Toward the 'golden' era: The status in uncovering the regulatory control of carotenoid accumulation in plants

Exogenous CaCl2 reduces the oxidative cleavage of carotenoids in shredded carrots by targeting CAMTA4-mediated transcriptional repression of carotenoid degradation pathway

Carotenoid oxidative cleavage is a significant factor contributing to the color changes of shredded carrots and treatment with calcium chloride (CaCl2, 1% w/v) has been observed to alleviate the whitening symptom and color loss. However, the specific mechanism by which CaCl2 treatment suppresses carotenoid degradation remains unclear. In this study, the effect of CaCl2 and EGTA (calcium ion chelating agent) treatment on carotenoid biosynthesis and degradation in shredded carrots and the mechanism involved was investigated. CaCl2 treatment promoted the expression and activity of carotenoid biosynthetic enzyme (phytoene synthase, PSY), but inhibited the increases of the degradative enzyme activity of carotenoid cleavage dioxygenase (CCD) and down-regulated the corresponding transcripts, thus delayed the degradation of total carotenoid and maintaining higher levels of major carotenoid compounds including β-carotene, α-carotene, lycopene, and lutein in shredded carrots during storage. However, EGTA treatment promoted the gene expression and enzyme activity of CCD and increased the degradation of carotenoid compounds in shredded carrots during storage. Furthermore, the CaCl2 treatment induced DcCAMTA4, identified as a calcium decoder in shredded carrots, which, in turn, suppressed the expressions of DcCCD1 and DcCCD4 by interacting with their promoters. The transient overexpression of DcCAMTA4 in tobacco leaves led to reduced expression of NtCCD1 and NtCCD4, maintaining a higher content of carotenoids. Thus, CaCl2 alleviated the oxidative cleavage of carotenoids in shredded carrots through the DcCAMTA4-mediated carotenoid degradation pathway.

Integrative Analysis of Metabolome and Transcriptome of Carotenoid Biosynthesis Reveals the Mechanism of Fruit Color Change in Tomato (Solanum lycopersicum)

Tomato fruit ripening is accompanied by carotenoid accumulation and color changes. To elucidate the regulatory mechanisms underlying carotenoid synthesis during fruit ripening, a combined transcriptomic and metabolomic analysis was conducted on red-fruited tomato (WP190) and orange-fruited tomato (ZH108). A total of twenty-nine (29) different carotenoid compounds were identified in tomato fruits at six different stages. The abundance of the majority of the carotenoids was enhanced significantly with fruit ripening, with higher levels of lycopene; (E/Z)-lycopene; and α-, β- and γ-carotenoids detected in the fruits of WP190 at 50 and 60 days post anthesis (DPA). Transcriptome analysis revealed that the fruits of two varieties exhibited the highest number of differentially expressed genes (DEGs) at 50 DPA, and a module of co-expressed genes related to the fruit carotenoid content was established by WGCNA. qRT-PCR analysis validated the transcriptome result with a significantly elevated transcript level of lycopene biosynthesis genes (including SlPSY2, SlZCIS, SlPDS, SlZDS and SlCRTSO2) observed in WP190 at 50 DPA in comparison to ZH108. In addition, during the ripening process, the expression of ethylene biosynthesis (SlACSs and SlACOs) and signaling (SlEIN3 and SlERF1) genes was also increased, and these mechanisms may regulate carotenoid accumulation and fruit ripening in tomato. Differential expression of several key genes in the fruit of two tomato varieties at different stages regulates the accumulation of carotenoids and leads to differences in color between the two varieties of tomato. The results of this study provide a comprehensive understanding of carotenoid accumulation and ethylene biosynthesis and signal transduction pathway regulatory mechanisms during tomato fruit development.

Nudix hydrolase 23 post-translationally regulates carotenoid biosynthesis in plants

Carotenoids are essential for photosynthesis and photoprotection. Plants must evolve multifaceted regulatory mechanisms to control carotenoid biosynthesis. However, the regulatory mechanisms and the regulators conserved among plant species remain elusive. Phytoene synthase (PSY) catalyzes the highly regulated step of carotenogenesis and geranylgeranyl diphosphate synthase (GGPPS) acts as a hub to interact with GGPP-utilizing enzymes for the synthesis of specific downstream isoprenoids. Here, we report a function of Nudix hydrolase 23 (NUDX23), a Nudix domain-containing protein, in post-translational regulation of PSY and GGPPS for carotenoid biosynthesis. NUDX23 expresses highly in Arabidopsis (Arabidopsis thaliana) leaves. Overexpression of NUDX23 significantly increases PSY and GGPPS protein levels and carotenoid production, whereas knockout of NUDX23 dramatically reduces their abundances and carotenoid accumulation in Arabidopsis. NUDX23 regulates carotenoid biosynthesis via direct interactions with PSY and GGPPS in chloroplasts, which enhances PSY and GGPPS protein stability in a large PSY-GGPPS enzyme complex. NUDX23 was found to co-migrate with PSY and GGPPS proteins and to be required for the enzyme complex assembly. Our findings uncover a regulatory mechanism underlying carotenoid biosynthesis in plants and offer promising genetic tools for developing carotenoid-enriched food crops.

QTL mapping and candidate gene analysis reveal two major loci regulating green leaf color in non-heading Chinese cabbage

KEY MESSAGE

Two major-effect QTL GlcA07.1 and GlcA09.1 for green leaf color were fine mapped into 170.25 kb and 191.41 kb intervals on chromosomes A07 and A09, respectively, and were validated by transcriptome analysis. Non-heading Chinese cabbage (NHCC) is a leafy vegetable with a wide range of green colors. Understanding the genetic mechanism behind broad spectrum of green may facilitate the breeding of high-quality NHCC. Here, we used F2 and F7:8 recombination inbred line (RIL) population from a cross between Wutacai (dark-green) and Erqing (lime-green) to undertake the genetic analysis and quantitative trait locus (QTL) mapping in NHCC. The genetic investigation of the F2 population revealed that the variation of green leaf color was controlled by two recessive genes. Six pigments associated with green leaf color, including total chlorophyll, chlorophyll a, chlorophyll b, total carotenoids, lutein, and carotene were quantified and applied for QTL mapping in the RIL population. A total of 7 QTL were detected across the whole genome. Among them, two major-effect QTL were mapped on chromosomes A07 (GlcA07.1) and A09 (GlcA09.1) corresponding to two QTL identified in the F2 population. The QTL GlcA07.1 and GlcA09.1 were further fine mapped into 170.25 kb and 191.41 kb genomic regions, respectively. By comparing gene expression level and gene annotation, BraC07g023810 and BraC07g023970 were proposed as the best candidates for GlcA07.1, while BraC09g052220 and BraC09g052270 were suggested for GlcA09.1. Two InDel molecular markers (GlcA07.1-BcGUN4 and GlcA09.1-BcSG1) associated with BraC07gA023810 and BraC09g052220 were developed and could effectively identify leaf color in natural NHCC accessions, suggesting their potential for marker-assisted leaf color selection in NHCC breeding.

Integrated transcriptomics and metabolomics analyses of the effects of bagging treatment on carotenoid biosynthesis and regulation of Areca catechu L

Introduction

Fresh Aareca nut fruit for fresh fruit chewing commonly found in green or dark green hues. Despite its economic significance, there is currently insufficient research on the study of color and luster of areca. And the areca nut fruits after bagging showed obvious color change from green to tender yellow. In the study, we tried to explain this interesting variation in exocarp color.

Methods

Fruits were bagged (with a double-layered black interior and yellow exterior) 45 days after pollination and subsequently harvested 120 days after pollination. In this study, we examined the the chlorophyll and carotenoid content of pericarp exocarp, integrated transcriptomics and metabolomics to study the effects of bagging on the carotenoid pathway at the molecular level.

