Zeaxanthin epoxidase 3 Knockout Mutants of the Model Diatom Phaeodactylum tricornutum Enable Commercial Production of the Bioactive Carotenoid Diatoxanthin

Carotenoids are pigments that have a range of functions in human health. The carotenoid diatoxanthin is suggested to have antioxidant, anti-inflammatory and chemo-preventive properties. Diatoxanthin is only produced by a few groups of microalgae, where it functions in photoprotection. Its large-scale production in microalgae is currently not feasible. In fact, rapid conversion into the inactive pigment diadinoxanthin is triggered when cells are removed from a high-intensity light source, which is the case during large-scale harvesting of microalgae biomass. Zeaxanthin epoxidase (ZEP) 2 and/or ZEP3 have been suggested to be responsible for the back-conversion of high-light accumulated diatoxanthin to diadinoxanthin in low-light in diatoms. Using CRISPR/Cas9 gene editing technology, we knocked out the ZEP2 and ZEP3 genes in the marine diatom Phaeodactylum tricornutum to investigate their role in the diadinoxanthin-diatoxanthin cycle and determine if one of the mutant strains could function as a diatoxanthin production line. Light-shift experiments proved that ZEP3 encodes the enzyme converting diatoxanthin to diadinoxanthin in low light. Loss of ZEP3 caused the high-light-accumulated diatoxanthin to be stable for several hours after the cultures had been returned to low light, suggesting that zep3 mutant strains could be suitable as commercial production lines of diatoxanthin.

Suppression of ZEAXANTHIN EPOXIDASE 1 restricts stripe rust growth in wheat

Reducing losses caused by pathogens is an effective strategy for stabilizing crop yields. Daunting challenges remain in cloning and characterizing genes that inhibit stripe rust, a devastating disease of wheat (Triticum aestivum) caused by Puccinia striiformis f. sp. tritici (Pst). We found that suppression of wheat zeaxanthin epoxidase 1 (ZEP1) increased wheat defense against Pst. We isolated the yellow rust slower 1 (yrs1) mutant of tetraploid wheat in which a premature stop mutation in ZEP1-B underpins the phenotype. Genetic analyses revealed increased H2O2 accumulation in zep1 mutants and demonstrated a correlation between ZEP1 dysfunction and slower Pst growth in wheat. Moreover, wheat kinase START 1.1 (WKS1.1, Yr36) bound, phosphorylated, and suppressed the biochemical activity of ZEP1. A rare natural allele in the hexaploid wheat ZEP1-B promoter reduced its transcription and Pst growth. Our study thus identified a novel suppressor of Pst, characterized its mechanism of action, and revealed beneficial variants for wheat disease control. This work opens the door to stacking wheat ZEP1 variants with other known Pst resistance genes in future breeding programs to enhance wheat tolerance to pathogens.

Shedding light on the dark side of xanthophyll cycles

Xanthophyll cycles are broadly important in photoprotection, and the reversible de-epoxidation of xanthophylls typically occurs in excess light conditions. However, as presented in this review, compiling evidence in a wide range of photosynthetic eukaryotes shows that xanthophyll de-epoxidation also occurs under diverse abiotic stress conditions in darkness. Light-driven photochemistry usually leads to the pH changes that activate de-epoxidases (e.g. violaxanthin de-epoxidase), but in darkness alternative electron transport pathways and luminal domains enriched in monogalactosyl diacyl glycerol (which enhance de-epoxidase activity) likely enable de-epoxidation. Another 'dark side' to sustaining xanthophyll de-epoxidation is inactivation and/or degradation of epoxidases (e.g. zeaxanthin epoxidase). There are obvious benefits of such activity regarding stress tolerance, and indeed this phenomenon has only been reported in stressful conditions. However, more research is required to unravel the mechanisms and understand the physiological roles of dark-induced formation of zeaxanthin. Notably, the de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin in darkness is still a frequently ignored process, perhaps because it questions a previous paradigm. With that in mind, this review seeks to shed some light on the dark side of xanthophyll de-epoxidation, and point out areas for future work.

