Synthesis and Function of Apocarotenoid Signals in Plants

Synthetic conversion of leaf chloroplasts into carotenoid-rich plastids reveals mechanistic basis of natural chromoplast development

Plastids, the defining organelles of plant cells, undergo physiological and morphological changes to fulfill distinct biological functions. In particular, the differentiation of chloroplasts into chromoplasts results in an enhanced storage capacity for carotenoids with industrial and nutritional value such as beta-carotene (provitamin A). Here, we show that synthetically inducing a burst in the production of phytoene, the first committed intermediate of the carotenoid pathway, elicits an artificial chloroplast-to-chromoplast differentiation in leaves. Phytoene overproduction initially interferes with photosynthesis, acting as a metabolic threshold switch mechanism that weakens chloroplast identity. In a second stage, phytoene conversion into downstream carotenoids is required for the differentiation of chromoplasts, a process that involves a concurrent reprogramming of nuclear gene expression and plastid morphology for improved carotenoid storage. We hence demonstrate that loss of photosynthetic competence and enhanced production of carotenoids are not just consequences but requirements for chloroplasts to differentiate into chromoplasts.

Mycorrhiza induced resistance (MIR): a defence developed through synergistic engagement of phytohormones, metabolites and rhizosphere

Plants get phosphorus, water and other soil nutrients at the cost of sugar through mycorrhizal symbiotic association. A common mycorrhizal network (CMN) - a dense network of mycorrhizal hyphae - provides a passage for exchange of chemicals and signals between the plants sharing CMN. Mycorrhisation impact plants at hormonal, physiological and metabolic level and successful symbiosis also regulates ecology of the plant rhizosphere. Apart from nutritional benefits, mycorrhisation provides an induced resistance to the plants known as mycorrhiza induced resistance (MIR). MIR is effective against soil as well as foliar pathogens and pest insects. In this review, molecular mechanisms underlying MIR such as role of phytohormones, their cross talk and priming effect are discussed. Evidence of MIR against economically important pathogens and pest insects in different plants is summarised. Mycorrhiza induces many plant secondary metabolites, many of which have a role in plant defence. Involvement of these secondary metabolites in mycorrhisation and their putative role in MIR are further reviewed. Controversies about MIR are also briefly discussed in order to provide insights on the scope for research about MIR. We have further extended our review with an open ended discussion about the possibilities for transgenerational MIR.

Combined metabolome and transcriptome analyses of photosynthetic pigments in red maple

Leaf color is affected primarily by the content and proportion of various pigments. Not only do photosynthetic pigments display various colors, they are also key to the physiological activities of plants. To precisely elucidate the mechanisms of chlorophyll and carotenoid biosynthesis and their interactions in red maple, transcriptomic and metabolomic profiling were performed on green, red and yellow leaves from a selected red maple mutant. In the chlorophyll pathway, ArPOR1, ArCLH2, and ArCLH4 positively regulated chlorophyll a, and made it the dominant pigment in green leaves, whereas ArNOL8 and ArNOL14 negatively regulated chlorophyll synthesis. In the carotenoid pathway, under the positive regulation of ArLUT5-1 and ArLUT5-4, the synthesis of carotenoids changed from α-branch to β-branch during the gradual senescence of leaves. Through integrated data analysis, we obtained genes that directly coordinated between carotenoid and chlorophyll biosynthesis. Ultimately, the results of this paper will be a valuable resource for future research on gene-to-metabolite correlations in red maple, and provide basic information toward facilitating the improvement of color in foliage plants.

Heat stress response in the closest algal relatives of land plants reveals conserved stress signaling circuits

All land plants (embryophytes) share a common ancestor that likely evolved from a filamentous freshwater alga. Elucidating the transition from algae to embryophytes - and the eventual conquering of Earth's surface - is one of the most fundamental questions in plant evolutionary biology. Here, we investigated one of the organismal properties that might have enabled this transition: resistance to drastic temperature shifts. We explored the effect of heat stress in Mougeotia and Spirogyra, two representatives of Zygnematophyceae - the closest known algal sister lineage to land plants. Heat stress induced pronounced phenotypic alterations in their plastids, and high-performance liquid chromatography-tandem mass spectroscopy-based profiling of 565 transitions for the analysis of main central metabolites revealed significant shifts in 43 compounds. We also analyzed the global differential gene expression responses triggered by heat, generating 92.8 Gbp of sequence data and assembling a combined set of 8905 well-expressed genes. Each organism had its own distinct gene expression profile; less than one-half of their shared genes showed concordant gene expression trends. We nevertheless detected common signature responses to heat such as elevated transcript levels for molecular chaperones, thylakoid components, and - corroborating our metabolomic data - amino acid metabolism. We also uncovered the heat-stress responsiveness of genes for phosphorelay-based signal transduction that links environmental cues, calcium signatures and plastid biology. Our data allow us to infer the molecular heat stress response that the earliest land plants might have used when facing the rapidly shifting temperature conditions of the terrestrial habitat.

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

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

On the biosynthesis and evolution of apocarotenoid plant growth regulators

Carotenoids are an important source of metabolites with regulatory function, which include the plant hormones abscisic acid (ABA) and strigolactones (SLs), and several recently identified growth regulators and signaling molecules. These carotenoid-derivatives originate from oxidative breakdown of double bonds in the carotenoid polyene, a common metabolic process that gives rise to diverse carbonyl cleavage-products known as apocarotenoids. Apocarotenoids exert biologically important functions in all taxa. In plants, they are a major regulator of plant growth, development and response to biotic and abiotic environmental stimuli, and mediate plant's communication with surrounding organisms. In this article, we provide a general overview on the biology of plant apocarotenoids, focusing on ABA, SLs, and recently identified apocarotenoid growth regulators. Following an introduction on carotenoids, we describe plant apocarotenoid biosynthesis, signal transduction, and evolution and summarize their biological functions. Moreover, we discuss the evolution of these intriguing metabolites, which has not been adequately addressed in the literature.

Apocarotenoids profiling in different Capsicum species

The present study report on the application of an on line supercritical fluid extraction-supercritical fluid chromatography-triple quadrupole/mass spectrometry methodology to the first apocarotenoids profiling of seventeen different chilli peppers cultivars belonging to Capsicum annuum, Capsicum baccatum and Capsicum chinense species. A total of 19 free apocarotenoids and 8 apocarotenoids fatty acid esters were identified; β-Apo-8'-carotenal and Apo-8'-zeaxanthinal were also quantified and the β-Apo-8'-carotenal occurrence was in the percentage ranges relative to β-carotene of 0.03-3.87%. PCA was performed as a multivariate display method on the quantified carotenoids and apocarotenoids, in order to visualize the data structure. Moreover, different ε-apoluteinals and 4-oxo-apo-β-carotenals were detected in Capsicum species also for the first time and, to the best of authors knowledge, in any food matrix.