Results

It was found that the chlorophyll and carotenoid content of bagged areca nut (YP) exocarp was significantly reduced. A total of 21 differentially expressed metabolites (DEMs) and 1784 differentially expressed genes (DEGs) were screened by transcriptomics and metabolomics. Three key genes in the carotenoid biosynthesis pathway as candidate genes for qPCR validation by co-analysis, which suggested their role in the regulation of pathways related to crtB, crtZ and CYP707A.

Discussion

We described that light intensity may appear as a main factor influencing the noted shift from green to yellow and the ensuing reduction in carotenoid content after bagging.

Abiotic stress as a dynamic strategy for enhancing high value phytochemicals in microalgae: Critical insights, challenges and future prospects

Microalgae showcase an extraordinary capacity for synthesizing high-value phytochemicals (HVPCs), offering substantial potential for diverse applications across various industries. Emerging research suggests that subjecting microalgae to abiotic stress during cultivation and the harvesting stages can further enhance the accumulation of valuable metabolites within their cells, including carotenoids, antioxidants, and vitamins. This study delves into the pivotal impacts of manipulating abiotic stress on microalgae yields, with a particular focus on biomass and selected HVPCs that have received limited attention in the existing literature. Moreover, approaches to utilising abiotic stress to increase HVPCs production while minimising adverse effects on biomass productivity were discussed. The present study also encompasses a techno-economic assessment (TEA) aimed at pinpointing significant bottlenecks in the conversion of microalgae biomass into high-value products and evaluating the desirability of various conversion pathways. The TEA methodology serves as a valuable tool for both researchers and practitioners in the quest to identify sustainable strategies for transforming microalgae biomass into high-value products and goods. Overall, this comprehensive review sheds light on the pivotal role of abiotic stress in microalgae cultivation, promising insights that could lead to more efficient and sustainable approaches for HVPCs production.

Involvement of Transcription Factors and Regulatory Proteins in the Regulation of Carotenoid Accumulation in Plants and Algae

Carotenoids are essential for photosynthesis and photoprotection in photosynthetic organisms, which are widely used in food coloring, feed additives, nutraceuticals, cosmetics, and pharmaceuticals. Carotenoid biofortification in crop plants or algae has been considered as a sustainable strategy to improve human nutrition and health. However, the regulatory mechanisms of carotenoid accumulation are still not systematic and particularly scarce in algae. This article focuses on the regulatory mechanisms of carotenoid accumulation in plants and algae through regulatory factors (transcription factors and regulatory proteins), demonstrating the complexity of homeostasis regulation of carotenoids, mainly including transcriptional regulation as the primary mechanism, subsequent post-translational regulation, and cross-linking with other metabolic processes. Different organs of plants and different plant/algal species usually have specific regulatory mechanisms for the biosynthesis, storage, and degradation of carotenoids in response to the environmental and developmental signals. In plants and algae, regulators such as MYB, bHLH, MADS, bZIP, AP2/ERF, WRKY, and orange proteins can be involved in the regulation of carotenoid metabolism. And many more regulators, regulatory networks, and mechanisms need to be explored. Our paper will provide a basis for multitarget or multipathway engineering for carotenoid biofortification in plants and algae.

Irrigation level and substrate type on the acclimatization and development of mandacaru (Cereus jamacaru DC.): an emblematic cactus from Brazilian semiarid region

Mandacaru is a cactus with great socioeconomic potential, but lack of information about its cultivation hinders its domestication. Here, we aimed to evaluate the acclimatization and vegetative development of mandacaru under different substrates and irrigation levels. For this, seeds inoculated in vitro were grown for 120 days, being transplanted to pots containing four types of substrate (S1-caatinga soil + gravel; S2-washed sand + organic matter + soil + charcoal; S3-washed sand + cattle manure + soil + sand; S4-commercial organic substrate). Pots were irrigated with 100% of the field capacity (FC) once-a-week, or with 50% FC twice-a-week, and kept in a greenhouse for six months. The experimental design was completely randomized, in a 4 × 2 factorial scheme, with six replications. Plant height and diameter, axial and radial growth rate, fresh and dry mass of stem and root, water content, and photosynthetic pigments were determined. Growth was affected mainly by the substrate, with S4 resulting in higher growth and pigment content, while S1 was impaired and S2 and S3 resulted in intermediate growth. The use of S4 and 100% FC once per week was the best condition for mandacaru.

Integrative Metabolome and Transcriptome Analyses Provide Insights into Carotenoid Variation in Different-Colored Peppers

Carotenoids are important pigments in pepper fruits. The colors of each pepper are mainly determined by the composition and content of carotenoid. The 'ZY' variety, which has yellow fruit, is a natural mutant derived from a branch mutant of 'ZR' with different colors. ZY and ZR exhibit obvious differences in fruit color, but no other obvious differences in other traits. To investigate the main reasons for the formation of different colored pepper fruits, transcriptome and metabolome analyses were performed in three developmental stages (S1-S3) in two cultivars. The results revealed that these structural genes (PSY1, CRTISO, CCD1, CYP97C1, VDE1, CCS, NCED1 and NCED2) related to carotenoid biosynthesis were expressed differentially in the two cultivars. Capsanthin and capsorubin mainly accumulated in ZR and were almost non-existent in ZY. S2 is the fruit color-changing stage; this may be a critical period for the development of different color formation of ZY and ZR. A combination of transcriptome and metabolome analyses indicated that CCS, NCED2, AAO4, VDE1 and CYP97C1 genes were key to the differences in the total carotenoid content. These new insights into pepper fruit coloration may help to improve fruit breeding strategies.

Transcription factor VvWRKY70 inhibits both norisoprenoid and flavonol biosynthesis in grape

Norisoprenoids and flavonols are important secondary metabolites in grape berries (Vitis vinifera L.). The former is a class of ubiquitous flavor and fragrance compounds produced by the cleavage of carotenoids, and the latter, which is derived from the flavonoid metabolic pathway, has been proposed as a general quality marker for red grapes. However, the transcriptional regulatory mechanisms underlying norisoprenoid and flavonol production are still not fully understood. In this study, we characterized a transcription factor, VvWRKY70, as a repressor of both norisoprenoid and flavonol biosynthesis in grape berries, and its expression was downregulated by light and high-temperature treatment. Overexpressing VvWRKY70 in grape calli reduced norisoprenoid and flavonol production, particularly under light exposure or at high temperature, by repressing the expression of several related genes in the isoprenoid and flavonoid metabolic pathways. VvWRKY70 downregulated β-CAROTENE HYDROXYLASE 2 (VvBCH2) and CHALCONE SYNTHASE 3 (VvCHS3) expression based on yeast 1-hybrid analysis combined with electrophoretic mobility shift assay and chromatin immunoprecipitation-quantitative PCR. We discuss the role of VvWRKY70 in the coordinated regulatory network of isoprenoid and flavonoid metabolism. These findings provide a theoretical basis to improve flavor, color, and other comprehensive qualities of fruit crops and their processing products.

Light regulation of the biosynthesis of phenolics, terpenoids, and alkaloids in plants

Biosynthesis of specialized metabolites (SM), including phenolics, terpenoids, and alkaloids, is stimulated by many environmental factors including light. In recent years, significant progress has been made in understanding the regulatory mechanisms involved in light-stimulated SM biosynthesis at the transcriptional, posttranscriptional, and posttranslational levels of regulation. While several excellent recent reviews have primarily focused on the impacts of general environmental factors, including light, on biosynthesis of an individual class of SM, here we highlight the regulation of three major SM biosynthesis pathways by light-responsive gene expression, microRNA regulation, and posttranslational modification of regulatory proteins. In addition, we present our future perspectives on this topic.

