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

ORANGE family proteins: multifunctional chaperones shaping plant carotenoid level, plastid development, stress tolerance, and more

ORANGE (OR) family proteins are DnaJE1 molecular chaperones ubiquitous and highly conserved in all plant species, indicating their important roles in plant growth and development. OR proteins have been found to exert multiple functions in regulating carotenoid and chlorophyll biosynthesis, plastid development, and stress tolerance, with additional functions expected to be discovered. As molecular chaperones, OR proteins directly influence the stability of their target proteins via their holdase activity and may perform other molecular roles through unknown mechanisms. Exploration of OR has uncovered novel mechanisms underlying core plant metabolism pathways and expanded our understanding of processes linked to plastid development. Continued investigation of OR family proteins will not only reveal new functions of molecular chaperones but also provide pioneering tools for crop improvement. Thus, OR family proteins offer a distinctive opportunity to comprehend molecular chaperones in modulating various metabolic and developmental processes and exemplify the importance of chaperones in crop development and adaptability. This review briefly details the history of OR family proteins, highlights recent advancements in understanding their myriad of functions, and discusses the prospects of this fascinating group of chaperones towards generating innovative, more nutritious, and resilient crops alongside other agronomically important traits.

Molecular protocol to develop β-carotene-biofortified rice events via molecular optimization

Providing food with nutrition and functionality is crucial for sustaining human life. Rice (Oryza sativa L.) is a representative staple crop with high carbohydrate content but low amounts of essential amino acids, micronutrients, and carotenoids such as provitamin A. To improve the nutritional quality, rice endosperm was biofortified to accumulate carotenoids such as β-carotene through genetic engineering (i.e., using synthetic carotenoid biosynthetic genes, a nonmammalian viral polycistronic sequence, and an optimized promoter and transit peptide) and high-throughput rice transformation (approximately 300 transgenic plants per construct). To facilitate the safety assessment of genetically modified food, molecular characterization was performed to select elite lines equipped with a single intergenic insertion of T-DNA, high transgene expression, in this case leading to high carotenoid content, and with phenotypic and compositional substantial equivalence. In this study, we present β-carotene-biofortified rice event candidate lines eligible for commercial use and a disclosed molecular protocol for the development of biotech rice crops.

Strategies and bibliometric analysis of legumes biofortification to address malnutrition

MAIN CONCLUSION

Biofortification of legumes using diverse techniques such as plant breeding, agronomic practices, genetic modification, and nano-technological approaches presents a sustainable strategy to address micronutrient deficiencies of underprivileged populations. The widespread issue of chronic malnutrition, commonly referred to as "hidden hunger," arises from the consumption of poor-quality food, leading to various health and cognitive impairments. Biofortified food crops have been a sustainable solution to address micronutrient deficiencies. This review highlights multiple biofortification techniques, such as plant breeding, agronomic practices, genetic modification, and nano-technological approaches, aimed at enhancing the nutrient content of commonly consumed crops. Emphasizing the biofortification of legumes, this review employs bibliometric analysis to examine research trends from 2000 to 2023. It identifies key authors, influential journals, contributing countries, publication trends, and prevalent keywords in this field. The review highlights the progress in developing biofortified crops and their potential to improve global nutrition and help underprivileged populations.

De novo domestication: what about the weeds?

Most high-yielding crops are susceptible to abiotic and biotic stresses, making them particularly vulnerable to the potential effects of climate change. A possible alternative is to accelerate the domestication of wild plants that are already tolerant to harsh conditions and to increase their yields by methods such as gene editing. We foresee that crops' wild progenitors could potentially compete with the resulting de novo domesticated plants, reducing yields. To improve the recognition of weeds, we propose using gene editing techniques to introduce traits into de novo domesticated crops that will allow for visual recognition of the crops by weeding robots that have been trained by machine learning.

Biofortification of Triticum species: a stepping stone to combat malnutrition

BACKGROUND

Biofortification represents a promising and sustainable strategy for mitigating global nutrient deficiencies. However, its successful implementation poses significant challenges. Among staple crops, wheat emerges as a prime candidate to address these nutritional gaps. Wheat biofortification offers a robust approach to enhance wheat cultivars by elevating the micronutrient levels in grains, addressing one of the most crucial global concerns in the present era.

MAIN TEXT

Biofortification is a promising, but complex avenue, with numerous limitations and challenges to face. Notably, micronutrients such as iron (Fe), zinc (Zn), selenium (Se), and copper (Cu) can significantly impact human health. Improving Fe, Zn, Se, and Cu contents in wheat could be therefore relevant to combat malnutrition. In this review, particular emphasis has been placed on understanding the extent of genetic variability of micronutrients in diverse Triticum species, along with their associated mechanisms of uptake, translocation, accumulation and different classical to advanced approaches for wheat biofortification.

CONCLUSIONS

By delving into micronutrient variability in Triticum species and their associated mechanisms, this review underscores the potential for targeted wheat biofortification. By integrating various approaches, from conventional breeding to modern biotechnological interventions, the path is paved towards enhancing the nutritional value of this vital crop, promising a brighter and healthier future for global food security and human well-being.

