionarily conserved genes, the early response is dominated by species-specific genes. These orphan genes may encode as yet unknown acclimation processes.

Enrichment of provitamin A content in wheat (Triticum aestivum L.) by introduction of the bacterial carotenoid biosynthetic genes CrtB and CrtI

Carotenoid content is a primary determinant of wheat nutritional value and affects its end-use quality. Wheat grains contain very low carotenoid levels and trace amounts of provitamin A content. In order to enrich the carotenoid content in wheat grains, the bacterial phytoene synthase gene (CrtB) and carotene desaturase gene (CrtI) were transformed into the common wheat cultivar Bobwhite. Expression of CrtB or CrtI alone slightly increased the carotenoid content in the grains of transgenic wheat, while co-expression of both genes resulted in a darker red/yellow grain phenotype, accompanied by a total carotenoid content increase of approximately 8-fold achieving 4.76 μg g(-1) of seed dry weight, a β-carotene increase of 65-fold to 3.21 μg g(-1) of seed dry weight, and a provitamin A content (sum of α-carotene, β-carotene, and β-cryptoxanthin) increase of 76-fold to 3.82 μg g(-1) of seed dry weight. The high provitamin A content in the transgenic wheat was stably inherited over four generations. Quantitative PCR analysis revealed that enhancement of provitamin A content in transgenic wheat was also a result of the highly coordinated regulation of endogenous carotenoid biosynthetic genes, suggesting a metabolic feedback regulation in the wheat carotenoid biosynthetic pathway. These transgenic wheat lines are not only valuable for breeding wheat varieties with nutritional benefits for human health but also for understanding the mechanism regulating carotenoid biosynthesis in wheat endosperm.

A novel carotenoid, 4-keto-α-carotene, as an unexpected by-product during genetic engineering of carotenogenesis in rice callus

Rice endosperm is devoid of carotenoids because the initial biosynthetic steps are absent. The early carotenogenesis reactions were constituted through co-transformation of endosperm-derived rice callus with phytoene synthase and phytoene desaturase transgenes. Subsequent steps in the pathway such as cyclization and hydroxylation reactions were catalyzed by endogenous rice enzymes in the endosperm. The carotenoid pathway was extended further by including a bacterial ketolase gene able to form astaxanthin, a high value carotenoid which is not a typical plant carotenoid. In addition to astaxanthin and precursors, a carotenoid accumulated in the transgenic callus which did not fit into the pathway to astaxanthin. This was subsequently identified as 4-keto-α-carotene by HPLC co-chromatography, chemical modification, mass spectrometry and the reconstruction of its biosynthesis pathway in Escherichia coli. We postulate that this keto carotenoid is formed from α-carotene which accumulates by combined reactions of the heterologous gene products and endogenous rice endosperm cyclization reactions.

Engineering complex metabolic pathways in plants

Metabolic engineering can be used to modulate endogenous metabolic pathways in plants or introduce new metabolic capabilities in order to increase the production of a desirable compound or reduce the accumulation of an undesirable one. In practice, there are several major challenges that need to be overcome, such as gaining enough knowledge about the endogenous pathways to understand the best intervention points, identifying and sourcing the most suitable metabolic genes, expressing those genes in such a way as to produce a functional enzyme in a heterologous background, and, finally, achieving the accumulation of target compounds without harming the host plant. This article discusses the strategies that have been developed to engineer complex metabolic pathways in plants, focusing on recent technological developments that allow the most significant bottlenecks to be overcome.

Carotenoids of biotechnological importance

Carotenoids are natural pigments with antioxidative functions that protect against oxidative stress. They are essential for humans and must be supplied through the diet. Carotenoids are the precursors for the visual pigment rhodopsin, and lutein and zeaxanthin must be accumulated in the yellow eye spot to protect the retina from excess light and ultraviolet damage. There is a global market for carotenoids as food colorants, animal feed, and nutraceuticals. Some carotenoids are chemically synthesized, whereas others are from natural sources. Microbial mass production systems of industrial interest for carotenoids are in use, and new ones are being developed by metabolic pathway engineering of bacteria, fungi, and plants. Several examples will be highlighted in this chapter.