Carotenoid sequestration protein FIBRILLIN participates in CmOR-regulated β-carotene accumulation in melon

Chromoplasts are plant organelles with a unique ability to sequester and store massive carotenoids. Chromoplasts have been hypothesized to enable high levels of carotenoid accumulation due to enhanced sequestration ability or sequestration substructure formation. However, the regulators that control the substructure component accumulation and substructure formation in chromoplasts remain unknown. In melon (Cucumis melo) fruit, β-carotene accumulation in chromoplasts is governed by ORANGE (OR), a key regulator for carotenoid accumulation in chromoplasts. By using comparative proteomic analysis of a high β-carotene melon variety and its isogenic line low-β mutant that is defective in CmOr with impaired chromoplast formation, we identified carotenoid sequestration protein FIBRILLIN1 (CmFBN1) as differentially expressed. CmFBN1 expresses highly in melon fruit tissue. Overexpression of CmFBN1 in transgenic Arabidopsis (Arabidopsis thaliana) containing ORHis that genetically mimics CmOr significantly enhances carotenoid accumulation, demonstrating its involvement in CmOR-induced carotenoid accumulation. Both in vitro and in vivo evidence showed that CmOR physically interacts with CmFBN1. Such an interaction occurs in plastoglobules and results in promoting CmFBN1 accumulation. CmOR greatly stabilizes CmFBN1, which stimulates plastoglobule proliferation and subsequently carotenoid accumulation in chromoplasts. Our findings show that CmOR directly regulates CmFBN1 protein levels and suggest a fundamental role of CmFBN1 in facilitating plastoglobule proliferation for carotenoid sequestration. This study also reveals an important genetic tool to further enhance OR-induced carotenoid accumulation in chromoplasts in crops.

Lycopene ε-cyclase mediated transition of α-carotene and β-carotene metabolic flow in carrot fleshy root

The accumulation of carotenoids, such as xanthophylls, lycopene, and carotenes, is responsible for the color of carrot (Daucus carota subsp. sativus) fleshy roots. The potential role of DcLCYE, encoding a lycopene ε-cyclase associated with carrot root color, was investigated using cultivars with orange and red roots. The expression of DcLCYE in red carrot varieties was significantly lower than that in orange carrots at the mature stage. Furthermore, red carrots accumulated larger amounts of lycopene and lower levels of α-carotene. Sequence comparison and prokaryotic expression analysis revealed that amino acid differences in red carrots did not affect the cyclization function of DcLCYE. Analysis of the catalytic activity of DcLCYE revealed that it mainly formed ε-carotene, while a side activity on α-carotene and γ-carotene was also observed. Comparative analysis of the promoter region sequences indicated that differences in the promoter region may affect the transcription of DcLCYE. DcLCYE was overexpressed in the red carrot 'Benhongjinshi' under the control of the CaMV35S promoter. Lycopene in transgenic carrot roots was cyclized, resulting in the accumulation of higher levels of α-carotene and xanthophylls, while the β-carotene content was significantly decreased. The expression levels of other genes in the carotenoid pathway were simultaneously upregulated. Knockout of DcLCYE in the orange carrot 'Kurodagosun' by CRISPR/Cas9 technology resulted in a decrease in the α-carotene and xanthophyll contents. The relative expression levels of DcPSY1, DcPSY2, and DcCHXE were sharply increased in DcLCYE knockout mutants. The results of this study provide insights into the function of DcLCYE in carrots, which could serve as a basis for creating colorful carrot germplasms.

High temperature elevates carotenoid accumulation of banana fruit via upregulation of MaEIL9 module

Banana is a good source of carotenoids, which are bioactive metabolites with health beneficial properties for human. However, the molecular mechanism of carotenoid accumulation in banana fruit is largely unclear. In this study, we found that high temperature elevated carotenoid production in banana pulp, which is presumably due to upregulation of a subset of carotenogenic genes as well as a carotenoid biosynthesis regulator MaSPL16. Moreover, an ethylene signaling component MaEIL9 was identified, whose transcript and protein contents were also induced by high temperature. In addition, MaEIL9 positively regulates transcription of MaDXR1, MaPDS1, MaZDS1 and MaSPL16 through directly targeting their promoters. Overexpression of MaEIL9 in tomato fruit substantially increased the expression of carotenoid formation genes and elevated carotenoid content. Importantly, transiently silencing MaEIL9 in banana fruit weakened carotenoid production caused by high temperature. Taken together, these results indicate that high temperature induces carotenoid production in banana fruit, at least in part, through MaEIL9-mediated activation of MaDXR1, MaPDS1, MaZDS1 and MaSPL16 expression.

More Than Pigments: The Potential of Astaxanthin and Bacterioruberin-Based Nanomedicines

Carotenoids are natural products regulated by the food sector, currently used as feed dyes and as antioxidants in dietary supplements and composing functional foods for human consumption. Of the nearly one thousand carotenoids described to date, only retinoids, derived from beta carotene, have the status of a drug and are regulated by the pharmaceutical sector. In this review, we address a novel field: the transformation of xanthophylls, particularly the highly marketed astaxanthin and the practically unknown bacterioruberin, in therapeutic agents by altering their pharmacokinetics, biodistribution, and pharmacodynamics through their formulation as nanomedicines. The antioxidant activity of xanthophylls is mediated by routes different from those of the classical oral anti-inflammatory drugs such as corticosteroids and non-steroidal anti-inflammatory drugs (NSAIDs): remarkably, xanthophylls lack therapeutic activity but also lack toxicity. Formulated as nanomedicines, xanthophylls gain therapeutic activity by mechanisms other than increased bioavailability. Loaded into ad hoc tailored nanoparticles to protect their structure throughout storage and during gastrointestinal transit or skin penetration, xanthophylls can be targeted and delivered to selected inflamed cell groups, achieving a massive intracellular concentration after endocytosis of small doses of formulation. Most first reports showing the activities of oral and topical anti-inflammatory xanthophyll-based nanomedicines against chronic diseases such as inflammatory bowel disease, psoriasis, atopic dermatitis, and dry eye disease emerged between 2020 and 2023. Here we discuss in detail their preclinical performance, mostly targeted vesicular and polymeric nanoparticles, on cellular models and in vivo. The results, although preliminary, are auspicious enough to speculate upon their potential use for oral or topical administration in the treatment of chronic inflammatory diseases.

Grape phytochrome-interacting factor VvPIF1 negatively regulates carotenoid biosynthesis by repressing VvPSY expression

Phytochrome-interacting factors (PIFs) play important roles in light-mediated secondary metabolism; however, the roles of PIFs in grape fruit carotenogenesis remain unclear. Here, by identifying the PIF family genes in grapes, we focused on the role of VvPIF1 in carotenoid metabolism. During grape berry development, VvPIF1 expression was negatively correlated with carotenoid accumulation and the transcription of phytoene synthase 1/2 (VvPSY1/2), which encodes the major flux-controlling enzymes for carotenoid biosynthesis. Light significantly repressed VvPIF1 expression, but induced the expression of carotenogenic genes including VvPSY1/2. VvPIF1 functioned as a nucleus-localized protein and interacted with the light photoreceptor VvphyB. Overexpression of VvPIF1 resulted in the downregulation of the endogenous PIF1 gene, which may unexpectedly induce carotenoid accumulation and PSY expression in tobacco leaves. The transgenic grape leaves and tomato fruits with high VvPIF1 expression produced a significant decrease in carotenoid concentrations, with suppressed transcription of PSY and other carotenogenic genes. Further biochemical assays demonstrated that VvPIF1 bound directly to the promoters of VvPSY1/2 to inhibit their transcription. Collectively, we conclude that VvPIF1 negatively regulates carotenoid biosynthesis by repressing VvPSY expression in grapes. These findings shed light on the role and mode of action of PIFs in the carotenoid regulatory network of grapes.