Biofortification as a solution for addressing nutrient deficiencies and malnutrition

Malnutrition, defined as both undernutrition and overnutrition, is a major global health concern affecting millions of people. One possible way to address nutrient deficiency and combat malnutrition is through biofortification. A comprehensive review of the literature was conducted to explore the current state of biofortification research, including techniques, applications, effectiveness and challenges. Biofortification is a promising strategy for enhancing the nutritional condition of at-risk populations. Biofortified varieties of basic crops, including rice, wheat, maize and beans, with elevated amounts of vital micronutrients, such as iron, zinc, vitamin A and vitamin C, have been successfully developed using conventional and advanced technologies. Additionally, the ability to specifically modify crop genomes to improve their nutritional profiles has been made possible by recent developments in genetic engineering, such as CRISPR-Cas9 technology. The health conditions of people have been shown to improve and nutrient deficiencies were reduced when biofortified crops were grown. Particularly in environments with limited resources, biofortification showed considerable promise as a long-term and economical solution to nutrient shortages and malnutrition. To fully exploit the potential of biofortified crops to enhance public health and global nutrition, issues such as consumer acceptance, regulatory permitting and production and distribution scaling up need to be resolved. Collaboration among governments, researchers, non-governmental organizations and the private sector is essential to overcome these challenges and promote the widespread adoption of biofortification as a key part of global food security and nutrition strategies.

Antioxidants of Non-Enzymatic Nature: Their Function in Higher Plant Cells and the Ways of Boosting Their Biosynthesis

Plants are exposed to a variety of abiotic and biotic stresses leading to increased formation of reactive oxygen species (ROS) in plant cells. ROS are capable of oxidizing proteins, pigments, lipids, nucleic acids, and other cell molecules, disrupting their functional activity. During the process of evolution, numerous antioxidant systems were formed in plants, including antioxidant enzymes and low molecular weight non-enzymatic antioxidants. Antioxidant systems perform neutralization of ROS and therefore prevent oxidative damage of cell components. In the present review, we focus on the biosynthesis of non-enzymatic antioxidants in higher plants cells such as ascorbic acid (vitamin C), glutathione, flavonoids, isoprenoids, carotenoids, tocopherol (vitamin E), ubiquinone, and plastoquinone. Their functioning and their reactivity with respect to individual ROS will be described. This review is also devoted to the modern genetic engineering methods, which are widely used to change the quantitative and qualitative content of the non-enzymatic antioxidants in cultivated plants. These methods allow various plant lines with given properties to be obtained in a rather short time. The most successful approaches for plant transgenesis and plant genome editing for the enhancement of biosynthesis and the content of these antioxidants are discussed.

A multi-omic resource of wheat seed tissues for nutrient deposition and improvement for human health

As a globally important staple crop, wheat seeds provide us with nutrients and proteins. The trend of healthy dietary has become popular recently, emphasizing the consumption of whole-grain wheat products and the dietary benefits. However, the dynamic changes in nutritional profiles of different wheat seed regions (i.e., the embryo, endosperm and outer layers) during developmental stages and the molecular regulation have not been well studied. Here, we provide this multi-omic resource of wheat seeds and describe the generation, technical assessment and preliminary analyses. This resource includes a time-series RNA-seq dataset of the embryo, endosperm and outer layers of wheat seeds and their corresponding metabolomic dataset, covering the middle and late stages of seed development. Our RNA-seq experiments profile the expression of 63,708 genes, while the metabolomic data includes the abundance of 984 metabolites. We believe that this was the first reported transcriptome and metabolome dataset of wheat seeds that helps understand the molecular regulation of the deposition of beneficial nutrients and hence improvements for nutritional and processing quality traits.

An established protocol for generating transgenic wheat for wheat functional genomics via particle bombardment

Wheat is one of the most important food crops in the world and is considered one of the top targets in crop biotechnology. With the high-quality reference genomes of wheat and its relative species and the recent burst of genomic resources in Triticeae, demands to perform gene functional studies in wheat and genetic improvement have been rapidly increasing, requiring that production of transgenic wheat should become a routine technique. While established for more than 20 years, the particle bombardment-mediated wheat transformation has not become routine yet, with only a handful of labs being proficient in this technique. This could be due to, at least partly, the low transformation efficiency and the technical difficulties. Here, we describe the current version of this method through adaptation and optimization. We report the detailed protocol of producing transgenic wheat by the particle gun, including several critical steps, from the selection of appropriate explants (i.e., immature scutella), the preparation of DNA-coated gold particles, and several established strategies of tissue culture. More importantly, with over 20 years of experience in wheat transformation in our lab, we share the many technical details and recommendations and emphasize that the particle bombardment-mediated approach has fewer limitations in genotype dependency and vector construction when compared with the Agrobacterium-mediated methods. The particle bombardment-mediated method has been successful for over 30 wheat genotypes, from the tetraploid durum wheat to the hexaploid common wheat, from modern elite varieties to landraces. In conclusion, the particle bombardment-mediated wheat transformation has demonstrated its potential and wide applications, and the full set of protocol, experience, and successful reports in many wheat genotypes described here will further its impacts, making it a routine and robust technique in crop research labs worldwide.