Effect of constitutive expression of bacterial phytoene desaturase CRTI on photosynthetic electron transport in Arabidopsis thaliana

The constitutive expression of the bacterial carotene desaturase (CRTI) in Arabidopsis thaliana leads to increased susceptibility of leaves to light-induced damage. Changes in the photosynthetic electron transport chain rather than alterations of the carotenoid composition in the antenna were responsible for the increased photoinhibition. A much higher level of superoxide/hydrogen peroxide was generated in the light in thylakoid membranes from the CRTI expressing lines than in wild-type while the level of singlet oxygen generation remained unchanged. The increase in reactive oxygen species was related to the activity of plastid terminal oxidase (PTOX) since their generation was inhibited by the PTOX-inhibitor octyl gallate, and since the protein level of PTOX was increased in the CRTI-expressing lines. Furthermore, cyclic electron flow was suppressed in these lines. We propose that PTOX competes efficiently with cyclic electron flow for plastoquinol in the CRTI-expressing lines and that it plays a crucial role in the control of the reduction state of the plastoquinone pool.

Variation in contents of main active components and antioxidant activity in leaves of different pigeon pea cultivars during growth

Pigeon pea is an important and multiuse grain legume crop, and its leaves are a very valuable natural resource. To obtain a high-quality biological resource, it is necessary to choose the excellent cultivar and determine the appropriate harvest time. In this study, the variation in contents of main active components and antioxidant activity in leaves of six pigeon pea cultivars during growth were investigated. The level of each individual active component significantly varied during growth, but with a different pattern, and this variation was different among cultivars. Flavonoid glycosides orientin, vitexin, and apigenin-6,8-di-C-α-L-arabinopyranoside showed two peak values at mid-late and final stages of growth in most cases. Pinostrobin chalcone, longistyline C, and cajaninstilbene acid showed remarkablely higher values at the mid-late stage of growth than at other stages. Pinostrobin had an extremely different variation pattern compared to other active components. Its content was the highest at the earlier stage of growth. Principal component analysis (PCA) revealed that vitexin and apigenin-6,8-di-C-α-L-arabinopyranoside were mainly responsible for distinguishing cultivars analyzed. In a comprehensive consideration, the leaves should preferentially be harvested at the 135th day after sowing when the level of active components and antioxidant activity reached higher values. Cultivars ICP 13092, ICPL 87091, and ICPL 96053 were considered to be excellent cultivars with high antioxidant activity. Our findings can provide valuable information for producing a high-quality pigeon pea resource.

The carotenoid biosynthetic pathway: thinking in all dimensions

The carotenoid biosynthetic pathway serves manifold roles in plants related to photosynthesis, photoprotection, development, stress hormones, and various volatiles and signaling apocarotenoids. The pathway also produces compounds that impact human nutrition and metabolic products that contribute to fragrance and flavor of food and non-food crops. It is no surprise that the pathway has been a target of metabolic engineering, most prominently in the case of Golden Rice. The future success and predictability of metabolic engineering of carotenoids rests in the ability to target carotenoids for specific physiological purposes as well as to simultaneously modify carotenoids along with other desired traits. Here, we ask whether predictive metabolic engineering of the carotenoid pathway is indeed possible. Despite a long history of research on the pathway, at this point in time we can only describe the pathway as a parts list and have almost no knowledge of the location of the complete pathway, how it is assembled, and whether there exists any trafficking of the enzymes or the carotenoids themselves. We discuss the current state of knowledge regarding the "complete" pathway and make the argument that predictive metabolic engineering of the carotenoid pathway (and other pathways) will require investigation of the three dimensional state of the pathway as it may exist in plastids of different ultrastructures. Along with this message we point out the need to develop new types of visualization tools and resources that better reflect the dynamic nature of biosynthetic pathways.

Biofortification of plants with altered antioxidant content and composition: genetic engineering strategies

Antioxidants are protective molecules that neutralize reactive oxygen species and prevent oxidative damage to cellular components such as membranes, proteins and nucleic acids, therefore reducing the rate of cell death and hence the effects of ageing and ageing-related diseases. The fortification of food with antioxidants represents an overlap between two diverse environments, namely fortification of staple foods with essential nutrients that happen to have antioxidant properties (e.g. vitamins C and E) and the fortification of luxury foods with health-promoting but non-essential antioxidants such as flavonoids as part of the nutraceuticals/functional foods industry. Although processed foods can be artificially fortified with vitamins, minerals and nutraceuticals, a more sustainable approach is to introduce the traits for such health-promoting compounds at source, an approach known as biofortification. Regardless of the target compound, the same challenges arise when considering the biofortification of plants with antioxidants, that is the need to modulate endogenous metabolic pathways to increase the production of specific antioxidants without affecting plant growth and development and without collateral effects on other metabolic pathways. These challenges become even more intricate as we move from the engineering of individual pathways to several pathways simultaneously. In this review, we consider the state of the art in antioxidant biofortification and discuss the challenges that remain to be overcome in the development of nutritionally complete and health-promoting functional foods.