Metabolomic and Transcriptomic Analysis Reveal the Role of Metabolites and Genes in Modulating Flower Color of Paphiopedilum micranthum

Food-deceptive flowers primarily use visual signals (such as color) to mimic model plants and deceive insects into achieving pollination. Paphiopedilum micranthum is a food-deceptive orchid that has a pink labellum and two purple petals with a yellow base and has been proven to be pollinated by bumblebees. However, the chemical and molecular bases of the floral color are not well understood. We conducted targeted metabolite profiling and transcriptomic analysis to determine the color signal and its genetic basis in P. micranthum. We found that both anthocyanins and carotenoids contribute significantly to the formation of floral color that determines the color signal. Higher concentrations of anthocyanins (cyanidin and peonidin) and carotenoids (primarily lutein and zeaxanthin) were detected in the petal compared to the labellum. The upregulation of structural genes of CHS, F3'H, DFR and ANS on the anthocyanin biosynthesis pathway in petals was identified, as well as three genes of LCYE, BCH, and CCD4 on the carotenoid biosynthesis pathway. Furthermore, we discovered that three R2R3-MYBs and one bHLH transcription factors were co-expressed with the expression of different genes. These genes and transcription factors may be responsible for the spatial color difference of P. micranthum. Our study emphasizes that the color of this food-deceptive orchids is achieved through specific genes and transcription factors associated with the pigment biosynthesis pathway.

Unraveling transcriptomics of sorghum grain carotenoids: a step forward for biofortification

BACKGROUND

Sorghum (Sorghum bicolor [L.] Moench) is a promising target for pro-vitamin A biofortification as it is a global staple crop, particularly in regions where vitamin A deficiency is prevalent. As with most cereal grains, carotenoid concentrations are low in sorghum, and breeding could be a feasible strategy to increase pro-vitamin A carotenoids to biologically relevant concentrations. However, there are knowledge gaps in the biosynthesis and regulation of sorghum grain carotenoids, which can limit breeding effectiveness. The aim of this research was to gain an understanding of the transcriptional regulation of a priori candidate genes in carotenoid precursor, biosynthesis, and degradation pathways.

RESULTS

We used RNA sequencing of grain to compare the transcriptional profile of four sorghum accessions with contrasting carotenoid profiles through grain development. Most a priori candidate genes involved in the precursor MEP, carotenoid biosynthesis, and carotenoid degradation pathways were found to be differentially expressed between sorghum grain developmental stages. There was also differential expression of some of the a priori candidate genes between high and low carotenoid content groups at each developmental time point. Among these, we propose geranyl geranyl pyrophosphate synthase (GGPPS), phytoene synthase (PSY), and phytoene desaturase (PDS) as promising targets for pro-vitamin A carotenoid biofortification efforts in sorghum grain.

CONCLUSIONS

A deeper understanding of the controls underlying biosynthesis and degradation of sorghum grain carotenoids is needed to advance biofortification efforts. This study provides the first insights into the regulation of sorghum grain carotenoid biosynthesis and degradation, suggesting potential gene targets to prioritize for molecular breeding.

AgMYB5, an MYB transcription factor from celery, enhanced β-carotene synthesis and promoted drought tolerance in transgenic Arabidopsis

BACKGROUND

Water shortage caused by global warming seriously affects the yield and quality of vegetable crops. β-carotene, the lipid-soluble natural product with important pharmacological value, is abundant in celery. Transcription factor MYB family extensively disperses in plants and plays regulatory roles in carotenoid metabolism and water scarcity response.

RESULTS

Here, the AgMYB5 gene encoding 196 amino acids was amplified from celery cv. 'Jinnanshiqin'. In celery, the expression of AgMYB5 exhibited transactivation activity, tissue specificity, and drought-condition responsiveness. Further analysis proved that ectopic expression of AgMYB5 increased β-carotene content and promoted drought tolerance in transgenic Arabidopsis thaliana. Moreover, AgMYB5 expression promoted β-carotene biosynthesis by triggering the expression of AtCRTISO and AtLCYB, which in turn increased antioxidant enzyme activities, and led to the decreased contents of H₂O₂ and MDA, and the inhibition of O₂^- generation. Meanwhile, β-carotene accumulation promoted endogenous ABA biosynthesis of transgenic Arabidopsis, which resulted in ABA-induced stomatal closing and delayed water loss. In addition, ectopic expression of AgMYB5 increased expression levels of AtERD1, AtP5CS1, AtRD22, and AtRD29.

CONCLUSIONS

The findings indicated that AgMYB5 up-regulated β-carotene biosynthesis and drought tolerance of Arabidopsis.

A xanthophyll-derived apocarotenoid regulates carotenogenesis in tomato chromoplasts

Carotenoids possess important biological functions that make them essential components of the human diet. β-Carotene and some other carotenoids have vitamin A activity while lutein and zeaxanthin, typically referred to as the macular pigments, are involved in good vision and in delaying the onset of age-related eye diseases. In order to create a zeaxanthin-producing tomato fruit, two transgenic lines, one with a high β-carotene cyclase activity and the other with a high β-carotene hydroxylase activity, have been genetically crossed. Ripe fruits from the resulting progeny contained significant levels of violaxanthin, antheraxanthin, and xanthophyll esters. However, their zeaxanthin content was not as high as expected, and the total level of carotenoids was only 25% of the carotenoids found in ripe fruits of the comparator line. Targeted transcript analysis and apocarotenoids determinations indicated that transcriptional regulation of the pathway or degradation of synthesized carotenoids were not responsible for the low carotenoid content of hybrid fruits which instead appeared to result from a substantial reduction of carotenoid biosynthesis. Notably, the content of an unidentified hydroxylated cyclic (C13) apocarotenoid was 13 times higher in the hybrid fruits than in the control fruits. Furthermore, a GC-MS-based metabolite profiling demonstrated a perturbation of carotenogenesis in ripening hybrid fruits compatible with a block of the pathway. Moreover, carotenoid profiling on leaf, fruit, and petal samples from a set of experimental lines carrying the hp3 mutation, in combination with the two transgenes, indicated that the carotenoid biosynthesis in petal and fruit chromoplasts could be regulated. Altogether the data were consistent with the hypothesis of the regulation of the carotenoid pathway in tomato chromoplasts through a mechanism of feedback inhibition mediated by a xanthophyll-derived apocarotenoid. This chromoplast-specific post-transcriptional mechanism was disclosed in transgenic fruits of HU hybrid owing to the abnormal production of zeaxanthin and antheraxanthin, the more probable precursors of the apocarotenoid signal. A model describing the regulation of carotenoid pathway in tomato chromoplasts is presented.

Roles of plastid-located phosphate transporters in carotenoid accumulation

Enhanced carotenoid accumulation in plants is crucial for the nutritional and health demands of the human body since these beneficial substances are acquired through dietary intake. Plastids are the major organelles to accumulate carotenoids in plants and it is reported that manipulation of a single plastid phosphate transporter gene enhances carotenoid accumulation. Amongst all phosphate transport proteins including phosphate transporters (PHTs), plastidial phosphate translocators (pPTs), PHOSPHATE1 (PHO1), vacuolar phosphate efflux transporter (VPE), and Sulfate transporter [SULTR]-like phosphorus distribution transporter (SPDT) in plants, plastidic PHTs (PHT2 & PHT4) are found as the only clade that is plastid located, and manipulation of which affects carotenoid accumulation. Manipulation of a single chromoplast PHT (PHT4;2) enhances carotenoid accumulation, whereas manipulation of a single chloroplast PHT has no impact on carotenoid accumulation. The underlying mechanism is mainly attributed to their different effects on plastid orthophosphate (Pi) concentration. PHT4;2 is the only chromoplast Pi efflux transporter, and manipulating this single chromoplast PHT significantly regulates chromoplast Pi concentration. This variation subsequently modulates the carotenoid accumulation by affecting the supply of glyceraldehyde 3-phosphate, a substrate for carotenoid biosynthesis, by modulating the transcript abundances of carotenoid biosynthesis limited enzyme genes, and by regulating chromoplast biogenesis (facilitating carotenoid storage). However, at least five orthophosphate influx PHTs are identified in the chloroplast, and manipulating one of the five does not substantially modulate the chloroplast Pi concentration in a long term due to their functional redundancy. This stable chloroplast Pi concentration upon one chloroplast PHT absence, therefore, is unable to modulate Pi-involved carotenoid accumulation processes and finally does affect carotenoid accumulation in photosynthetic tissues. Despite these advances, several cases including the precise location of plastid PHTs, the phosphate transport direction mediated by these plastid PHTs, the plastid PHTs participating in carotenoid accumulation signal pathway, the potential roles of these plastid PHTs in leaf carotenoid accumulation, and the roles of these plastid PHTs in other secondary metabolites are waiting for further research. The clarification of the above-mentioned cases is beneficial for breeding high-carotenoid accumulation plants (either in photosynthetic or non-photosynthetic edible parts of plants) through the gene engineering of these transporters.