Gene silencing of BnaA09.ZEP and BnaC09.ZEP confers orange color in Brassica napus flowers

Brassica napus is currently cultivated as an important ornamental crop in China. Flower color has attracted much attention in rapeseed genetics and breeding. Here, we characterize an orange-flowered mutant of B. napus that exhibits an altered carotenoid profile in its petals. As revealed by map-based cloning, the change in color from yellow to orange is attributed to the loss of BnaC09.ZEP (zeaxanthin epoxidase) and a 1695-bp deletion in BnaA09.ZEP. HPLC analysis, genetic complementation and CRISPR/Cas9 experiments demonstrated that BnaA09.ZEP and BnaC09.ZEP have similar functions, and the abolishment of both genes led to a substantial increase in lutein content and a sharp decline in violaxanthin content in petals but not leaves. BnaA09.ZEP and BnaC09.ZEP are predominantly expressed in floral tissues, whereas their homologs, BnaA07.ZEP and BnaC07.ZEP, mainly function in leaves, indicating redundancy and tissue-specific diversification of BnaZEP function. Transcriptome analysis in petals revealed differences in the expression of carotenoid and flavonoid biosynthesis-related genes between the mutant and its complementary lines. Flavonoid profiles in the petals of complementary lines were greatly altered compared to the mutant, indicating potential cross-talk between the regulatory networks underlying the carotenoid and flavonoid pathways. Additionally, our results indicate that there is functional compensation by BnaA07.ZEP and BnaC07.ZEP in the absence of BnaA09.ZEP and BnaC09.ZEP. Cloning and characterization of BnaZEPs provide insights into the molecular mechanisms underlying flower pigmentation in B. napus and would facilitate breeding of B. napus varieties with higher ornamental value.

The zeaxanthin epoxidase is degraded along with the D1 protein during photoinhibition of photosystem II

The xanthophyll zeaxanthin is synthesized in chloroplasts upon high light exposure of plants and serves central photoprotective functions. The reconversion of zeaxanthin to violaxanthin is catalyzed by the zeaxanthin epoxidase (ZEP). ZEP shows highest activity after short and moderate high light periods, but becomes gradually down-regulated in response to increasing high light stress along with down-regulation of photosystem II (PSII) activity. ZEP activity and ZEP protein levels were studied in response to high light stress in four plant species: Arabidopsis thaliana, Pisum sativum, Nicotiana benthamiana and Spinacia oleracea. In all species, ZEP protein was degraded during photoinhibition of PSII in parallel with the D1 protein of PSII. In the presence of streptomycin, an inhibitor of chloroplast protein synthesis, photoinhibition of PSII and ZEP activity as well as degradation of D1 and ZEP protein was strongly increased, indicating a close correlation of ZEP regulation with PSII photoinhibition and repair. The concomitant high light-induced inactivation/degradation of ZEP and D1 prevents the reconversion of zeaxanthin during photoinhibition and repair of PSII. This regulation of ZEP activity supports a coordinated degradation of D1 and ZEP during photoinhibition/repair of PSII and an essential photoprotective function of zeaxanthin during the PSII repair cycle.

Loss of Function in Zeaxanthin Epoxidase of Dunaliella tertiolecta Caused by a Single Amino Acid Mutation within the Substrate-Binding Site

The zea1 mutant of marine microalga Dunaliella tertiolecta accumulates zeaxanthin under normal growth conditions, and its phenotype has been speculated to be related to zeaxanthin epoxidase (ZEP). In this study, we isolated the ZEP gene from both wild-type D. tertiolecta and the mutant. We found that the zea1 mutant has a point mutation of the 1337th nucleotide of the ZEP sequence (a change from guanine to adenine), resulting in a change of glycine to aspartate in a highly conserved region in the catalytic domain. Similar expression levels of ZEP mRNA and protein in both wild-type and zea1 were confirmed by using qRT-PCR and western blot analysis, respectively. Additionally, the enzyme activity analysis of ZEPs in the presence of cofactors showed that the inactivation of ZEP in zea1 was not caused by deficiency in the levels of cofactors. From the predicted three-dimensional ZEP structure of zea1, we observed a conformational change on the substrate-binding site in the ZEP. A comparative analysis of the ZEP structures suggested that the conformational change induced by a single amino acid mutation might impact the interaction between the substrate and substrate-binding site, resulting in loss of zeaxanthin epoxidase function.

Overexpression of zeaxanthin epoxidase gene from Medicago sativa enhances the tolerance to low light in transgenic tobacco

Zeaxanthin epoxidase (ZEP) plays an important role in xanthophyll cycle which is a process closely related to photosynthesis. However, an impact of ZEP on low-light stress has not been studied. In this study, the functions of an alfalfa (Medicago sativa) zeaxanthin epoxidase gene, MsZEP, in response to low-light stress were investigated by heterologous expression in tobacco (Nicotiana tabacum). Under normal light conditions, the measured parameters were not significantly different between transgenic and wild-type (WT) plants except for non-photochemical quenching value and chlorophyll a content. However, the differences were detected under low-light stress. We found that MsZEP-overexpression tobacco grew faster than WT (p≤0.05). The leaf fresh weight and leaf area of transgenic plants were significantly higher, and the number of stomata was greater in MsZEP-overexpression tobacco. As for photosynthetic characteristics, quantum yield of PSII (ΦPSII) and maximal photochemical efficiency of PSII (Fv/Fm) were not significantly different, whereas non-photochemical quenching (NPQ), net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of MsZEP-overexpression tobacco were significantly higher than in WT plants. However, no significant difference was detected between the two types of tobacco in chlorophyll and carotenoids content. In conclusion, MsZEP can improve the ability of tobacco to withstand low-light stress, which might be due to its stronger photosynthetic activity and the improvement of stomatal density under low light.