Epistasis in tomato color mutations involves regulation of phytoene synthase 1 expression by cis-carotenoids

Tomato (Solanum lycopersicum) fruit accumulate the red carotenoid pigment lycopene. The recessive mutation yellow-flesh (locus r) in tomato eliminates fruit carotenoids by disrupting the activity of the fruit-specific phytoene synthase (PSY1), the first committed step in the carotenoid biosynthesis pathway. Fruits of the recessive mutation tangerine (t) appear orange due to accumulation of 7,9,7',9'-tetra-cis-lycopene (prolycopene) as a result of a mutation in the carotenoid cis-trans isomerase. It was established 60 y ago that tangerine is epistatic to yellow-flesh. This uncharacteristic epistasis interaction defies a paradigm in biochemical genetics arguing that mutations that disrupt enzymes acting early in a biosynthetic pathway are epistatic to other mutations that block downstream steps in the same pathway. To explain this conundrum, we have investigated the interaction between tangerine and yellow-flesh at the molecular level. Results presented here indicate that allele r(2997) of yellow-flesh eliminates transcription of PSY1 in fruits. In a genetic background of tangerine, transcription of PSY1 is partially restored to a level sufficient for producing phytoene and downstream carotenoids. Our results revealed the molecular mechanism underlying the epistasis of t over r and suggest the involvement of cis-carotenoid metabolites in a feedback regulation of PSY1 gene expression.

A GH3 family member, OsGH3-2, modulates auxin and abscisic acid levels and differentially affects drought and cold tolerance in rice

Plant responses to abiotic stresses are coordinated by arrays of growth and developmental processes. Indole-3-acetic acid (IAA) and abscisic acid (ABA) play critical roles in developmental programmes and environmental responses, respectively, through complex signalling and metabolism networks. However, crosstalk between the two phytohormones in the stress responses remains largely unknown. Here, it is reported that a GH3 family gene, OsGH3-2, encoding an enzyme catalysing IAA conjugation to amino acids, is involved in the modulation of ABA level and stress tolerance. Expression of OsGH3-2 was induced by drought but was suppressed by cold. Overexpression of OsGH3-2 in rice caused significant morphological aberrations related to IAA deficiency, such as dwarfism, smaller leaves, and fewer crown roots and root hairs. The overexpressing line showed significantly reduced carotene, ABA, and free IAA levels, greater stomata aperture, and faster water loss, and was hypersensitive to drought stress. However, the overexpressing line showed increased cold tolerance, which was due to the combined effects of reduced free IAA content, alleviated oxidative damage, and decreased membrane penetrability. Furthermore, expression levels of some ABA synthesis- and stress-related genes were significantly changed in the overexpression line. It was conclude that OsGH3-2 modulates both endogenous free IAA and ABA homeostasis and differentially affects drought and cold tolerance in rice.

Comprehending crystalline β-carotene accumulation by comparing engineered cell models and the natural carotenoid-rich system of citrus

Genetic manipulation of carotenoid biosynthesis has become a recent focus for the alleviation of vitamin A deficiency. However, the genetically modified phenotypes often challenge the expectation, suggesting the incomplete comprehension of carotenogenesis. Here, embryogenic calli were engineered from four citrus genotypes as engineered cell models (ECMs) by over-expressing a bacterial phytoene synthase gene (CrtB). Ripe flavedos (the coloured outer layer of citrus fruits), which exhibit diverse natural carotenoid patterns, were offered as a comparative system to the ECMs. In the ECMs, carotenoid patterns showed diversity depending on the genotypes and produced additional carotenoids, such as lycopene, that were absent from the wild-type lines. Especially in the ECMs from dark-grown culture, there emerged a favoured β,β-pathway characterized by a striking accumulation of β-carotene, which was dramatically different from those in the wild-type calli and ripe flavedos. Unlike flavedos that contained a typical chromoplast development, the ECMs sequestered most carotenoids in the amyloplasts in crystal form, which led the amyloplast morphology to show a chromoplast-like profile. Transcriptional analysis revealed a markedly flavedo-specific expression of the β-carotene hydroxylase gene (HYD), which was suppressed in the calli. Co-expression of CrtB and HYD in the ECMs confirmed that HYD predominantly mediated the preferred carotenoid patterns between the ECMs and flavedos, and also revealed that the carotenoid crystals in the ECMs were mainly composed of β-carotene. In addition, a model is proposed to interpret the common appearance of a favoured β,β-pathway and the likelihood of carotenoid degradation potentially mediated by photo-oxidation and vacuolar phagocytosis in the ECMs is discussed.