Eco-Evo-Devo of petal pigmentation patterning

Colourful spots, stripes and rings decorate the corolla of most flowering plants and fulfil important biotic and abiotic functions. Spatial differences in the pigmentation of epidermal cells can create these patterns. The last few years have yielded new data that have started to illuminate the mechanisms controlling the function, formation and evolution of petal patterns. These advances have broad impacts beyond the immediate field as pigmentation patterns are wonderful systems to explore multiscale biological problems: from understanding how cells make decisions at the microscale to examining the roots of biodiversity at the macroscale. These new results also reveal there is more to petal patterning than meets the eye, opening up a brand new area of investigation. In this mini-review, we summarise our current knowledge on the Eco-Evo-Devo of petal pigmentation patterns and discuss some of the most exciting yet unanswered questions that represent avenues for future research.

Down-regulation of NCED leads to the accumulation of carotenoids in the flesh of F1 generation of peach hybrid

Flesh color is an important target trait in peach [Prunus persica (L.) Batsch] breeding. In this study, two white-fleshed peach cultivars were crossed [Changsong Whitepeach (WP-1) × 'Xiacui'], and their hybrid F₁ generation showed color segregation of white flesh (BF1) and yellow flesh (HF1). Metabolome analysis revealed that the flesh color segregation in the hybrid F₁ generation was related to the carotenoid content. The decrease in β-carotene and β-cryptoxanthin in BF1 flesh and increase in β-cryptoxanthin oleate, rubixanthin caprate, rubixanthin laurate and zeaxanthin dipalmitate in HF1 flesh contributed to their difference in carotenoid accumulation. Transcriptome analysis demonstrated that compared with BF1, HF1 showed significant up-regulation and down-regulation of ZEP and CCD8 at the core-hardening stage, respectively, while significant down-regulation of NCED in the whole fruit development stage. The down-regulation of NCED might inhibit the breakdown of the violaxanthin and its upstream substances and further promote the accumulation of carotenoids, resulting in yellow flesh. Therefore, NCED may be a key gene controlling the fruit color traits of peach. In this study, targeted metabolomics and transcriptomics were used to jointly explore the mechanism controlling the fruit color of peach, which may help to identify the key genes for the differences in carotenoid accumulation and provide a reference for the breeding of yellow-fleshed peach.

Transcriptomic and Physiological Analyses Reveal Potential Genes Involved in Photoperiod-Regulated β-Carotene Accumulation Mechanisms in the Endocarp of Cucumber (Cucumis sativus L.) Fruit

The accumulation of carotenoids in plants is a key nutritional quality in many horticultural crops. Although the structural genes encoding the biosynthetic enzymes are well-characterized, little is known regarding photoperiod-mediated carotenoid accumulation in the fruits of some horticultural crops. Herein, we performed physiological and transcriptomic analyses using two cucumber genotypes, SWCC8 (XIS-orange-fleshed and photoperiod-sensitive) and CC3 (white-fleshed and photoperiod-non-sensitive), established under two photoperiod conditions (8L/16D vs. 12L/12D) at four fruit developmental stages. Day-neutral treatments significantly increased fruit β-carotene content by 42.1% compared to short day (SD) treatments in SWCC8 at 40 DAP with no significant changes in CC3. Day-neutral condition elevated sugar levels of fruits compared to short-day treatments. According to GO and KEGG analyses, the predominantly expressed genes were related to photosynthesis, carotenoid biosynthesis, plant hormone signaling, circadian rhythms, and carbohydrates. Consistent with β-carotene accumulation in SWCC8, the day-neutral condition elevated the expression of key carotenoid biosynthesis genes such as PSY1, PDS, ZDS1, LYCB, and CHYB1 during later stages between 30 to 40 days of fruit development. Compared to SWCC8, CC3 showed an expression of DEGs related to carotenoid cleavage and oxidative stresses, signifying reduced β-carotene levels in CC3 cucumber. Further, a WGCNA analysis revealed co-expression between carbohydrate-related genes (pentose-phosphatase synthase, β-glucosidase, and trehalose-6-phosphatase), photoperiod-signaling genes (LHY, APRR7/5, FKF1, PIF3, COP1, GIGANTEA, and CK2) and carotenoid-biosynthetic genes, thus suggesting that a cross-talk mechanism between carbohydrates and light-related genes induces β-carotene accumulation. The results highlighted herein provide a framework for future gene functional analyses and molecular breeding towards enhanced carotenoid accumulation in edible plant organs.

"Omics" insights into plastid behavior toward improved carotenoid accumulation

Plastids are a group of diverse organelles with conserved carotenoids synthesizing and sequestering functions in plants. They optimize the carotenoid composition and content in response to developmental transitions and environmental stimuli. In this review, we describe the turbulence and reforming of transcripts, proteins, and metabolic pathways for carotenoid metabolism and storage in various plastid types upon organogenesis and external influences, which have been studied using approaches including genomics, transcriptomics, proteomics, and metabonomics. Meanwhile, the coordination of plastid signaling and carotenoid metabolism including the effects of disturbed carotenoid biosynthesis on plastid morphology and function are also discussed. The "omics" insight extends our understanding of the interaction between plastids and carotenoids and provides significant implications for designing strategies for carotenoid-biofortified crops.

Arbuscular Mycorrhizal Fungi Mediated Alleviation of Drought Stress via Non-Enzymatic Antioxidants: A Meta-Analysis

Drought stress constrains plant cell metabolism and induces the production of reactive oxygen species (ROS). In response to drought stress, plants induce a series of physiological and biochemical changes, scavenging ROS. Among soil microbes, arbuscular mycorrhizal fungi (AMF) are found to be effective ameliorators of ROS under drought-stress conditions. However, the comprehensive roles of the oxidative stress ameliorators mediated by AMF in alleviating drought stress are not studied in detail. The present study aims to determine the oxidative stress ameliorators using meta-analysis highlighting AMF inoculation efficacy on drought stress alleviation. The results confirmed that AMF inoculation had a significant reduction in hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and electrolyte leakage (EL). Nevertheless, proline accumulation was found to have a non-significant correlation with AMF inoculation. Further, carotenoids and soluble sugars increased positively in AMF-inoculated plants under drought stress and there was a subsequent reduction of abscisic acid (ABA). The results of the meta-analysis reveal the benefits of AMF inoculation with reduced H₂O₂ levels leading to reduced lipid peroxidation (MDA) and increased membrane stability (EL). Thus, the present assessment reveals the sequence of events involved in eliciting drought stress alleviation due to AMF inoculation.

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.

Acibenzolar-S-methyl activates mitogen-activated protein kinase cascade to mediate chlorophyll and carotenoid metabolisms in the exocarp of Docteur Jules Guyot pears

BACKGROUND

Acibenzolar-S-methyl (ASM), a well-known plant activator, has been used to protect fruit and vegetable from fungal invasion and maintain quality. However, little is known about the molecular mechanism of ASM in regulating chlorophyll and carotenoid metabolisms. Therefore, Docteur Jules Guyot pears were used as the materials to study the changes of hydrogen peroxide (H₂ O₂ ) production, mitogen-activated protein kinase (MAPK) cascade, transcription factors, chlorophyll, and carotenoid metabolisms after ASM and PD98059 (a MAPK cascade blocker) treatments.