Bread wheat with enhanced grain carotenoid content: a novel option for wheat biofortification

Colored wheat has piqued the interest of breeders and consumers alike. The chromosomal segment from 7E of Thinopyrum ponticum, which carries a leaf rust resistant gene, Lr19, has been rarely employed in wheat breeding operations due to its association with the Y gene, which gives a yellow tint to the flour. By prioritizing nutritional content over color preferences, consumer acceptance has undergone a paradigm change. Through marker-assisted backcross breeding, we introduced an alien segment harboring the Y (PsyE1) gene into a high yielding commercial bread wheat (HD 2967) background to generate rust resistant carotenoid biofortified bread wheat. Agro-morphological characterization was also performed on a subset of developed 70 lines having enhanced grain carotene content. In the introgression lines, carotenoid profiling using HPLC analysis demonstrated a considerable increase in β-carotene levels (up to 12 ppm). Thus, the developed germplasm caters the threat to nutritional security and can be utilized to produce carotenoid fortified wheat.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01338-0.

Overexpression of PSY1 increases fruit skin and flesh carotenoid content and reveals associated transcription factors in apple (Malus × domestica)

Knowledge of the transcriptional regulation of the carotenoid metabolic pathway is still emerging and here, we have misexpressed a key biosynthetic gene in apple to highlight potential transcriptional regulators of this pathway. We overexpressed phytoene synthase (PSY1), which controls the key rate-limiting biosynthetic step, in apple and analyzed its effects in transgenic fruit skin and flesh using two approaches. Firstly, the effects of PSY overexpression on carotenoid accumulation and gene expression was assessed in fruit at different development stages. Secondly, the effect of light exclusion on PSY1-induced fruit carotenoid accumulation was examined. PSY1 overexpression increased carotenoid content in transgenic fruit skin and flesh, with beta-carotene being the most prevalent carotenoid compound. Light exclusion by fruit bagging reduced carotenoid content overall, but carotenoid content was still higher in bagged PSY fruit than in bagged controls. In tissues overexpressing PSY1, plastids showed accelerated chloroplast to chromoplast transition as well as high fluorescence intensity, consistent with increased number of chromoplasts and carotenoid accumulation. Surprisingly, the expression of other carotenoid pathway genes was elevated in PSY fruit, suggesting a feed-forward regulation of carotenogenesis when this enzyme step is mis-expressed. Transcriptome profiling of fruit flesh identified differentially expressed transcription factors (TFs) that also were co-expressed with carotenoid pathway genes. A comparison of differentially expressed genes from both the developmental series and light exclusion  treatment revealed six candidate TFs exhibiting strong correlation with carotenoid accumulation. This combination of physiological, transcriptomic and metabolite data sheds new light on plant carotenogenesis and TFs that may play a role in regulating apple carotenoid biosynthesis.

Differential hydroxylation efficiency of the two non-heme carotene hydroxylases: DcBCH1, rather than DcBCH2, plays a major role in carrot taproot

Carotene hydroxylase plays an important role in catalyzing the hydroxylation of carotene to xanthopylls, including two types: non-heme carotene hydroxylase (BCH type) and heme-containing cytochrome P450 hydroxylase (P450 type). Two BCH-encoding genes were annotated in the carrot genome. However, the role of BCHs and whether there are functional interactions between the duplicated BCHs in carrot remains unclear. In this study, two BCH encoding genes, DcBCH1 and DcBCH2, were cloned from carrot. The relative expression level of DcBCH1 was much higher than that of DcBCH2 in carrot taproots with different carotene accumulation levels. Overexpression of DcBCH1 in 'KRD' (high carotene accumulated) carrot changed the taproot color from orange to yellow, accompanied by substantial reductions in α-carotene and β-carotene. There was no obvious change in taproot color between transgenic 'KRD' carrot overexpressing DcBCH2 and control carrot. Simultaneously, the content of α-carotene in the taproot of DcBCH2-overexpressing carrot decreased, but the content of β-carotene did not change significantly in comparison with control carrot. Using the CRISPR/Cas9 system to knock out DcBCH1 in 'KRD' carrot lightened the taproot color from orange to pink-orange; the content of α-carotene in the taproot increased slightly, while the β-carotene content was still significantly decreased, compared with control carrot. In DcBCH1-knockout carrot, the transcript level of DcBCH2 was significantly increased. These results indicated that in carrot taproot, DcBCH1 played the main function of BCH enzyme, which could hydroxylate α-carotene and β-carotene; DcBCH1 and DcBCH2 had functional redundancy, and these two DcBCHs could partially compensate for each other.

Nucleotide variation in the phytoene synthase (ClPsy1) gene contributes to golden flesh in watermelon (Citrullus lanatus L.)

A gene controlling golden flesh trait in watermelon was discovered and fine mapped to a 39.08 Kb region on chromosome 1 through a forward genetic strategy, and Cla97C01G008760 (annotated as phytoene synthase protein, ClPsy1 ) was recognized as the most likely candidate gene. Vitamin A deficiency is a worldwide public nutrition problem, and β-carotene is the precursor for vitamin A synthesis. Watermelon with golden flesh (gf, which occurs due to an accumulated abundance of β-carotene) is an important germplasm resource. In this study, a genetic analysis of segregated gf gene populations indicated that gf was controlled by a single recessive gene. BSA-seq (Bulked segregation analysis) and an initial linkage analysis placed the gf locus in a 290-Kb region on watermelon chromosome 1. Further fine mapping in a large population including over 1000 F₂ plants narrowed this region to 39.08 Kb harboring two genes, Cla97C01G008760 and Cla97C01G008770, which encode phytoene synthase (ClPsy1) and GATA zinc finger domain-containing protein, respectively. Gene sequence alignment and expression analysis between parental lines revealed Cla97C01G008760 as the best possible candidate gene for the gf trait. Nonsynonymous SNP mutations in the first exon of ClPsy1 between parental lines co-segregated with the gf trait only among individuals in the genetic population and were not related to flesh color in natural watermelon panels. Promoter sequence analysis of 26 watermelon accessions revealed two SNPs in the cis-acting element sequences corresponding to MYB and MYC2 transcription factors. RNA-seq data and qRT-PCR verification showed that two MYBs exhibited expression trends similar to that of ClPsy1 in the parental lines and may regulate the ClPsy1 expression. Further research findings indicate that the gf trait is determined not only by ClPsy1 but also by ClLCYB, ClCRTISO and ClNCED7, which play important roles in watermelon β-carotene accumulation.