Tandem gene duplications drive divergent evolution of caffeine and crocin biosynthetic pathways in plants

Background

Plants have evolved a panoply of specialized metabolites that increase their environmental fitness. Two examples are caffeine, a purine psychotropic alkaloid, and crocins, a group of glycosylated apocarotenoid pigments. Both classes of compounds are found in a handful of distantly related plant genera (Coffea, Camellia, Paullinia, and Ilex for caffeine; Crocus, Buddleja, and Gardenia for crocins) wherein they presumably evolved through convergent evolution. The closely related Coffea and Gardenia genera belong to the Rubiaceae family and synthesize, respectively, caffeine and crocins in their fruits.

Results

Here, we report a chromosomal-level genome assembly of Gardenia jasminoides, a crocin-producing species, obtained using Oxford Nanopore sequencing and Hi-C technology. Through genomic and functional assays, we completely deciphered for the first time in any plant the dedicated pathway of crocin biosynthesis. Through comparative analyses with Coffea canephora and other eudicot genomes, we show that Coffea caffeine synthases and the first dedicated gene in the Gardenia crocin pathway, GjCCD4a, evolved through recent tandem gene duplications in the two different genera, respectively. In contrast, genes encoding later steps of the Gardenia crocin pathway, ALDH and UGT, evolved through more ancient gene duplications and were presumably recruited into the crocin biosynthetic pathway only after the evolution of the GjCCD4a gene.

Conclusions

This study shows duplication-based divergent evolution within the coffee family (Rubiaceae) of two characteristic secondary metabolic pathways, caffeine and crocin biosynthesis, from a common ancestor that possessed neither complete pathway. These findings provide significant insights on the role of tandem duplications in the evolution of plant specialized metabolism.

Research Advances on Biosynthesis, Regulation, and Biological Activities of Apocarotenoid Aroma in Horticultural Plants

Apocarotenoids, which play important roles in the growth and development of horticultural plants, are produced by the action of carotenoid cleavage oxygenase (CCO) family members or nonenzymatic cleavage actions. Apocarotenoids are commonly found in leaves, flowers, and fruits of many horticultural plants and participate in the formation of pigments, flavors, hormones, and signaling compounds. Some of them are recognized as important aroma components of fruit and flower with aromatic odor, such as βß-ionone, β-damascenone, and 6-methyl-5-hepten-2-one in tomato fruit, and have low odor thresholds with β-ionone having odor threshold of only 0.007 ppb. In this review, the main apocarotenoid aroma components in horticultural plants were listed, and factors influencing their production were discussed at first. Then, the biosynthetic pathway of apocarotenoid aromas was briefly introduced, and the CCDs gene family was highlighted, and the nonenzymatic production of apocarotenoid aromas was also mentioned. Next, chemical and molecular regulations of apocarotenoid aromas and their biological activities were summarized. Finally, further exploration aspects needed were suggested. We anticipate that this review can afford some crucial information for comprehensive application of apocarotenoid volatile compounds in horticultural plants.

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.

Distinct gene expression and secondary metabolite profiles in suppressor of prosystemin-mediated responses2 (spr2) tomato mutants having impaired mycorrhizal colonization

Arbuscular mycorrhizal fungi (AMF) colonization, sampled at 32-50 days post-inoculation (dpi), was significantly reduced in suppressor of prosystemin-mediated responses2 (spr2) mutant tomato plants impaired in the ω-3 FATTY ACID DESATURASE7 (FAD7) gene that limits the generation of linolenic acid and, consequently, the wound-responsive jasmonic acid (JA) burst. Contrary to wild-type (WT) plants, JA levels in root and leaves of spr2 mutants remained unchanged in response to AMF colonization, further supporting its regulatory role in the AM symbiosis. Decreased AMF colonization in spr2 plants was also linked to alterations associated with a disrupted FAD7 function, such as enhanced salicylic acid (SA) levels and SA-related defense gene expression and a reduction in fatty acid content in both mycorrhizal spr2 roots and leaves. Transcriptomic data revealed that lower mycorrhizal colonization efficiency in spr2 mutants coincided with the modified expression of key genes controlling gibberellin and ethylene signaling, brassinosteroid, ethylene, apocarotenoid and phenylpropanoid synthesis, and the wound response. Targeted metabolomic analysis, performed at 45 dpi, revealed augmented contents of L-threonic acid and DL-malic acid in colonized spr2 roots which suggested unfavorable conditions for AMF colonization. Additionally, time- and genotype-dependent changes in root steroid glycoalkaloid levels, including tomatine, suggested that these metabolites might positively regulate the AM symbiosis in tomato. Untargeted metabolomic analysis demonstrated that the tomato root metabolomes were distinctly affected by genotype, mycorrhizal colonization and colonization time. In conclusion, reduced AMF colonization efficiency in spr2 mutants is probably caused by multiple and interconnected JA-dependent and independent gene expression and metabolomic alterations.

Modeling Dynamic Conformations of Organic Molecules: Alkyne Carotenoids in Solution

Calculating the spectroscopic properties of complex conjugated organic molecules in their relaxed state is far from simple. An additional complexity arises for flexible molecules in solution, where the rotational energy barriers are low enough so that nonminimum conformations may become dynamically populated. These metastable conformations quickly relax during the minimization procedures preliminary to density functional theory calculations, and so accounting for their contribution to the experimentally observed properties is problematic. We describe a strategy for stabilizing these nonminimum conformations in silico, allowing their properties to be calculated. Diadinoxanthin and alloxanthin present atypical vibrational properties in solution, indicating the presence of several conformations. Performing energy calculations in vacuo and polarizable continuum model calculations in different solvents, we found three different conformations with values for the δ dihedral angle of the end ring ca. 0, 180, and 90° with respect to the plane of the conjugated chain. The latter conformation, a nonglobal minimum, is not stable during the minimization necessary for modeling its spectroscopic properties. To circumvent this classical problem, we used a Car–Parinello MD supermolecular approach, in which diadinoxanthin was solvated by water molecules so that metastable conformations were stabilized by hydrogen-bonding interactions. We progressively removed the number of solvating waters to find the minimum required for this stabilization. This strategy represents the first modeling of a carotenoid in a distorted conformation and provides an accurate interpretation of the experimental data.

Full-length transcriptome sequencing provides insights into the evolution of apocarotenoid biosynthesis in Crocus sativus

Crocus sativus, containing remarkably amounts of crocin, picrocrocin and safranal, is the source of saffron with tremendous medicinal, economic and cultural importance. Here, we present a high-quality full-length transcriptome of the sterile triploid C. sativus, using the PacBio SMRT sequencing technology. This yields 31,755 high-confidence predictions of protein-coding genes, with 50.1% forming paralogous gene pairs. Analysis on distribution of Ks values suggests that the current genome of C. sativus is probably a product resulting from at least two rounds of whole-genome duplication (WGD) events occurred at ~28 and ~114 million years ago (Mya), respectively. We provide evidence demonstrating that the recent β WGD event confers a major impact on family expansion of secondary metabolite genes, possibly leading to an enhanced accumulation of three distinct compounds: crocin, picrocrocin and safranal. Phylogenetic analysis unravels that the founding member (CCD2) of CCD enzymes necessary for the biosynthesis of apocarotenoids in C. sativus might be evolved from the CCD1 family via the β WGD event. Based on the gene expression profiling, CCD2 is found to be expressed at an extremely high level in the stigma. These findings may shed lights on further genomic refinement of the characteristic biosynthesis pathways and promote germplasm utilization for the improvement of saffron quality.