RESULTS

ASM increased NADPH oxidase (NOX) and superoxide dismutase (SOD) activities, and H₂ O₂ content, promoted PcMAPKKK1, PcMAPKK3, and PcMAPK6 expressions, and down-regulated PcMYC2, PcPIF1, PcPIF3, and PcPIF4 expressions in exocarp of pears. ASM also delayed the decrease of chlorophyll a and b contents, and inhibited the accumulation of β-carotene, lycopene and lutein, PcNYC1, PcHCAR, PcPPH, PcSGR1/2, PcPAO, PcPSY, PcLCYB, PcCRTZ2, PcCCS1 expressions, and promoted PcLCYE expression. PD98059 + ASM treatments depressed SOD and NOX activities and H₂ O₂ content, inhibited PcMAPKKK1, PcMAPKK3, PcMAPK6, PcPIF1, and PcPIF3 expressions, and promoted PcMYC2 and PcPIF4 expressions in exocarp of pears. Additionally, PD98059 + ASM accelerated PcNYC1, PcHCAR, PcPPH, PcSGR1/2, PcPAO, PcPSY, PcCYB, PcCRTZ2, and PcCCS1 expressions, thereby reducing chlorophyll a and b contents, and promoting β-carotene, lycopene and lutein contents.

CONCLUSIONS

Postharvest ASM treatment promoted the production of H₂ O₂ to activate the MAPK cascade, then phosphorylated/dephosphorylated transcription factors expression, and delayed chlorophyll decomposition and carotenoid synthesis in pears. © 2022 Society of Chemical Industry.

Engineering an oilseed crop for hyper-accumulation of carotenoids in the seeds without using a traditional marker gene

Ketocarotenoids were synthesized successfully in Camelina sativa seeds by genetic modification without using a traditional selection marker genes. This method provided an interesting tool for metabolic engineering of seed crops. Camelina sativa (L.) Crantz is an important oil crop with many excellent agronomic traits. This model oil plant has been exploited to accumulate value-added bioproducts using genetic manipulation that depends on antibiotic- or herbicide-based selection marker genes (SMG), one of the major concerns for genetically modified foods. Here we reported metabolic engineering of C. sativa to synthesize red ketocarotenoids that could serve as a reporter to visualize transgenic events without using a traditional SMG. Overexpression of a non-native β-carotene ketolase gene coupled with three other carotenogenous genes (phytoene synthase, β-carotene hydroxylase, and Orange) in C. sativa resulted in production of red seeds that were visibly distinguishable from the normal yellow ones. Constitutive expression of the transgenes led to delayed plant development and seed germination. In contrast, seed-specific transformants demonstrated normal growth and seed germination despite the accumulation of up to 70-fold the level of carotenoids in the seeds compared to the controls, including significant amounts of astaxanthin and keto-lutein. As a result, the transgenic seed oils exhibited much higher antioxidant activity. No significant changes were found in the profiles of fatty acids between transgenic and control seeds. This study provided an interesting tool for metabolic engineering of seed crops without using a disputed SMG.

Carotenoids: Dietary Sources, Extraction, Encapsulation, Bioavailability, and Health Benefits-A Review of Recent Advancements

Natural carotenoids (CARs), viz. β-carotene, lutein, astaxanthin, bixin, norbixin, capsanthin, lycopene, canthaxanthin, β-Apo-8-carotenal, zeaxanthin, and β-apo-8-carotenal-ester, are being studied as potential candidates in fields such as food, feed, nutraceuticals, and cosmeceuticals. CAR research is advancing in the following three major fields: (1) CAR production from natural sources and optimization of its downstream processing; (2) encapsulation for enhanced physical and chemical properties; and (3) preclinical, clinical, and epidemiological studies of CARs’ health benefits. This review critically discusses the recent developments in studies of the chemistry and antioxidant activity, marketing trends, dietary sources, extraction, bioaccessibility and bioavailability, encapsulation methods, dietary intake, and health benefits of CARs. Preclinical, clinical, and epidemiological studies on cancer, obesity, type 2 diabetes (T2D), cardiovascular diseases (CVD), osteoporosis, neurodegenerative disease, mental health, eye, and skin health are also discussed.

Phytoene Synthase: The Key Rate-Limiting Enzyme of Carotenoid Biosynthesis in Plants

Phytoene synthase (PSY) catalyzes the first committed step in the carotenoid biosynthesis pathway and is a major rate-limiting enzyme of carotenogenesis. PSY is highly regulated by various regulators and factors to modulate carotenoid biosynthesis in response to diverse developmental and environmental cues. Because of its critical role in controlling the total amount of synthesized carotenoids, PSY has been extensively investigated and engineered in plant species. However, much remains to be learned on its multifaceted regulatory control and its catalytic efficiency for carotenoid enrichment in crops. Here, we present current knowledge on the basic biology, the functional evolution, the dynamic regulation, and the metabolic engineering of PSY. We also discuss the open questions and gaps to stimulate additional research on this most studied gene/enzyme in the carotenogenic pathway.

Comparison of Aroma Trait of the White-Fleshed Peach ‘Hu Jing Mi Lu’ and the Yellow-Fleshed Peach ‘Jin Yuan’ Based on Odor Activity Value and Odor Characteristics

Peach flesh colors and aromas impact greatly on consumer behaviors and these two traits are closely associated in white- and yellow-fleshed peaches. However, current understanding of their aromas is rather limited and confined to the concentration differences of some volatiles. Therefore, this study aims to compare the overall aromas of the white-fleshed peach ‘Hu Jing Mi Lu’ (HJML) and yellow-fleshed peach ‘Jin Yuan’ (JY), two fresh cultivars with intense aromas and industrial influence by applications such as HS-SPME/GC-MS analysis, odor activity value evaluations, and odor note analysis. The significant contributions of 26 odor-active compounds to their aromas were revealed. Among which, 15 compounds showed no concentration differences and contributed to the fruity, floral, sweet, etc., odors in both HJML and JY; (E)-2-nonenal, 1-pentanol, and styrene showed significantly higher concentrations in HJML and conveyed much stronger fusel-like and balsamic odors; likewise, (Z)-3-hexenyl acetate, octanal, nonanal, and 3,5-octadien-2-one showed significantly higher concentrations in JY and conveyed much stronger banana, citrus-like, and honey odors; besides, benzyl alcohol, 1-heptanol, 1-octen-3-ol, and 3-octanone with woody, earthy, mushroom, and lavender odors were exclusively detected in HJML. Overall, apart from the common and stronger specific odors in either the white- or yellow-fleshed peach cultivar, the white-fleshed peach was endowed with a unique aroma.

The transcription factor complex CmAP3-CmPI-CmUIF1 modulates carotenoid metabolism by directly regulating carotenogenic gene CmCCD4a-2 in chrysanthemum

Carotenoids are one of the most important pigments for the coloring in many plants, fruits and flowers. Recently, significant progress has been made in carotenoid metabolism. However, the specific understanding on transcriptional regulation controlling the expression of carotenoid metabolic genes remains extremely limited. Anemone-type chrysanthemum, as a special group of chrysanthemum cultivars, contain elongated disc florets in capitulum, which usually appear in different colors compared with the ray florets since accumulating distinct content of carotenoids. In this study, the carotenoid composition and content of the ray and disc florets of an anemone-type chrysanthemum cultivar 'Dong Li Fen Gui' were analyzed by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) and the key structural gene CmCCD4a-2, of which differential expression resulted in the distinct content of carotenoids accumulated in these two types of florets, was identified. Then the promoter sequence of CmCCD4a-2 was used as bait to screen a chrysanthemum flower cDNA library and two transcription factors, CmAP3 and CmUIF1 were identified. Y2H, BiFC and Y3H experiments demonstrated that these two TFs were connected by CmPI to form CmAP3-CmPI-CmUIF1 TF complex. This TF complex regulated carotenoid metabolism through activating the expression of CmCCD4a-2 directly. Furthermore, a large number of target genes regulated directly by the CmAP3-CmPI-CmUIF1 TF complex, including carotenoid biosynthetic genes, flavonoid biosynthetic genes and flower development-related genes, were identified by DNA-affinity purification sequencing (DAP-seq), which indicated that the CmAP3-CmPI-CmUIF1 TF complex might participate in multiple processes. These findings expand our knowledge for the transcriptional regulation of carotenoid metabolism in plants and will be helpful to manipulating carotenoid accumulation in chrysanthemum.