Assessment of Biofortification Approaches Used to Improve Micronutrient-Dense Plants That Are a Sustainable Solution to Combat Hidden Hunger

Malnutrition causes diseases, immune system disorders, deterioration in physical growth, mental development, and learning capacity worldwide. Micronutrient deficiency, known as hidden hunger, is a serious global problem. Biofortification is a cost-effective and sustainable agricultural strategy for increasing the concentrations or bioavailability of essential elements in the edible parts of plants, minimizing the risks of toxic metals, and thus reducing malnutrition. It has the advantage of delivering micronutrient-dense food crops to a large part of the global population, especially poor populations. Agronomic biofortification and biofertilization, traditional plant breeding, and optimized fertilizer applications are more globally accepted methods today; however, genetic biofortification based on genetic engineering such as increasing or manipulating (such as CRISPR-Cas9) the expression of genes that affect the regulation of metal homeostasis and carrier proteins that serve to increase the micronutrient content for higher nutrient concentration and greater productivity or that affect bioavailability is also seen as a promising high-potential strategy in solving this micronutrient deficiency problem. Data that micronutrients can help strengthen the immune system against the COVID-19 pandemic and other diseases has highlighted the importance of tackling micronutrient deficiencies. In this study, biofortification approaches such as plant breeding, agronomic techniques, microbial fertilization, and some genetic and nanotechnological methods used in the fight against micronutrient deficiency worldwide were compiled.

Enrichment of provitamin A content in durum wheat grain by suppressing β-carotene hydroxylase 1 genes with a TILLING approach

The suppression of the HYD-1 gene by a TILLING approach increases the amount of β-carotene in durum wheat kernel. Vitamin A deficiency is a major public health problem that affects numerous countries in the world. As humans are not able to synthesize vitamin A, it must be daily assimilated along with other micro- and macronutrients through the diet. Durum wheat is an important crop for Mediterranean countries and provides a discrete amount of nutrients, such as carbohydrates and proteins, but it is deficient in some essential micronutrients, including provitamin A. In the present work, a targeting induced local lesions in genomes strategy has been undertaken to obtain durum wheat genotypes biofortified in provitamin A. In detail, we focused on the suppression of the β-carotene hydroxylase 1 (HYD1) genes, encoding enzymes involved in the redirection of β-carotene toward the synthesis of the downstream xanthophylls (neoxanthin, violaxanthin and zeaxanthin). Expression analysis of genes involved in carotenoid biosynthesis revealed a reduction of the abundance of HYD1 transcripts greater than 50% in mutant grain compared to the control. The biochemical profiling of carotenoid in the wheat mutant genotypes highlighted a significant increase of more than 70% of β-carotene compared to the wild-type sibling lines, with no change in lutein, α-carotene and zeaxanthin content. This study sheds new light on the molecular mechanism governing carotenoid biosynthesis in durum wheat and provides new genotypes that represent a good genetic resource for future breeding programs focused on the provitamin A biofortification through non-transgenic approaches.

Multi-strategy engineering greatly enhances provitamin A carotenoid accumulation and stability in Arabidopsis seeds

Staple grains with low levels of provitamin A carotenoids contribute to the global prevalence of vitamin A deficiency and therefore are the main targets for provitamin A biofortification. However, carotenoid stability during both seed maturation and postharvest storage is a serious concern for the full benefits of carotenoid biofortified grains. In this study, we utilized Arabidopsis as a model to establish carotenoid biofortification strategies in seeds. We discovered that manipulation of carotenoid biosynthetic activity by seed-specific expression of Phytoene synthase (PSY) increases both provitamin A and total carotenoid levels but the increased carotenoids are prone to degradation during seed maturation and storage, consistent with previous studies of provitamin A biofortified grains. In contrast, stacking with Orange (OR ^His ), a gene that initiates chromoplast biogenesis, dramatically enhances provitamin A and total carotenoid content and stability. Up to 65- and 10-fold increases of β-carotene and total carotenoids, respectively, with provitamin A carotenoids composing over 63% were observed in the seeds containing OR ^His and PSY. Co-expression of Homogentisate geranylgeranyl transferase (HGGT) with OR ^His and PSY further increases carotenoid accumulation and stability during seed maturation and storage. Moreover, knocking-out of β-carotene hydroxylase 2 (BCH2) by CRISPR/Cas9 not only potentially facilitates β-carotene accumulation but also minimizes the negative effect of carotenoid over production on seed germination. Our findings provide new insights into various processes on carotenoid accumulation and stability in seeds and establish a multiplexed strategy to simultaneously target carotenoid biosynthesis, turnover, and stable storage for carotenoid biofortification in crop seeds.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-021-00046-1.