The physiological mechanism underlying root elongation in response to nitrogen deficiency in crop plants

In response to low nitrogen stress, multiple hormones together with nitric oxide signaling pathways work synergistically and antagonistically in crop root elongation. Changing root morphology allows plants to adapt to soil nutrient availability. Nitrogen is the most important essential nutrient for plant growth. An important adaptive strategy for crops responding to nitrogen deficiency is root elongation, thereby accessing increased soil space and nitrogen resources. Multiple signaling pathways are involved in this regulatory network, working together to fine-tune root elongation in response to soil nitrogen availability. Based on existing research, we propose a model to explain how different signaling pathways interact to regulate root elongation in response to low nitrogen stress. In response to a low shoot nitrogen status signal, auxin transport from the shoot to the root increases. High auxin levels in the root tip stimulate the production of nitric oxide, which promotes the synthesis of strigolactones to accelerate cell division. In this process, cytokinin, ethylene, and abscisic acid play an antagonistic role, while brassinosteroids and auxin play a synergistic role in regulating root elongation. Further study is required to identify the QTLs, genes, and favorable alleles which control the root elongation response to low nitrogen stress in crops.

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.

Banana MaSPL16 Modulates Carotenoid Biosynthesis during Fruit Ripening through Activating the Transcription of Lycopene β-Cyclase Genes

Carotenoids are a class of bioactive compounds that exhibit health-promoting properties for humans, but their regulation in bananas during fruit ripening remains largely unclear. Here, we found that the total carotenoid content continued to be elevated along the course of banana ripening and peaked at the ripening stage followed by a decrease, which is presumably caused by the transcript abundances of carotenoid biosynthetic genes MaLCYB1.1 and MaLCYB1.2. Moreover, a ripening-inducible transcription factor MaSPL16 was characterized, which was a nuclear protein with transactivation activity. Transient transformation of MaSPL16 in banana fruits led to enhanced transcript levels of MaLCYB1.1 and MaLCYB1.2 and hence the total carotenoid accumulation. Importantly, MaSPL16 stimulated the transcription of MaLCYB1.1 and MaLCYB1.2 through directly binding to their promoters. Collectively, our findings indicate that MaSPL16 behaves as an activator to modulate banana carotenoid biosynthesis, which may provide a new target for molecular improvement of the nutritional and bioactive qualities of agricultural crops that accumulate carotenoids.

A cis-carotene derived apocarotenoid regulates etioplast and chloroplast development

Carotenoids are a core plastid component and yet their regulatory function during plastid biogenesis remains enigmatic. A unique carotenoid biosynthesis mutant, carotenoid chloroplast regulation 2 (ccr2), that has no prolamellar body (PLB) and normal PROTOCHLOROPHYLLIDE OXIDOREDUCTASE (POR) levels, was used to demonstrate a regulatory function for carotenoids and their derivatives under varied dark-light regimes. A forward genetics approach revealed how an epistatic interaction between a ζ-carotene isomerase mutant (ziso-155) and ccr2 blocked the biosynthesis of specific cis-carotenes and restored PLB formation in etioplasts. We attributed this to a novel apocarotenoid retrograde signal, as chemical inhibition of carotenoid cleavage dioxygenase activity restored PLB formation in ccr2 etioplasts during skotomorphogenesis. The apocarotenoid acted in parallel to the repressor of photomorphogenesis, DEETIOLATED1 (DET1), to transcriptionally regulate PROTOCHLOROPHYLLIDE OXIDOREDUCTASE (POR), PHYTOCHROME INTERACTING FACTOR3 (PIF3) and ELONGATED HYPOCOTYL5 (HY5). The unknown apocarotenoid signal restored POR protein levels and PLB formation in det1, thereby controlling plastid development.

CRISPR/Cas9 directed editing of lycopene epsilon-cyclase modulates metabolic flux for β-carotene biosynthesis in banana fruit

Banana is one of the most economically important fruit crops worldwide. Genetic improvement in banana is a challenging task due to its parthenocarpic nature and triploid genome. Genetic modification of crops via the CRISPR/Cas9 module has emerged as a promising tool to develop important traits. In the present work, a CRISPR/Cas9-based approach was used to develop the β-carotene-enriched Cavendish banana cultivar (cv.) Grand Naine (AAA genome). The fifth exon of the lycopene epsilon-cyclase (LCYε) gene was targeted. The targeting specificity of the designed guide-RNA was also tested by its ability to create indels in the LCYε gene at the A genome of cv. Rasthali (AAB genome). Sequence analysis revealed multiple types of indels in the genomic region of Grand Naine LCYε (GN-LCYε). Metabolic profiling of the fruit pulp of selected edited lines showed enhanced accumulation of β-carotene content up to 6-fold (~24 μg/g) compared with the unedited plants. These lines also showed either an absence or a drastic reduction in the levels of lutein and α-carotene, suggesting metabolic reprogramming, without any significant effect on the agro-morphological parameters. In addition, differential expression of carotenoid pathway genes was observed in the edited lines in comparison to unedited plants. Overall, this is the first report in banana to improve nutritional trait by using a precise genome editing approach.

Determination of free apocarotenoids and apocarotenoid esters in human colostrum

The presence of carotenoids in human colostrum has been reported in the literature, and xanthophyll esters in human colostrum were recently detected for the first time. However, no published studies have reported whether apocarotenoids, which are metabolites derived from carotenoid enzymatic or nonenzymatic oxidative cleavage, are present in human colostrum. Therefore, the purpose of the present study was to search for the possible occurrence of apocarotenoids, including apocarotenoid esters, in human colostrum for the first time by applying an online supercritical fluid extraction-supercritical fluid chromatography-tandem mass spectrometry methodology. Recent evidence related to apocarotenoid transcriptional activity has suggested that they may have beneficial health properties superior to those of their parent carotenoids. Three different apocarotenoids, namely apo-8'-β-carotenal, apo-8'-lycopenal, and β-citraurin, were identified in intact human colostrum samples, with average concentrations of 85 nmol L^-1, 54.6 nmol L^-1, and 75.4 nmol L^-1, respectively. The overall detection of 16 different free apocarotenoids and 10 different apocarotenoid fatty acid esters in human colostrum was achieved here for the first time. Their occurrence in human colostrum certainly has implications for newborn health status, since colostrum is the only form of food for the newborn during the very first days of life. Graphical abstract.

cis/trans Carotenoid Extraction, Purification, Detection, Quantification, and Profiling in Plant Tissues

Reverse phase high-performance liquid chromatography (HPLC) is the method of choice used in biological, health, and food research to identify, quantify, and profile carotenoid species. The identification and quantification of cis- and/or trans-carotene and xanthophyll isomers in plant tissues can be affected by the method of sample preparation and extraction, as well as the HPLC column chemistry and the solvent gradient. There is a high degree of heterogeneity in existing methods in terms of their ease, efficiency, and accuracy. We describe a simple carotenoid extraction method and two different optimised HPLC methods utilizing C18 or C30 reverse-phase columns. We outline applications, advantages, and disadvantages for using these reverse phase columns to detect xanthophylls and cis-carotenes in wild-type photosynthetic leaves and mutant dark-grown etiolated seedlings, respectively. Resources are provided to profile individual species based upon their spectral properties and retention time, as well as quantify carotenoids by their composition and absolute levels in different plant tissues.