Plant carotenoids: recent advances and future perspectives

Carotenoids are isoprenoid metabolites synthesized de novo in all photosynthetic organisms. Carotenoids are essential for plants with diverse functions in photosynthesis, photoprotection, pigmentation, phytohormone synthesis, and signaling. They are also critically important for humans as precursors of vitamin A synthesis and as dietary antioxidants. The vital roles of carotenoids to plants and humans have prompted significant progress toward our understanding of carotenoid metabolism and regulation. New regulators and novel roles of carotenoid metabolites are continuously revealed. This review focuses on current status of carotenoid metabolism and highlights recent advances in comprehension of the intrinsic and multi-dimensional regulation of carotenoid accumulation. We also discuss the functional evolution of carotenoids, the agricultural and horticultural application, and some key areas for future research.

Analyses of key gene networks controlling carotenoid metabolism in Xiangfen 1 banana

BACKGROUND

Banana fruits are rich in various high-value metabolites and play a key role in the human diet. Of these components, carotenoids have attracted considerable attention due to their physiological role and human health care functions. However, the accumulation patterns of carotenoids and genome-wide analysis of gene expression during banana fruit development have not been comprehensively evaluated.

RESULTS

In the present study, an integrative analysis of metabolites and transcriptome profiles in banana fruit with three different development stages was performed. A total of 11 carotenoid compounds were identified, and most of these compounds showed markedly higher abundances in mature green and/or mature fruit than in young fruit. Results were linked to the high expression of carotenoid synthesis and regulatory genes in the middle and late stages of fruit development. Co-expression network analysis revealed that 79 differentially expressed transcription factor genes may be responsible for the regulation of LCYB (lycopene β-cyclase), a key enzyme catalyzing the biosynthesis of α- and β-carotene.

CONCLUSIONS

Collectively, the study provided new insights into the understanding of dynamic changes in carotenoid content and gene expression level during banana fruit development.

Fruit ripening: dynamics and integrated analysis of carotenoids and anthocyanins

BACKGROUND

Fruits are vital food resources as they are loaded with bioactive compounds varying with different stages of ripening. As the fruit ripens, a dynamic color change is observed from green to yellow to red due to the biosynthesis of pigments like chlorophyll, carotenoids, and anthocyanins. Apart from making the fruit attractive and being a visual indicator of the ripening status, pigments add value to a ripened fruit by making them a source of nutraceuticals and industrial products. As the fruit matures, it undergoes biochemical changes which alter the pigment composition of fruits.

RESULTS

The synthesis, degradation and retention pathways of fruit pigments are mediated by hormonal, genetic, and environmental factors. Manipulation of the underlying regulatory mechanisms during fruit ripening suggests ways to enhance the desired pigments in fruits by biotechnological interventions. Here we report, in-depth insight into the dynamics of a pigment change in ripening and the regulatory mechanisms in action.

CONCLUSIONS

This review emphasizes the role of pigments as an asset to a ripened fruit as they augment the nutritive value, antioxidant levels and the net carbon gain of fruits; pigments are a source for fruit biofortification have tremendous industrial value along with being a tool to predict the harvest. This report will be of great utility to the harvesters, traders, consumers, and natural product divisions to extract the leading nutraceutical and industrial potential of preferred pigments biosynthesized at different fruit ripening stages.

The Genetic Components of a Natural Color Palette: A Comprehensive List of Carotenoid Pathway Mutations in Plants

Carotenoids comprise the most widely distributed natural pigments. In plants, they play indispensable roles in photosynthesis, furnish colors to flowers and fruit and serve as precursor molecules for the synthesis of apocarotenoids, including aroma and scent, phytohormones and other signaling molecules. Dietary carotenoids are vital to human health as a source of provitamin A and antioxidants. Hence, the enormous interest in carotenoids of crop plants. Over the past three decades, the carotenoid biosynthesis pathway has been mainly deciphered due to the characterization of natural and induced mutations that impair this process. Over the year, numerous mutations have been studied in dozens of plant species. Their phenotypes have significantly expanded our understanding of the biochemical and molecular processes underlying carotenoid accumulation in crops. Several of them were employed in the breeding of crops with higher nutritional value. This compendium of all known random and targeted mutants available in the carotenoid metabolic pathway in plants provides a valuable resource for future research on carotenoid biosynthesis in plant species.

Carotenoid Production from Microalgae: Biosynthesis, Salinity Responses and Novel Biotechnologies

Microalgae are excellent biological factories for high-value products and contain biofunctional carotenoids. Carotenoids are a group of natural pigments with high value in social production and human health. They have been widely used in food additives, pharmaceutics and cosmetics. Astaxanthin, β-carotene and lutein are currently the three carotenoids with the largest market share. Meanwhile, other less studied pigments, such as fucoxanthin and zeaxanthin, also exist in microalgae and have great biofunctional potentials. Since carotenoid accumulation is related to environments and cultivation of microalgae in seawater is a difficult biotechnological problem, the contributions of salt stress on carotenoid accumulation in microalgae need to be revealed for large-scale production. This review comprehensively summarizes the carotenoid biosynthesis and salinity responses of microalgae. Applications of salt stress to induce carotenoid accumulation, potentials of the Internet of Things in microalgae cultivation and future aspects for seawater cultivation are also discussed. As the global market share of carotenoids is still ascending, large-scale, economical and intelligent biotechnologies for carotenoid production play vital roles in the future microalgal economy.

Making extra room for carotenoids in plant cells: New opportunities for biofortification

Plant carotenoids are essential for photosynthesis and photoprotection and provide colors in the yellow to red range to non-photosynthetic organs such as petals and ripe fruits. They are also the precursors of biologically active molecules not only in plants (including hormones and retrograde signals) but also in animals (including retinoids such as vitamin A). A carotenoid-rich diet has been associated with improved health and cognitive capacity in humans, whereas the use of carotenoids as natural pigments is widespread in the agrofood and cosmetic industries. The nutritional and economic relevance of carotenoids has spurred a large number of biotechnological strategies to enrich plant tissues with carotenoids. Most of such approaches to alter carotenoid contents in plants have been focused on manipulating their biosynthesis or degradation, whereas improving carotenoid sink capacity in plant tissues has received much less attention. Our knowledge on the molecular mechanisms influencing carotenoid storage in plants has substantially grown in the last years, opening new opportunities for carotenoid biofortification. Here we will review these advances with a particular focus on those creating extra room for carotenoids in plant cells either by promoting the differentiation of carotenoid-sequestering structures within plastids or by transferring carotenoid production to the cytosol.

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.

Unravelling the Effect of Provitamin A Enrichment on Agronomic Performance of Tropical Maize Hybrids

Maize is consumed in different traditional diets as a source of macro- and micro-nutrients across Africa. Significant investment has thus been made to develop maize with high provitamin A content to complement other interventions for alleviating vitamin A deficiencies. The current breeding focus on increasing β-carotene levels to develop biofortified maize may affect the synthesis of other beneficial carotenoids. The changes in carotenoid profiles, which are commonly affected by environmental factors, may also lead to a trade-off with agronomic performance. The present study was therefore conducted to evaluate provitamin A biofortified maize hybrids across diverse field environments. The results showed that the difference in accumulating provitamin A and other beneficial carotenoids across variable growing environments was mainly regulated by the genetic backgrounds of the hybrids. Many hybrids, accumulating more than 10 µg/g of provitamin A, produced higher grain yields (>3600 kg/ha) than the orange commercial maize hybrid (3051 kg/ha). These hybrids were also competitive, compared to the orange commercial maize hybrid, in accumulating lutein and zeaxanthins. Our study showed that breeding for enhanced provitamin A content had no adverse effect on grain yield in the biofortified hybrids evaluated in the regional trials. Furthermore, the results highlighted the possibility of developing broadly adapted hybrids containing high levels of beneficial carotenoids for commercialization in areas with variable maize growing conditions in Africa.