Vitamins in Cereals: A Critical Review of Content, Health Effects, Processing Losses, Bioaccessibility, Fortification, and Biofortification Strategies for Their Improvement

Around the world, cereals are stapled foods and good sources of vitamins A, B, and E. As cereals are inexpensive and consumed in large quantities, attempts are being made to enrich cereals using fortification and biofortification in order to address vitamin deficiency disorders in a vulnerable population. The processing and cooking of cereals significantly affect vitamin content. Depending on grain structure, milling can substantially reduce vitamin content, while cooking methods can significantly impact vitamin retention and bioaccessibility. Pressure cooking has been reported to result in large vitamin losses, whereas minimal vitamin loss was observed following boiling. The fortification of cereal flour with vitamins B1, B2, B3, and B9, which are commonly deficient, has been recommended; and in addition, region-specific fortification using either synthetic or biological vitamins has been suggested. Biofortification is a relatively new concept and has been explored as a method to generate vitamin-rich crops. Once developed, biofortified crops can be utilized for several years. A recent cereal biofortification success story is the enrichment of maize with provitamin A carotenoids.

GmCCD4 controls carotenoid content in soybeans

To better understand the mechanisms regulating plant carotenoid metabolism in staple crop, we report the map-based cloning and functional characterization of the Glycine max carotenoid cleavage dioxygenase 4 (GmCCD4) gene, which encodes a carotenoid cleavage dioxygenase enzyme involved in metabolizing carotenoids into volatile β-ionone. Loss of GmCCD4 protein function in four Glycine max increased carotenoid content (gmicc) mutants resulted in yellow flowers due to excessive accumulation of carotenoids in flower petals. The carotenoid contents also increase three times in gmicc1 seeds. A genome-wide association study indicated that the GmCCD4 locus was one major locus associated with carotenoid content in natural population. Further analysis indicated that the haplotype-1 of GmCCD4 gene was positively associated with higher carotenoid levels in soybean cultivars and accumulated more β-carotene in engineered E. coli with ectopic expression of different GmCCD4 haplotypes. These observations uncovered that GmCCD4 was a negative regulator of carotenoid content in soybean, and its various haplotypes provide useful resources for future soybean breeding practice.

[The content of yellow pigments in durum wheat (Titicum durum Desf.) grains: biosynthesis, genetic control, marker selection]

Зерно с высоким содержанием каротиноидных пигментов ценится за ярко-желтый цвет пасты, производимой из него, и провитаминную (витамин А) и антиоксидантную активность пигментов. Цель настоя- щего обзора – обобщение современных знаний о биосинтезе и генетическом контроле накопления пигментов в зерне твердой пшеницы и оценка основных результатов исследований и селекции за последние двадцать лет за рубежом и в России. Признак «концентрация каротиноидных пигментов в зерне» (Ypc) относится к раз- ряду количественных. Тем не менее превалирование сильных аддитивных эффектов генов и высокая насле- дуемость способствовали значительному прогрессу в селекции по этому признаку. Методами молекулярного маркирования локусов количественных признаков (QTL), контролирующих синтез каротиноидных пигментов и значения индекса желтизны (IY), установлено их распределение по всем хромосомам генома твердой пшеницы. Основные генетические локусы, определяющие более 60 % варьирования признака, были картированы в хромосомах 7AL и 7BL. Вклад этих локусов связан с аллельными вариациями, влияющими на активность фермента фитоенсинтетазы (PSY). В других хромосомах были локализованы минорные генетические факторы, из которых наиболее значимы QTL, расположенные в хромосомах 3AS (ассоциирован с геном LCYE-ликопин- ε-циклаза) и 4ВS (аллель Lpx-B1.1c). При этом показано, что аллель Lpx-B1.1c вносит вклад в снижение актив- ности липоксигеназы, окисляющей каротиноиды в процессе изготовления конечных продуктов. Рассмотрены и обсуждены проблемы использования молекулярных маркеров в селекционных программах, нацеленных на увеличение концентрации пигментов в зерне и улучшение цветовых характеристик пасты.

Metabolomics Intervention Towards Better Understanding of Plant Traits

The majority of the most economically important plant and crop species are enriched with the availability of high-quality reference genome sequences forming the basis of gene discovery which control the important biochemical pathways. The transcriptomics and proteomics resources have also been made available for many of these plant species that intensify the understanding at expression levels. However, still we lack integrated studies spanning genomics–transcriptomics–proteomics, connected to metabolomics, the most complicated phase in phenotype expression. Nevertheless, for the past few decades, emphasis has been more on metabolome which plays a crucial role in defining the phenotype (trait) during crop improvement. The emergence of modern high throughput metabolome analyzing platforms have accelerated the discovery of a wide variety of biochemical types of metabolites and new pathways, also helped in improving the understanding of known existing pathways. Pinpointing the causal gene(s) and elucidation of metabolic pathways are very important for development of improved lines with high precision in crop breeding. Along with other-omics sciences, metabolomics studies have helped in characterization and annotation of a new gene(s) function. Hereby, we summarize several areas in the field of crop development where metabolomics studies have made its remarkable impact. We also assess the recent research on metabolomics, together with other omics, contributing toward genetic engineering to target traits and key pathway(s).