Vitamin deficiencies in humans: can plant science help?

The term vitamin describes a small group of organic compounds that are absolutely required in the human diet. Although for the most part, dependency criteria are met in developed countries through balanced diets, this is not the case for the five billion people in developing countries who depend predominantly on a single staple crop for survival. Thus, providing a more balanced vitamin intake from high-quality food remains one of the grandest challenges for global human nutrition in the coming decade(s). Here, we describe the known importance of vitamins in human health and current knowledge on their metabolism in plants. Deficits in developing countries are a combined consequence of a paucity of specific vitamins in major food staple crops, losses during crop processing, and/or overreliance on a single species as a primary food source. We discuss the role that plant science can play in addressing this problem and review successful engineering of vitamin pathways. We conclude that while considerable advances have been made in understanding vitamin metabolic pathways in plants, more cross-disciplinary approaches must be adopted to provide adequate levels of all vitamins in the major staple crops to eradicate vitamin deficiencies from the global population.

Carotenoid accumulation in orange-pigmented Capsicum annuum fruit, regulated at multiple levels

The pericarp of Capsicum fruit is a rich dietary source of carotenoids. Accumulation of these compounds may be controlled, in part, by gene transcription of biosynthetic enzymes. The carotenoid composition in a number of orange-coloured C. annuum cultivars was determined using HPLC and compared with transcript abundances for four carotenogenic enzymes, Psy, LcyB, CrtZ-2, and Ccs determined by qRT-PCR. There were unique carotenoid profiles as well as distinct patterns of transcription of carotenogenic enzymes within the seven orange-coloured cultivars. In one cultivar, 'Fogo', carrying the mutant ccs-3 allele, transcripts were detected for this gene, but no CCS protein accumulated. The premature stop termination in ccs-3 prevented expression of the biosynthetic activity to synthesize the capsanthin and capsorubin forms of carotenoids. In two other orange-coloured cultivars, 'Orange Grande' and 'Oriole', both with wild-type versions of all four carotenogenic enzymes, no transcripts for Ccs were detected and no red pigments accumulated. Finally, in a third case, the orange-coloured cultivar, Canary, transcripts for all four of the wild-type carotenogenic enzymes were readily detected yet no CCS protein appeared to accumulate and no red carotenoids were synthesized. In the past, mutations in Psy and Ccs have been identified as the loci controlling colour in the fruit. Now there is evidence that a non-structural gene may control colour development in Capsicum.

Carotenoids and their cleavage products: biosynthesis and functions

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

Characterization of the beta-carotene hydroxylase gene DSM2 conferring drought and oxidative stress resistance by increasing xanthophylls and abscisic acid synthesis in rice

Drought is a major limiting factor for crop production. To identify critical genes for drought resistance in rice (Oryza sativa), we screened T-DNA mutants and identified a drought-hypersensitive mutant, dsm2. The mutant phenotype was caused by a T-DNA insertion in a gene encoding a putative β-carotene hydroxylase (BCH). BCH is predicted for the biosynthesis of zeaxanthin, a carotenoid precursor of abscisic acid (ABA). The amounts of zeaxanthin and ABA were significantly reduced in two allelic dsm2 mutants after drought stress compared with the wild type. Under drought stress conditions, the mutant leaves lost water faster than the wild type and the photosynthesis rate, biomass, and grain yield were significantly reduced, whereas malondialdehyde level and stomata aperture were increased in the mutant. The mutant is also hypersensitive to oxidative stresses. The mutant had significantly lower maximal efficiency of photosystem II photochemistry and nonphotochemical quenching capacity than the wild type, indicating photoinhibition in photosystem II and decreased capacity for eliminating excess energy by thermal dissipation. Overexpression of DSM2 in rice resulted in significantly increased resistance to drought and oxidative stresses and increases of the xanthophylls and nonphotochemical quenching. Some stress-related ABA-responsive genes were up-regulated in the overexpression line. DSM2 is a chloroplast protein, and the response of DSM2 to environmental stimuli is distinctive from the other two BCH members in rice. We conclude that the DSM2 gene significantly contributes to control of the xanthophyll cycle and ABA synthesis, both of which play critical roles in the establishment of drought resistance in rice.

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

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