Pathways for Carotenoid Biosynthesis, Degradation, and Storage

Carotenoids are isoprenoid compounds synthesized de novo in all photosynthetic organisms as well as in some nonphotosynthetic bacteria and fungi. In plants, carotenoids are essential for light harvesting and photoprotection. They contribute to the vivid color found in many plant organs. The cleavage of carotenoids produces small molecules (apocarotenoids) that serve as aroma compounds, as well as phytohormones and signals to affect plant growth and development. Since carotenoids provide valuable nutrition and health benefits for humans, understanding of carotenoid biosynthesis, catabolism and storage is important for biofortification of crops with improved nutritional quality. This chapter primarily introduces our current knowledge about carotenoid biosynthesis and degradation pathways as well as carotenoid storage in plants.

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

Carotenoids are essential pigments to plants and important natural products to humans. Carotenoids as both primary and specialized metabolites fulfill multifaceted functions in plants. As such, carotenoid accumulation (a net process of biosynthesis, degradation and sequestration) is subjected to complicated regulation throughout plant life cycle in response to developmental and environmental signals. Investigation of transcriptional regulation of carotenoid metabolic genes remains the focus in understanding the regulatory control of carotenoid accumulation. While discovery of bona fide carotenoid metabolic regulators is still challenging, the recent progress of identification of various transcription factors and regulators helps us to construct hierarchical regulatory network of carotenoid accumulation. The elucidation of carotenoid regulatory mechanisms at protein level and in chromoplast provides some insights into post-translational regulation of carotenogenic enzymes and carotenoid sequestration in plastid sink. This review briefly describes the pathways and main flux-controlling steps for carotenoid accumulation in plants. It highlights our recent understanding of the regulatory mechanisms underlying carotenoid accumulation at both transcriptional and post-translational levels. It also discusses the opportunities to expand toolbox for further shedding light upon the intrinsic regulation of carotenoid accumulation in plants.

Transportomics for the Characterization of Plant Apocarotenoid Transmembrane Transporters

Apocarotenoids are carotenoid derivatives produced by the nonenzymatic or enzymatic cleavage of carotenoids, followed by different enzymatic modifications. In plants, apocarotenoids play different roles, such as attraction of pollinators and seeds dispersal, defense against pathogens and herbivores, protection against photo-oxidative stresses, stimulation and inhibition of plant growth and regulation of biological processes in the case of phytohormones abscisic acid and strigolactones. While carotenoids are in general plastid-localized metabolites, apocarotenoids can reach different final destinations inside or outside the cell. The mechanisms of apocarotenoid transport through biological membranes have been poorly studied. This chapter describes a method to characterize transmembrane transporters involved in the transport of polar and amphipathic apocarotenoids. This protocol was successfully used to in vitro characterize the transport activity of ATP-binding cassette (ABC) and multidrug and toxic extrusion (MATE) in microsomes isolated from Saccharomyces cerevisiae expressing these plant transporters.

C13 Megastigmane Derivatives From Epipremnum pinnatum: β-Damascenone Inhibits the Expression of Pro-Inflammatory Cytokines and Leukocyte Adhesion Molecules as Well as NF-κB Signaling

In order to identify active constituents and to gain some information regarding their mode of action, extracts from leaves of Epipremnum pinnatum were tested for their ability to inhibit inflammatory gene expression in endothelial- and monocyte-like cells (HUVECtert and THP-1, respectively). Bioactivity-guided fractionation using expression of PTGS2 (COX-2) mRNA as a readout resulted in the isolation of two C13 megastigmane glycosides, gusanlungionoside C (1) and citroside A (3), and the phenylalcohol glycoside phenylmethyl-2-O-(6-O-rhamnosyl)-ß-D-galactopyranoside (2). Further analysis identified six additional megastigmane glycosides and the aglycones β-damascenone (10), megastigmatrienone (11), 3-hydroxy-β-damascenone (12), and 3-oxo-7,8-dihydro-α-ionol (13). Pharmacological analysis demonstrated that 10 inhibits LPS-stimulated induction of mRNAs encoding for proinflammatory cytokines and leukocyte adhesion molecules, such as TNF-α, IL-1β, IL-8, COX-2, E-selectin, ICAM-1, and VCAM-1 in HUVECtert and THP-1 cells. 10 inhibited induction of inflammatory genes in HUVECtert and THP-1 cells treated with different agonists, such as TNF-α, IL-1β, and LPS. In addition to mRNA, also the upregulation of inflammatory proteins was inhibited by 10 as demonstrated by immune assays for cell surface E-selectin and secreted TNF-α. Finally, using a luciferase reporter construct, it was shown, that 10 inhibits NF-κB-dependent transcription. Therefore, we hypothesize that inhibition of NF-κB by β-damascenone (10) may represent one of the mechanisms underlying the in vitro anti-inflammatory activity of Epipremnum pinnatum extracts.

Integrating Enzyme and Metabolic Engineering Tools for Enhanced α-Ionone Production

Metabolic engineering aims to balance intracellular pathways and increase the precursor supply. However, some heterologous enzymes are not evolved to support high flux. To remove the limitation, the catalytic properties of rate-limiting enzymes must be enhanced. Here, we engineered carotenoid cleavage dioxygenase 1 (CCD1), whose intrinsic promiscuity and low activity limited the production of α-ionone in Escherichia coli. Site-directed mutagenesis was carried out to mutate three structural elements of CCD1: an active site loop, η-helices, and α-helices. Furthermore, mutated CCD1 was fused with lycopene ε-cyclase to facilitate substrate channelling. Collectively, these methods improved the α-ionone concentration by >2.5-fold compared to our previously optimized strain. Lastly, the engineered enzyme was used in conjunction with the metabolic engineering strategy to further boost the α-ionone concentration by another 20%. This work deepens our understanding of CCD1 catalytic properties and proves that integrating enzyme and metabolic engineering can be synergistic for a higher microbial production yield.