Multiplying the efficiency and impact of biofortification through metabolic engineering

Ending all forms of hunger by 2030, as set forward in the UN-Sustainable Development Goal 2 (UN-SDG2), is a daunting but essential task, given the limited timeline ahead and the negative global health and socio-economic impact of hunger. Malnutrition or hidden hunger due to micronutrient deficiencies affects about one third of the world population and severely jeopardizes economic development. Staple crop biofortification through gene stacking, using a rational combination of conventional breeding and metabolic engineering strategies, should enable a leap forward within the coming decade. A number of specific actions and policy interventions are proposed to reach this goal.

Prospects for Carotenoid Biofortification Targeting Retention and Catabolism

Due to the ongoing prevalence of vitamin A deficiency (VAD) in developing countries there has been a large effort towards increasing the carotenoid content of staple foods via biofortification. Common strategies used for carotenoid biofortification include altering flux through the biosynthesis pathway to direct synthesis to a specific product, generally β-carotene, or via increasing the expression of genes early in the carotenoid biosynthesis pathway. Recently, carotenoid biofortification strategies are turning towards increasing the retention of carotenoids in plant tissues either via altering sequestration within the cell or via downregulating enzymes known to cause degradation of carotenoids. To date, little attention has focused on increasing the stability of carotenoids, which may be a promising method of increasing carotenoid content in staple foods.

Recent developments and applications of genetic transformation and genome editing technologies in wheat

Wheat (Triticum aestivum) is a staple crop across the world and plays a remarkable role in food supplying security. Over the past few decades, basic and applied research on wheat has lagged behind other cereal crops due to the complex and polyploid genome and difficulties in genetic transformation. A breakthrough called as PureWheat was made in the genetic transformation of wheat in 2014 in Asia, leading to a noticeable progress of wheat genome editing. Due to this great achievement, it is predicated that wheat biotechnology revolution is arriving. Genome editing technologies using zinc finger nucleases, transcription activator-like effector nuclease, and clustered regularly interspaced short palindromic repeats-associated endonucleases (CRISR/Cas) are becoming powerful tools for crop modification which can help biologists and biotechnologists better understand the processes of mutagenesis and genomic alteration. Among the three genome editing systems, CRISR/Cas has high specificity and activity, and therefore it is widely used in genetic engineering. Generally, the genome editing technologies depend on an efficient genetic transformation system. In this paper, we summarize recent progresses and applications on genetic transformation and genome editing in wheat. We also examine the future aspects of genetic transformation and genome editing. We believe that the technologies for wheat efficient genetic engineering and functional studies will become routine with the emergence of high-quality genomic sequences.

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.

Carotenoids in Cereal Food Crops: Composition and Retention throughout Grain Storage and Food Processing

Carotenoids are C40 isoprenoids synthesized by plants, as well as some bacteria, fungi and algae, that have been reported to be responsible for a number of benefits conferred on human health. The inability of animals and humans to synthesize de novo these compounds is the reason why they must be introduced from dietary sources. In cereal grains, carotenoids are important phytochemicals responsible for the characteristic yellow colour of the endosperm, which confers nutritional and aesthetic quality to cereal-based products. Cereals are staple foods for a large portion of the world population, and the biofortification of cereal grains with carotenoids may represent a simple way to prevent many human diseases and disorders. Unfortunately, evidence exists that the storage and processing of cereal grains into food products may negatively impact their carotenoid content; so, this loss should be taken into consideration when analysing the potential health benefits of the cereal-based products. Focusing on the recent updates, this review summarizes the chemical composition of the carotenoids in the grains of staple cereals, including wheat, maize, rice and sorghum, the main factors that affect their carotenoid content during storage and processing and the most fruitful strategies used improve the grain carotenoid content and limit the carotenoid post-harvest losses.

Provitamin A Biofortification of Durum Wheat through a TILLING Approach

Macro- and micronutrients, essential for the maintenance of human metabolism, are assimilated daily through the diet. Wheat and other major cereals are a good source of nutrients, such as carbohydrates and proteins, but cannot supply a sufficient amount of essential micronutrients, including provitamin A. As vitamin A deficiency (VAD) leads to several serious diseases throughout the world, the biofortification of a major staple crop, such as wheat, represents an effective way to preserve human health in developing countries. In the present work, a key enzyme involved in the branch of carotenoids pathway producing β-carotene, lycopene epsilon cyclase, has been targeted by a Targeting Induced Local Lesions in Genomes (TILLING) approach in a "block strategy" perspective. The null mutant genotype showed a strong reduction in the expression of the lcyE gene and also interesting pleiotropic effects on an enzyme (β-ring hydroxylase) acting downstream in the pathway. Biochemical profiling of carotenoids in the wheat mutant lines showed an increase of roughly 75% in β-carotene in the grains of the complete mutant line compared with the control. In conclusion, we describe here the production and characterization of a new wheat line biofortified with provitamin A obtained through a nontransgenic approach, which also sheds new light on the molecular mechanism governing carotenoid biosynthesis in durum wheat.