Understanding colour retention in red chilli pepper fruit using a metabolite profiling approach

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Image analysis can be used to speed up identification of high retention phenotypes.

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High and low retention phenotypes can be identified by their volatomes.

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Low retention lines have increased unsaturated fatty acids.

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Post-harvest colour loss in chilli fruit involves lipid peroxidation.

Carotenoids are the pigments responsible for conferring the characteristic deep red colour to chilli pepper. The post-harvest retention of this colour is a key trait that governs the price of the produce. Determining colour retention and the associated underlying biochemical mechanisms are important issues that require investigation. In this present study, the ability of image analysis to determine colour change in ground chilli fruit was evaluated. This method enabled differentiation of extreme retention phenotypes whilst also reducing the duration of storage required to make accurate determinations. The analysis of volatiles indicated different levels of lipid and carotenoid derived volatiles in lines with different retention properties. Metabolite profiling of intermediary metabolism supported these findings, with increased levels of unsaturated fatty acids present in lines with low retention properties. Collectively, these data have led us to propose that in chilli fruit lipid peroxidation is one of the progenitors of carotenoid degradation.

Diversity of Plastid Types and Their Interconversions

Plastids are pivotal subcellular organelles that have evolved to perform specialized functions in plant cells, including photosynthesis and the production and storage of metabolites. They come in a variety of forms with different characteristics, enabling them to function in a diverse array of organ/tissue/cell-specific developmental processes and with a variety of environmental signals. Here, we have comprehensively reviewed the distinctive roles of plastids and their transition statuses, according to their features. Furthermore, the most recent understanding of their regulatory mechanisms is highlighted at both transcriptional and post-translational levels, with a focus on the greening and non-greening phenotypes.

Carotenoid cleavage dioxygenases (HD-CCD1A and B) contribute as strong negative regulators of β-carotene in Indian bread wheat (cv. HD2967)

Wheat (Triticum aestivum L.) is the most common cereal crop that is considered to be deficient in provitamin A carotenoids. Carotenoids are prone to degrade into apocarotenoids by the activity of carotenoid cleavage dioxygenases (CCDs). Hence, in this study, multiple CCDs were cloned from commercial Indian wheat cultivar HD2967 to understand their role in provitamin A carotenoids degradation. The homoeolog specific expression of HD-CCD1 and HD-CCD4 at different grain filling stages revealed the higher expression of transcripts arising from the A and B subgenomes of HD-CCD1. Furthermore, the grain development stages showed a strong negative correlation of HD-CCD1A (r = - 0.969) and B (r = - 0.970) homoeologs expression to that of β-carotene accumulation. It suggested that they could be potentially involved in deciding the turn-over of β-carotene in wheat grain. Three-dimensional (3D) structures for all six homoeologs of HD-CCD1 and HD-CCD4 were predicted using maize VP14 template to gain better insight into their molecular mechanism. Ramachandran plot assessment revealed that ~ 90% of residues are in the most favoured region. Docking studies with various carotenoid substrates revealed the higher affinity of HD-CCD1A and B for β-carotene and β-cryptoxanthin. Bacterial complementation analysis validated the functional role of all six homoeologs with HD-CCD1B showing the highest activity followed by HD-CCD1A for β-carotene degradation. Results of this study provide valuable insights into the characteristics of HD-CCDs in wheat and thereby justifying them (HD-CCD1A and B) as the candidate genes for employing genome editing tools for developing β-carotene enriched wheat grains.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02775-y.

Comparative transcriptome analyses shed light on carotenoid production and plastid development in melon fruit

Carotenoids, such as β-carotene, accumulate in chromoplasts of various fleshy fruits, awarding them with colors, aromas, and nutrients. The Orange (CmOr) gene controls β-carotene accumulation in melon fruit by posttranslationally enhancing carotenogenesis and repressing β-carotene turnover in chromoplasts. Carotenoid isomerase (CRTISO) isomerizes yellow prolycopene into red lycopene, a prerequisite for further metabolism into β-carotene. We comparatively analyzed the developing fruit transcriptomes of orange-colored melon and its two isogenic EMS-induced mutants, low-β (Cmor) and yofi (Cmcrtiso). The Cmor mutation in low-β caused a major transcriptomic change in the mature fruit. In contrast, the Cmcrtiso mutation in yofi significantly changed the transcriptome only in early fruit developmental stages. These findings indicate that melon fruit transcriptome is primarily altered by changes in carotenoid metabolic flux and plastid conversion, but minimally by carotenoid composition in the ripe fruit. Clustering of the differentially expressed genes into functional groups revealed an association between fruit carotenoid metabolic flux with the maintenance of the photosynthetic apparatus in fruit chloroplasts. Moreover, large numbers of thylakoid localized photosynthetic genes were differentially expressed in low-β. CmOR family proteins were found to physically interact with light-harvesting chlorophyll a-b binding proteins, suggesting a new role of CmOR for chloroplast maintenance in melon fruit. This study brings more insights into the cellular and metabolic processes associated with fruit carotenoid accumulation in melon fruit and reveals a new maintenance mechanism of the photosynthetic apparatus for plastid development.

Structural and functional features of phytoene synthase isoforms PSY1 and PSY2 in pepper Capsicum annuum L. cultivars

The fruits of various pepper cultivars are characterized by a different color, which is determined by the pigment ratio; carotenoids dominate in ripe fruits, while chlorophylls, in immature fruits. A key regulator of carotenoid biosynthesis is the phytoene synthase encoded by the PSY gene. The Capsicum annuum genome contains two isoforms of this enzyme, localized in leaf (PSY2) and fruit (PSY1) plastids. In this work, the complete PSY1 and PSY2 genes were identified in nine C. annuum cultivars, which differ in ripe fruit color. PSY1 and PSY2 sequence variability was 2.43 % (69 SNPs) and 1.21 % (36 SNPs). The most variable were PSY1 proteins of the cultivars 'Maria' (red-fruited) and 'Sladkij shokolad' (red-brown-fruited). All identified PSY1 and PSY2 homologs contained the phytoene synthase domain HH-IPPS and the transit peptide. In the PSY1 and PSY2 HH-IPPS domains, functionally significant sites were determined. For all accessions studied, the active sites (YAKTF and RAYV), aspartate-rich substrate-Mg²⁺-binding sites (DELVD and DVGED), and other functional residues were shown to be conserved. Transit peptides were more variable, and their similarity in the PSY1 and PSY2 proteins did not exceed 78.68 %. According to the biochemical data obtained, the largest amounts of chlorophylls and carotenoids across the cultivars studied were detected in immature and ripe fruits of the cv. 'Sladkij shokolad' and 'Shokoladnyj'. Also, ripe fruits of the cv. 'Nesozrevayuschij' (green-fruited) were marked by significant chlorophyll content, but a minimum of carotenoids. The PSY1 and PSY2 expression patterns were determined in the fruit pericarp at three ripening stages in 'Zheltyj buket', 'Sladkij shokolad', 'Karmin' and 'Nesozrevayuschij', which have different ripe fruit colors: yellow, red-brown, dark red and green, respectively. In the leaves of the cultivars studied, PSY1 expression levels varied significantly. All cultivars were characterized by increased PSY1 transcription as the fruit ripened; the maximum transcription level was found in the ripe fruit of 'Sladkij shokolad', and the lowest, in 'Nesozrevayuschij'. PSY2 transcripts were detected not only in the leaves and immature fruits, but also in ripe fruits. Assessment of a possible correlation of PSY1 and PSY2 transcription with carotenoid and chlorophyll content revealed a direct relationship between PSY1 expression level and carotenoid pigmentation during fruit ripening. It has been suggested that the absence of a typical pericarp pigmentation pattern in 'Nesozrevayuschij' may be associated with impaired chromoplast formation.