Carotenoid Cleavage Dioxygenases: Identification, Expression, and Evolutionary Analysis of This Gene Family in Tobacco

Carotenoid cleavage dioxygenases (CCDs) selectively catalyze carotenoids, forming smaller apocarotenoids that are essential for the synthesis of apocarotenoid flavor, aroma volatiles, and phytohormone ABA/SLs, as well as responses to abiotic stresses. Here, 19, 11, and 10 CCD genes were identified in Nicotiana tabacum, Nicotiana tomentosiformis, and Nicotiana sylvestris, respectively. For this family, we systematically analyzed phylogeny, gene structure, conserved motifs, gene duplications, cis-elements, subcellular and chromosomal localization, miRNA-target sites, expression patterns with different treatments, and molecular evolution. CCD genes were classified into two subfamilies and nine groups. Gene structures, motifs, and tertiary structures showed similarities within the same groups. Subcellular localization analysis predicted that CCD family genes are cytoplasmic and plastid-localized, which was confirmed experimentally. Evolutionary analysis showed that purifying selection dominated the evolution of these genes. Meanwhile, seven positive sites were identified on the ancestor branch of the tobacco CCD subfamily. Cis-regulatory elements of the CCD promoters were mainly involved in light-responsiveness, hormone treatment, and physiological stress. Different CCD family genes were predominantly expressed separately in roots, flowers, seeds, and leaves and exhibited divergent expression patterns with different hormones (ABA, MeJA, IAA, SA) and abiotic (drought, cold, heat) stresses. This study provides a comprehensive overview of the NtCCD gene family and a foundation for future functional characterization of individual genes.

Apocarotenoids Involved in Plant Development and Stress Response

Carotenoids are isoprenoid pigments synthesized by all photosynthetic organisms and many heterotrophic microorganisms. They are equipped with a conjugated double-bond system that builds the basis for their role in harvesting light energy and in protecting the cell from photo-oxidation. In addition, the carotenoids polyene makes them susceptible to oxidative cleavage, yielding carbonyl products called apocarotenoids. This oxidation can be catalyzed by carotenoid cleavage dioxygenases or triggered nonenzymatically by reactive oxygen species. The group of plant apocarotenoids includes important phytohormones, such as abscisic acid and strigolactones, and signaling molecules, such as β-cyclocitral. Abscisic acid is a key regulator of plant's response to abiotic stress and is involved in different developmental processes, such as seed dormancy. Strigolactone is a main regulator of plant architecture and an important signaling molecule in the plant-rhizosphere communication. β-Cyclocitral, a volatile derived from β-carotene oxidation, mediates the response of cells to singlet oxygen stress. Besides these well-known examples, recent research unraveled novel apocarotenoid growth regulators and suggests the presence of yet unidentified ones. In this review, we describe the biosynthesis and biological functions of established regulatory apocarotenoids and touch on the recently identified anchorene and zaxinone, with emphasis on their role in plant growth, development, and stress response.

Emerging strategies for the identification of protein-metabolite interactions

Interactions between biological molecules enable life. The significance of a cell-wide understanding of molecular complexes is thus obvious. In comparison to protein-protein interactions, protein-metabolite interactions remain under-studied. However, this has been gradually changing due to technological progress. Here, we focus on the interactions between ligands and receptors, the triggers of signalling events. While the number of small molecules with proven or proposed signalling roles is rapidly growing, most of their protein receptors remain unknown. Conversely, there are numerous signalling proteins with predicted ligand-binding domains for which the identities of the metabolite counterparts remain elusive. Here, we discuss the current biochemical strategies for identifying protein-metabolite interactions and how they can be used to characterize known metabolite regulators and identify novel ones.

Transcriptional Regulation of Carotenoid Biosynthesis in Plants: So Many Regulators, So Little Consensus

In plants, the carotenoid biosynthesis pathway (CBP) is essential for the production of photosynthetic and protective pigments, plant hormones, and visual/olfactory attractants for animal pollinators and seed dispersers. The regulation of carotenoid biosynthesis at the transcriptional level is vitally important for all of these functions and has been the subject of intensive research. Many putative transcriptional regulators, both direct and indirect, have been identified through conventional mutant analysis, transcriptome profiling, yeast one-hybrid screening, and candidate gene approaches. Despite this progress, our understanding of the transcriptional regulation of carotenoid biosynthesis remains fragmented and incomplete. Frequently, a stimulus or regulator is known, but the mechanism by which it affects transcription has not been elucidated. In other cases, mechanisms have been proposed (such as direct binding of a CBP gene promoter by a transcription factor), but function was tested only in vitro or in heterologous systems, making it unclear whether these proteins actually play a role in carotenoid regulation in their endogenous environments. Even in cases where the mechanism is relatively well understood, regulators are often studied in isolation, either in a single plant species or outside the context of other known regulators. This presents a conundrum: why so many candidate regulators but so little consensus? Here we summarize current knowledge on transcriptional regulation of the CBP, lay out the challenges contributing to this conundrum, identify remaining knowledge gaps, and suggest future research directions to address these challenges and knowledge gaps.

First Apocarotenoids Profiling of Four Microalgae Strains

Both enzymatic or oxidative carotenoids cleavages can often occur in nature and produce a wide range of bioactive apocarotenoids. Considering that no detailed information is available in the literature regarding the occurrence of apocarotenoids in microalgae species, the aim of this study was to study the extraction and characterization of apocarotenoids in four different microalgae strains: Chlamydomonas sp. CCMP 2294, Tetraselmis chuii SAG 8-6, Nannochloropsis gaditana CCMP 526, and Chlorella sorokiniana NIVA-CHL 176. This was done for the first time using an online method coupling supercritical fluid extraction and supercritical fluid chromatography tandem mass spectrometry. A total of 29 different apocarotenoids, including various apocarotenoid fatty acid esters, were detected: apo-12’-zeaxanthinal, β-apo-12’-carotenal, apo-12-luteinal, and apo-12’-violaxanthal. These were detected in all the investigated strains together with the two apocarotenoid esters, apo-10’-zeaxanthinal-C4:0 and apo-8’-zeaxanthinal-C8:0. The overall extraction and detection time for the apocarotenoids was less than 10 min, including apocarotenoids esters, with an overall analysis time of less than 20 min. Moreover, preliminary quantitative data showed that the β-apo-8’-carotenal content was around 0.8% and 2.4% of the parent carotenoid, in the C. sorokiniana and T. chuii strains, respectively. This methodology could be applied as a selective and efficient method for the apocarotenoids detection.

From Central to Specialized Metabolism: An Overview of Some Secondary Compounds Derived From the Primary Metabolism for Their Role in Conferring Nutritional and Organoleptic Characteristics to Fruit

Fruit flavor and nutritional characteristics are key quality traits and ones of the main factors influencing consumer preference. Central carbon metabolism, also known as primary metabolism, contributes to the synthesis of intermediate compounds that act as precursors for plant secondary metabolism. Specific and specialized metabolic pathways that evolved from primary metabolism play a key role in the plant's interaction with its environment. In particular, secondary metabolites present in the fruit serve to increase its attractiveness to seed dispersers and to protect it against biotic and abiotic stresses. As a consequence, several important organoleptic characteristics, such as aroma, color, and fruit nutritional value, rely upon secondary metabolite content. Phenolic and terpenoid compounds are large and diverse classes of secondary metabolites that contribute to fruit quality and have their origin in primary metabolic pathways, while the delicate aroma of ripe fruits is formed by a unique combination of hundreds of volatiles that are derived from primary metabolites. In this review, we show that the manipulation of primary metabolism is a powerful tool to engineer quality traits in fruits, such as the phenolic, terpenoid, and volatile content. The enzymatic reactions responsible for the accumulation of primary precursors are bottlenecks in the transfer of metabolic flux from central to specialized metabolism and should be taken into account to increase the yield of the final products of the biosynthetic pathways. In addition, understanding the connection and regulation of the carbon flow between primary and secondary metabolism is a key factor for the development of fruit cultivars with enhanced organoleptic and nutritional traits.