Carotenoid Pigment Content in Durum Wheat (Triticum turgidum L. var durum): An Overview of Quantitative Trait Loci and Candidate Genes

Carotenoid pigment content is an important quality trait as it confers a natural bright yellow color to pasta preferred by consumers (whiteness vs. yellowness) and nutrients, such as provitamin A and antioxidants, essential for human diet. The main goal of the present review is to summarize the knowledge about the genetic regulation of the accumulation of pigment content in durum wheat grain and describe the genetic improvements obtained by using breeding approaches in the last two decades. Although carotenoid pigment content is a quantitative character regulated by various genes with additive effects, its high heritability has facilitated the durum breeding progress for this quality trait. Mapping research for yellow index and yellow pigment content has identified quantitative trait loci (QTL) on all wheat chromosomes. The major QTL, accounting for up to 60%, were mapped on 7L homoeologous chromosome arms, and they are explained by allelic variations of the phytoene synthase (PSY) genes. Minor QTL were detected on all chromosomes and associated to significant molecular markers, indicating the complexity of the trait. Despite there being currently a better knowledge of the mechanisms controlling carotenoid content and composition, there are gaps that require further investigation and bridging to better understand the genetic architecture of this important trait. The development and the utilization of molecular markers in marker-assisted selection (MAS) programs for improving grain quality have been reviewed and discussed.

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

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

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.

RNAi-mediated suppression of three carotenoid-cleavage dioxygenase genes, OsCCD1, 4a, and 4b, increases carotenoid content in rice

Carotenoids of staple food crops have a high nutritional value as provitamin A components in the daily diet. To increase the levels of carotenoids, inhibition of carotenoid-cleavage dioxygenases (CCDs), which degrade carotenoids, has been considered as a promising target in crop biotechnology. In this study, suppression of the OsCCD1, OsCCD4a, and OsCCD4b genes using RNAi was verified in transgenic rice plants by quantitative RT-PCR and small RNA detection. Leaf carotenoids were significantly increased overall in OsCCD4a-RNAi lines of the T1 generation, and the highest accumulation of 1.3-fold relative to non-transgenic plants was found in a line of the T2 generation. The effects on seed carotenoids were determined via cross-fertilization between β-carotene-producing transgenic rice and one of two independent homozygous lines of OsCCD1-RNAi, OsCCD4a-RNAi, or OsCCD4b-RNAi. This showed that carotenoids were increased to a maximum of 1.4- and 1.6-fold in OsCCD1-RNAi and OsCCD4a-RNAi, respectively, with a different preference toward α-ring and β-ring carotenoids; levels could not be established in OsCCD4b-RNAi. In addition, the contents of four carotenoids decreased when OsCCD1, OsCCD4a, and OsCCD4b were overexpressed in E. coli strains accumulating phytoene, lycopene, β-carotene, and zeaxanthin. OsCCD1 and OsCCD4a had a similar high carotenoid degrading activity, followed by OsCCD4b without substrate specificity. Overall, our results suggest that suppresing OsCCD4a activity may have potential as a tool for enhancing the carotenoid content of seed endosperms and leaves in rice.

A global perspective on carotenoids: Metabolism, biotechnology, and benefits for nutrition and health

Carotenoids are lipophilic isoprenoid compounds synthesized by all photosynthetic organisms and some non-photosynthetic prokaryotes and fungi. With some notable exceptions, animals (including humans) do not produce carotenoids de novo but take them in their diets. In photosynthetic systems carotenoids are essential for photoprotection against excess light and contribute to light harvesting, but perhaps they are best known for their properties as natural pigments in the yellow to red range. Carotenoids can be associated to fatty acids, sugars, proteins, or other compounds that can change their physical and chemical properties and influence their biological roles. Furthermore, oxidative cleavage of carotenoids produces smaller molecules such as apocarotenoids, some of which are important pigments and volatile (aroma) compounds. Enzymatic breakage of carotenoids can also produce biologically active molecules in both plants (hormones, retrograde signals) and animals (retinoids). Both carotenoids and their enzymatic cleavage products are associated with other processes positively impacting human health. Carotenoids are widely used in the industry as food ingredients, feed additives, and supplements. This review, contributed by scientists of complementary disciplines related to carotenoid research, covers recent advances and provides a perspective on future directions on the subjects of carotenoid metabolism, biotechnology, and nutritional and health benefits.

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.

The Silencing of Carotenoid β-Hydroxylases by RNA Interference in Different Maize Genetic Backgrounds Increases the β-Carotene Content of the Endosperm

Maize (Zea mays L.) is a staple food in many parts of Africa, but the endosperm generally contains low levels of the pro-vitamin A carotenoid β-carotene, leading to vitamin A deficiency disease in populations relying on cereal-based diets. However, maize endosperm does accumulate high levels of other carotenoids, including zeaxanthin, which is derived from β-carotene via two hydroxylation reactions. Blocking these reactions could therefore improve the endosperm β-carotene content. Accordingly, we used RNA interference (RNAi) to silence the endogenous ZmBCH1 and ZmBCH2 genes, which encode two non-heme di-iron carotenoid β-hydroxylases. The genes were silenced in a range of maize genetic backgrounds by introgressing the RNAi cassette, allowing us to determine the impact of ZmBCH1/ZmBCH2 silencing in diverse hybrids. The β-carotene content of the endosperm increased substantially in all hybrids in which ZmBCH2 was silenced, regardless of whether or not ZmBCH1 was silenced simultaneously. However, the β-carotene content did not change significantly in C17 hybrids (M7 × C17 and M13 × C17) compared to C17 alone, because ZmBCH2 is already expressed at negligible levels in the C17 parent. Our data indicate that ZmBCH2 is primarily responsible for the conversion of β-carotene to zeaxanthin in maize endosperm.