Effects of leaf colorness, pigment contents and allelochemicals on the orientation of the Asian citrus psyllid among four Rutaceae host plants

BACKGROUND

Asian citrus psyllid (ACP) is the primary vector responsible for the transmission of the phloem-limited bacteria Candidatus Liberibacter spp., associated with huanglongbing (HLB), which causes great loss to the citrus industry. Although the roles of leaf color and volatile compounds in the orientation of ACP have been proven, the quantification of color and allelochemicals in the host plant are kept unclear, especially in wild citrus germplasms.

RESULTS

Chongyi wild mandarin significantly attracted more ACP than wild Hong Kong kumquat, 'Gannan zao' navel orange and orange jasmine did in the four-choice and olfactometer assays. The color parameters of the tender leaves from Chongyi wild mandarin and 'Gannan zao' were similar. The yellow color in both of them was less saturated than that of the other two plants species, but Chongyi wild mandarin had significant lower carotenoid content (P < 0.05). Notably metabolic profiling differences were observed among the healthy tender shoots from the four tested plants via UPLC-QQQ-MS and GC-MS analyses. Comparing with the other three plant species, 66 and 50 metabolites with significantly different contents in Chongyi wild mandarin were selected as UPLC-identified and GC-identified metabolites of interest (P < 0.05), respectively. Flavonoids accounted for a large group of secondary metabolites of interest, which may function as stimulants or repellents of ACP. Higher content of salicylic acid o-hexoside and lower content of (+)-jasmonic acid in Chongyi wild mandarin may lead to higher amount of methyl salicylate (an ACP attractant) and lower amount of trans-ocimene (an attractant to herbivores' natural enemies) as well as the suppression of JA-mediated wounding response. This kind of synergistic or antagonistic effect among the metabolites differentially accumulated in Chongyi wild mandarin made it a more attractive host plant to ACP.

CONCLUSIONS

Less saturated yellow color, high amount of attractants, low amount of repellents and insensitivity of JA-mediated wounding response are the four possible reasons why Chongyi wild mandarin attracted more ACP. This work may shed light on the olfactory and visual response of ACP to wild citrus germplasm hosts, and suggest the feasibility of developing ACP attractants or repellents patterned on potential metabolites.

Inside and Beyond Color: Comparative Overview of Functional Quality of Tomato and Watermelon Fruits

The quali-quantitative evaluation and the improvement of the levels of plant bioactive secondary metabolites are increasingly gaining consideration by growers, breeders and processors, particularly in those fruits and vegetables that, due to their supposed health promoting properties, are considered "functional." Worldwide, tomato and watermelon are among the main grown and consumed crops and represent important sources not only of dietary lycopene but also of other health beneficial bioactives. Tomato and watermelon synthesize and store lycopene as their major ripe fruit carotenoid responsible of their typical red color at full maturity. It is also the precursor of some characteristic aroma volatiles in both fruits playing, thus, an important visual and olfactory impact in consumer choice. While sharing the same main pigment, tomato and watermelon fruits show substantial biochemical and physiological differences during ripening. Tomato is climacteric while watermelon is non-climacteric; unripe tomato fruit is green, mainly contributed by chlorophylls and xanthophylls, while young watermelon fruit mesocarp is white and contains only traces of carotenoids. Various studies comparatively evaluated in vivo pigment development in ripening tomato and watermelon fruits. However, in most cases, other classes of compounds have not been considered. We believe this knowledge is fundamental for targeted breeding aimed at improving the functional quality of elite cultivars. Hence, in this paper, we critically review the recent understanding underlying the biosynthesis, accumulation and regulation of different bioactive compounds (carotenoids, phenolics, aroma volatiles, and vitamin C) during tomato and watermelon fruit ripening. We also highlight some concerns about possible harmful effects of excessive uptake of bioactive compound on human health. We found that a complex interweaving of anabolic, catabolic and recycling reactions, finely regulated at multiple levels and with temporal and spatial precision, ensures a certain homeostasis in the concentrations of carotenoids, phenolics, aroma volatiles and Vitamin C within the fruit tissues. Nevertheless, several exogenous factors including light and temperature conditions, pathogen attack, as well as pre- and post-harvest manipulations can drive their amounts far away from homeostasis. These adaptive responses allow crops to better cope with abiotic and biotic stresses but may severely affect the supposed functional quality of fruits.

Trichoplusia ni (Lepidoptera: Noctuidae) Qualitative and Quantitative Sequestration of Host Plant Carotenoids

Carotenoids are fundamental precursors for hormones and antioxidants, and insects must acquire carotenoids from their diet. Previous research has shown that insects can selectively absorb dietary carotenoids, often modifying them qualitatively or quantitatively, and quantities may be proportional to those found in the diet. Trichoplusia ni Hübner is a generalist herbivore with host plants varying greatly in carotenoid profiles and concentrations. Larvae sequester carotenoids in their hemolymph, and carotenoid sequestration contributes to their cryptic green coloration. Our objectives were to compare the types of carotenoids found in T. ni and their host plants to determine whether qualitative changes occurred, and compare the amounts of sequestered carotenoids in T. ni reared upon different host plants to determine whether quantitative variation influences sequestration. To fulfill these objectives, larvae were fed romaine lettuce (Lactuca sativa L. [Asterales: Asteraceae] var. longifolia) or kale (Brassica oleracea L. [Brassicales: Brassicaceae] var. sabellica) for a period of 5 d, and sequestered carotenoids from the entire insect were resolved with thin-layer chromatography and measured with spectrophotometer. All carotenoids resolved from plants were also resolved from larvae, and although the carotenoids of plants differed quantitatively, the sequestered carotenoids did not differ between host plants. Regardless of host plant species, T. ni sequestered carotenoids at concentrations up to 20 times higher than the concentrations found in the plants. Future research may be able to explicitly identify enzyme systems involved in the transport and modification of carotenoids in T. ni and other animals.

Genetic dissection of cell wall defects and the strigolactone pathway in Arabidopsis

Defects in the biosynthesis and/or deposition of secondary plant cell wall polymers result in the collapse of xylem vessels causing a dwarfed plant stature and an altered plant architecture termed irregular xylem (irx) syndrome. For example, reduced xylan O-acetylation causes strong developmental defects and increased freezing tolerance. Recently, we demonstrated that the irx syndrome in the trichome birefringence-like 29/eskimo1 (tbl29/esk1) mutant is dependent on MORE AXILLARY GROWTH 4 (MAX4), a key enzyme in the biosynthesis of the phytohormone strigolactone (SL). In this report, we show that other xylan- and cellulose-deficient secondary wall mutants exhibit increased freezing tolerance correlated with the irx syndrome. In addition, these phenotypes are also dependent on MAX4, suggesting a more general interaction between secondary wall defects and SL biosynthesis. In contrast, MAX4 does not play a role in developmental defects triggered by primary wall deficiencies, suggesting that the interaction is restricted to vascular tissue. Through a reverse genetics approach, the requirement of different components of the SL pathway impacting the irx syndrome in tbl29 was evaluated. Our results show that the tbl29-associated irx phenotypes are dependent on the MAX3 and MAX4 enzymes, involved in the early steps of SL biosynthesis. In contrast, this signaling is independent on downstream enzymes in the biosynthesis and perception of SL such as MAX1 and MAX2.

Volatile apocarotenoid discovery and quantification in Arabidopsis thaliana: optimized sensitive analysis via HS-SPME-GC/MS

INTRODUCTION

In the field of carotenoid metabolism researchers' focus has been directed recently toward the discovery and quantification of carotenoid cleavage products (i.e. apocarotenoids, excluding the well-studied carotenoid-derived hormones abscisic acid and strigolactones), due to their emerging roles as putative signaling molecules. Gas chromatography mass spectrometry (GC/MS) and sample preparation via headspace solid phase micro-extraction (HS-SPME) are widely used analytical techniques for broad untargeted metabolomics studies and until now, no optimized quantitative targeted HS-SPME-GC/MS method has been developed specifically for volatile apocarotenoids (VAs) in planta.

OBJECTIVES

Optimization and subsequent validation of the HS-SPME technique for extracting and quantifying volatile apocarotenoids in planta.

METHODS

Factors considered during method optimization were HS-SPME parameters; vial storage conditions; different adsorbent SPME fibre coating chemistries; plant tissue matrix effects; and fresh tissues to be analyzed.

RESULTS

Mean linear regression in planta calibration correlation coefficients (R²) for VAs was 0.974. The resultant method mean limits of detection (LOD) and lower limits of quantification (LLOQ) for VAs using in planta standard additions were 0.384 ± 0.139 and 0.640 ± 0.231 µg/L, respectively. VAs remained stable at elevated SPME incubation temperatures, with no observable effects of thermal and photo-stereoisomerisation and oxidation. The bipolar 50/30 µm divinylbenzene/carboxen on polydimethylsiloxane (PDMS/DVB/CAR) was identified as the optimal fibre for broad molecular weight range VA analysis.

CONCLUSIONS

An optimized HS-SPME-GC/MS method for VA detection and quantification was validated in vitro and in planta: based on biological replicates and stringent QA/QC approaches, thereby providing robust detection and quantification of VAs across a broad range of Arabidopsis tissues, fifteen of which were identified for the first time in Arabidopsis.

Cloning and expression analysis of LoCCD8 during IAA-induced bulbils outgrowth in lily (Oriental Hybrid 'Sorbonne')

Aerial bulbils, which resemble tiny bulbs, develop from axillary buds and facilitate rapid propagation of Lilium. In most species of lily, bulbils are perpetually dormant and little is known about induction of these vegetative structures. Herein, we proposed that strigolactones (SLs) may regulate the induction of bulbils in Lilium. We tested this hypothesis by isolating and investigating the expression patterns of 2 copies of the carotenoid cleavage dioxygenase8 (CCD8) gene in lily-LoCCD8a and LoCCD8b-with regard to biosynthesis of SLs. Expression analyses revealed that LoCCD8a principally is expressed during vegetative growth, whereas LoCCD8b mainly is expressed during reproductive growth. The maximum quantity of LoCCD8a transcripts was observed in the basal plate in most developmental stages, which suggests that SLs may originate from underground parts, especially the basal plate, and move upward. The effects of treatments with indole-3-acetic acid or SL analog (GR24) on outgrowth of bulbils and expression of LoCCD8 genes suggested that SLs function downstream of auxin to inhibit the outgrowth of bulbil. The expression patterns of LoCCD8a and LoCCD8b at sprouting and bulblet weighting stages also implied that SLs may function in nutrient redistribution. Our findings are expected to promote the utilization of bulbils as vegetative propagules for commercial practice.

Multi-species transcriptome analyses for the regulation of crocins biosynthesis in Crocus

BACKGROUND

Crocins are soluble apocarotenoids that mainly accumulate in the stigma tissue of Crocus sativus and provide the characteristic red color to saffron spice, in addition to being responsible for many of the medicinal properties of saffron. Crocin biosynthesis and accumulation in saffron is developmentally controlled, and the concentration of crocins increases as the stigma develops. Until now, little has been known about the molecular mechanisms governing crocin biosynthesis and accumulation. This study aimed to identify the first set of gene regulatory processes implicated in apocarotenoid biosynthesis and accumulation.

RESULTS

A large-scale crocin-mediated RNA-seq analysis was performed on saffron and two other Crocus species at two early developmental stages coincident with the initiation of crocin biosynthesis and accumulation. Pairwise comparison of unigene abundance among the samples identified potential regulatory transcription factors (TFs) involved in crocin biosynthesis and accumulation. We found a total of 131 (up- and downregulated) TFs representing a broad range of TF families in the analyzed transcriptomes; by comparison with the transcriptomes from the same developmental stages from other Crocus species, a total of 11 TF were selected as candidate regulators controlling crocin biosynthesis and accumulation.

CONCLUSIONS

Our study generated gene expression profiles of stigmas at two key developmental stages for apocarotenoid accumulation in three different Crocus species. Differential gene expression analyses allowed the identification of transcription factors that provide evidence of environmental and developmental control of the apocarotenoid biosynthetic pathway at the molecular level.

The apocarotenoid metabolite zaxinone regulates growth and strigolactone biosynthesis in rice

Carotenoid cleavage dioxygenases (CCDs) form hormones and signaling molecules. Here we show that a member of an overlooked plant CCD subfamily from rice, that we name Zaxinone Synthase (ZAS), can produce zaxinone, a novel apocarotenoid metabolite in vitro. Loss-of-function mutants (zas) contain less zaxinone, exhibit retarded growth and showed elevated levels of strigolactones (SLs), a hormone that determines plant architecture, mediates mycorrhization and facilitates infestation by root parasitic weeds, such as Striga spp. Application of zaxinone can rescue zas phenotypes, decrease SL content and release and promote root growth in wild-type seedlings. In conclusion, we show that zaxinone is a key regulator of rice development and biotic interactions and has potential for increasing crop growth and combating Striga, a severe threat to global food security.

Apocarotenoids: Old and New Mediators of the Arbuscular Mycorrhizal Symbiosis

Plants utilize hormones and other small molecules to trigger and coordinate their growth and developmental processes, adapt and respond to environmental cues, and communicate with surrounding organisms. Some of these molecules originate from carotenoids that act as universal precursors of bioactive metabolites arising through oxidation of the carotenoid backbone. This metabolic conversion produces a large set of compounds known as apocarotenoids, which includes the plant hormones abscisic acid (ABA) and strigolactones (SLs) and different signaling molecules. An increasing body of evidence suggests a crucial role of previously identified and recently discovered carotenoid-derived metabolites in the communication with arbuscular mycorrhizal (AM) fungi and the establishment of the corresponding symbiosis, which is one of the most relevant plant-fungus mutualistic interactions in nature. In this review, we provide an update on the function of apocarotenoid hormones and regulatory metabolites in AM symbiosis, highlighting their effect on both partners.