Overexpression of the Squalene Epoxidase Gene Alone and in Combination with the 3-Hydroxy-3-methylglutaryl Coenzyme A Gene Increases Ganoderic Acid Production in Ganoderma lingzhi

The squalene epoxidase (SE) gene from the biosynthetic pathway of ganoderic acid (GA) was cloned and overexpressed in Ganoderma lingzhi. The strain that overexpressed the SE produced approximately 2 times more GA molecules than the wild-type (WT) strain. Moreover, SE overexpression upregulated lanosterol synthase gene expression in the biosynthetic pathway. These results indicated that SE stimulates GA accumulation. Then, the SE and 3-hydroxy-3-methylglutaryl coenzyme A (HMGR) genes were simultaneously overexpressed in G. lingzhi. Compared with the individual overexpression of SE or HMGR, the combined overexpression of the two genes further enhanced individual GA production. The overexpressing strain produced maximum GA-T, GA-S, GA-Mk, and GA-Me contents of 90.4 ± 7.5, 35.9 ± 5.4, 6.2 ± 0.5, and 61.8 ± 5.8 μg/100 mg dry weight, respectively. These values were 5.9, 4.5, 2.4, and 5.8 times higher than those produced by the WT strain. This is the first example of the successful manipulation of multiple biosynthetic genes to improve GA content in G. lingzhi.

Transcriptomic Analyses of Ascorbic Acid and Carotenoid Metabolites Influenced by Root Restriction during Grape Berry Development and Ripening

Ascorbic acid (AsA) and carotenoids are recognized as crucial metabolites for various biological processes in plants. The contents of AsA and carotenoids in fruits are influenced by external environmental stimuli, such as water, temperature, light, and hormones. However, it is still not clear whether it can be affected by root restriction (RR) treatment. In this study, "Summer Black" grape berries (Vitis vinifera × V. labrusca) under RR and control treatments during development and ripening were used as materials. The results showed that RR significantly increased the contents of AsA, and the transcript VIT_08s0040g03150 related to AsA recycling pathways may be the main regulator for AsA. Similarly, the contents of most of the carotenoids at the earlier stages significantly increased by RR; the enzyme crtB encoded by VIT_12s0028g00960 and the enzyme crtZ encoded by VIT_02s0025g00240 and VIT_16s0050g01090 were inferred to play major roles in the carotenoid metabolic pathways.

The carotenoid biosynthetic and catabolic genes in wheat and their association with yellow pigments

BACKGROUND

In plants carotenoids play an important role in the photosynthetic process and photo-oxidative protection, and are the substrate for the synthesis of abscisic acid and strigolactones. In addition to their protective role as antioxidants and precursors of vitamin A, in wheat carotenoids are important as they influence the colour (whiteness vs. yellowness) of the grain. Understanding the genetic basis of grain yellow pigments, and identifying associated markers provide the basis for improving wheat quality by molecular breeding.

RESULTS

Twenty-four candidate genes involved in the biosynthesis and catabolism of carotenoid compounds have been identified in wheat by comparative genomics. Single nucleotide polymorphisms (SNPs) found in the coding sequences of 19 candidate genes allowed their chromosomal location and accurate map position on two reference consensus maps to be determined. The genome-wide association study based on genotyping a tetraploid wheat collection with 81,587 gene-associated SNPs validated quantitative trait loci (QTLs) previously detected in biparental populations and discovered new QTLs for grain colour-related traits. Ten carotenoid genes mapped in chromosome regions underlying pigment content QTLs indicating possible functional relationships between candidate genes and the trait.

CONCLUSIONS

The availability of linked, candidate gene-based markers can facilitate breeding wheat cultivars with desirable levels of carotenoids. Identifying QTLs linked to carotenoid pigmentation can contribute to understanding genes underlying carotenoid accumulation in the wheat kernels. Together these outputs can be combined to exploit the genetic variability of colour-related traits for the nutritional and commercial improvement of wheat products.

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.

Overview of the Wheat Genetic Transformation and Breeding Status in China

In the past two decades, Chinese scientists have achieved significant progress on three aspects of wheat genetic transformation. First, the wheat transformation platform has been established and optimized to improve the transformation efficiency, shorten the time required from starting of transformation procedure to the fertile transgenic wheat plants obtained as well as to overcome the problem of genotype-dependent for wheat genetic transformation in wide range of wheat elite varieties. Second, with the help of many emerging techniques such as CRISPR/cas9 function of over 100 wheat genes has been investigated. Finally, modern technology has been combined with the traditional breeding technique such as crossing to accelerate the application of wheat transformation. Overall, the wheat end-use quality and the characteristics of wheat stress tolerance have been improved by wheat genetic engineering technique. So far, wheat transgenic lines integrated with quality-improved genes and stress tolerant genes have been on the way of Production Test stage in the field. The debates and the future studies on wheat transformation have been discussed, and the brief summary of Chinese wheat breeding research history has also been provided in this review.

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.

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.