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.

Limited appearance of apocarotenoids is observed in plasma after consumption of tomato juices: a randomized human clinical trial

Background

Nonvitamin A apocarotenoids occur in foods. Some function as retinoic acid receptor antagonists in vitro, though it is unclear if apocarotenoids are absorbed or accumulate to levels needed to elicit biological function.

Objective

The aim of this study was to quantify carotenoids and apocarotenoids (β-apo-8'-, -10'-, -12'-, and -14'-carotenal, apo-6'-, -8'-, -10'-, -12'-, and -14'-lycopenal, retinal, acycloretinal, β-apo-13-carotenone, and apo-13-lycopenone) in human plasma after controlled consumption of carotenoid-rich tomato juices.

Design

Healthy subjects (n = 35) consumed a low-carotenoid diet for 2 wk, then consumed 360 mL of high-β-carotene tomato juice (30.4 mg of β-carotene, 34.5 μg total β-apocarotenoids/d), high-lycopene tomato juice (42.5 mg of lycopene, 119.2 μg total apolycopenoids/d), or a carotenoid-free control (cucumber juice) per day for 4 wk. Plasma was sampled at baseline (after washout) and after 2 and 4 wk, and analyzed for carotenoids and apocarotenoids using high-pressure liquid chromatography (HPLC) and HPLC-tandem mass spectrometry, respectively. The methods used to analyze the apocarotenoids had limits of detection of ∼ 100 pmol/L.

Results

Apocarotenoids are present in tomato juices at 0.1-0.5% of the parent carotenoids. Plasma lycopene and β-carotene increased (P < 0.001) after consuming high-lycopene and β-carotene tomato juices, respectively, while retinol remained unchanged. β-Apo-13-carotenone was found in the blood of all subjects at every visit, although elevated (P < 0.001) after consuming β-carotene tomato juice for 4 wk (1.01 ± 0.27 nmol/L) compared with both baseline (0.37 ± 0.17 nmol/L) and control (0.46 ± 0.11 nmol/L). Apo-6'-lycopenal was detected or quantifiable in 29 subjects, while β-apo-10'- and 12'-carotenal were detected in 6 and 2 subjects, respectively. No other apolycopenoids or apocarotenoids were detected.

Conclusions

β-Apo-13-carotenone was the only apocarotenoid that was quantifiable in all subjects, and was elevated in those consuming high-β-carotene tomato juice. Levels were similar to previous reports of all-trans-retinoic acid. Other apocarotenoids are either poorly absorbed or rapidly metabolized or cleared, and so are absent or limited in blood. β-Apo-13-carotenone may form from vitamin A and its presence warrants further investigation. This trial was registered at clinicaltrials.gov as NCT02550483.

Overexpression of alfalfa Orange gene in tobacco enhances carotenoid accumulation and tolerance to multiple abiotic stresses

The multifunctional Orange (Or) protein plays crucial roles in carotenoid homeostasis, photosynthesis stabilization, and antioxidant activity in plants under various abiotic stress conditions. The Or gene has been cloned in several crops but not in alfalfa (Medicago sativa L.). Alfalfa is widely cultivated across the world; however, its cultivation is largely limited by various abiotic stresses, including drought. In this study, we isolated the Or gene from alfalfa (MsOr) cv. Xinjiang Daye. The amino acid sequence of the deduced MsOr protein revealed that the protein contained two trans-membrane domains and a DnaJ cysteine-rich zinc finger domain, and showed a high level of similarity with the Or protein of other plants species. The MsOr protein was localized in leaf chloroplasts of tobacco. The expression of MsOr was the highest in mature leaves and was significantly induced by abiotic stresses, especially drought. To perform functional analysis of the MsOr gene, we overexpressed MsOr gene in tobacco (Nicotiana benthamiana). Compared with wild-type (WT) plants, transgenic tobacco lines showed higher carotenoid accumulation and increased tolerance to various abiotic stresses, including drought, heat, salt, and methyl viologen-mediated oxidative stress. Additionally, contents of hydrogen peroxide and malondialdehyde were lower in the transgenic lines than in WT plants, suggesting superior membrane stability and antioxidant capacity of TOR lines under multiple abiotic stresses. These results indicate the MsOr gene as a potential target for the development of alfalfa cultivars with enhanced carotenoid content and tolerance to multiple environmental stresses.

Blumenols as shoot markers of root symbiosis with arbuscular mycorrhizal fungi

High-through-put (HTP) screening for functional arbuscular mycorrhizal fungi (AMF)-associations is challenging because roots must be excavated and colonization evaluated by transcript analysis or microscopy. Here we show that specific leaf-metabolites provide broadly applicable accurate proxies of these associations, suitable for HTP-screens. With a combination of untargeted and targeted metabolomics, we show that shoot accumulations of hydroxy- and carboxyblumenol C-glucosides mirror root AMF-colonization in Nicotiana attenuata plants. Genetic/pharmacologic manipulations indicate that these AMF-indicative foliar blumenols are synthesized and transported from roots to shoots. These blumenol-derived foliar markers, found in many di- and monocotyledonous crop and model plants (Solanum lycopersicum, Solanum tuberosum, Hordeum vulgare, Triticum aestivum, Medicago truncatula and Brachypodium distachyon), are not restricted to particular plant-AMF interactions, and are shown to be applicable for field-based QTL mapping of AMF-related genes.

The Biochemical Basis of Vitamin A Production from the Asymmetric Carotenoid β-Cryptoxanthin

Vitamin A serves essential functions in mammalian biology as a signaling molecule and chromophore. This lipid can be synthesized from more than 50 putative dietary provitamin A precursor molecules which contain at least one unsubstituted β-ionone ring. We here scrutinized the enzymatic properties and substrate specificities of the two structurally related carotenoid cleavage dioxygenases (CCDs) which catalyze this synthesis. Recombinant BCO1 split substrates across the C15,C15' double bond adjacent to a canonical β-ionone ring site to vitamin A aldehyde. Substitution of the ring with a hydroxyl group prevented this conversion. The removal of methyl groups from the polyene carbon backbone of the substrate did not impede enzyme activity. Homology modeling and site-directed mutagenesis identified amino acid residues at the entrance of the substrate tunnel, which determined BCO1's specificity for the canonical β-ionone ring site. In contrast, BCO2 split substrates across the C9,C10 double bond adjacent to assorted ionone ring sites. Kinetic analysis revealed a higher catalytic efficiency of BCO2 with substrates bearing 3-hydroxy-β-ionone rings. In the mouse intestine, the asymmetric carotenoid β-cryptoxanthin with one canonical and one 3-hydroxy-β-ionone ring site was meticulously converted to vitamin A. The tailoring of this asymmetric substrate occurred by a stepwise processing of the carotenoid substrate by both CCDs and involved a β-apo-10'-carotenal intermediate. Thus, opposite selectivity for ionone ring sites of the two mammalian CCDs complement each other in the metabolic challenge of vitamin A production from a chemically diverse set of precursor molecules.