A functional zeaxanthin epoxidase from red algae shedding light on the evolution of light-harvesting carotenoids and the xanthophyll cycle in photosynthetic eukaryotes

The epoxy-xanthophylls antheraxanthin and violaxanthin are key precursors of light-harvesting carotenoids and participate in the photoprotective xanthophyll cycle. Thus, the invention of zeaxanthin epoxidase (ZEP) catalyzing their formation from zeaxanthin has been a fundamental step in the evolution of photosynthetic eukaryotes. ZEP genes have only been found in Viridiplantae and chromalveolate algae with secondary plastids of red algal ancestry, suggesting that ZEP evolved in the Viridiplantae and spread to chromalveolates by lateral gene transfer. By searching publicly available sequence data from 11 red algae covering all currently recognized red algal classes we identified ZEP candidates in three species. Phylogenetic analyses showed that the red algal ZEP is most closely related to ZEP proteins from photosynthetic chromalveolates possessing secondary plastids of red algal origin. Its enzymatic activity was assessed by high performance liquid chromatography (HPLC) analyses of red algal pigment extracts and by cloning and functional expression of the ZEP gene from Madagascaria erythrocladioides in leaves of the ZEP-deficient aba2 mutant of Nicotiana plumbaginifolia. Unlike other ZEP enzymes examined so far, the red algal ZEP introduces only a single epoxy group into zeaxanthin, yielding antheraxanthin instead of violaxanthin. The results indicate that ZEP evolved before the split of Rhodophyta and Viridiplantae and that chromalveolates acquired ZEP from the red algal endosymbiont and not by lateral gene transfer. Moreover, the red algal ZEP enables engineering of transgenic plants incorporating antheraxanthin instead of violaxanthin in their photosynthetic machinery.

[Cloning and expression analysis on full length CDS of zeaxanthin epoxidase gene in Pseudostellaria heterophylla]

Zeaxanthin epoxidase plays an important role in indirect pathway of plant abscisic acid biosynthesis. According to the data of Pseudostellaria heterophylla transcriptome, zeaxanthin epoxidase gene was isolated and named as PhZEP. The results of bioinformatics analysis showed that the coding sequence of PhZEP was 1 263 bp long and encoded 420 amino acids. The putative protein molecular weight was 47.34 kDa and its theoretical isoelectric point was 6.64. The characteristic structure domains were predicted, including binding site of lipoprotein and flavoprotein monooxyenase. A signal peptide was discovered at the N-terminal of amino acids. The Real-time PCR revealed that PhZEP had a higher expression level in leaves than other tissues of P.heterophylla. Highly expressed PhZEP was also observed at 10 d and 40 d tuberous root after flowering. PhZEP presented a different expression after treatment with ABA, fluridone and ABA +fluridone compared to the control. The expression of PhZEP in tuberous root after ABA treatment was close to that in control group, while PhZEP showed significant up-regulation in the fluridone treatment group. In this study, the PhZEP gene from P. heterophylla was cloned and this result has important significance for its functional identification. This research provides a basis for the further analysis on functional mechanism of ABA during development of P. heterophylla.

Epoxycarotenoid-mediated synthesis of abscisic acid in Physcomitrella patens implicating conserved mechanisms for acclimation to hyperosmosis in embryophytes

Plants acclimate to environmental stress signals such as cold, drought and hypersalinity, and provoke internal protective mechanisms. Abscisic acid (ABA), a carotenoid-derived phytohormone, which increases in response to the stress signals above, has been suggested to play a key role in the acclimation process in angiosperms, but the role of ABA in basal land plants such as mosses, including its biosynthetic pathways, has not been clarified. Targeted gene disruption of PpABA1, encoding zeaxanthin epoxidase in the moss Physcomitrella patens was conducted to determine the role of endogenous ABA in acclimation processes in mosses. The generated ppaba1 plants were found to accumulate only a small amount of endogenous ABA. The ppaba1 plants showed reduced osmotic acclimation capacity in correlation with reduced dehydration tolerance and accumulation of late embryogenesis abundant proteins. By contrast, cold-induced freezing tolerance was less affected in ppaba1, indicating that endogenous ABA does not play a major role in the regulation of cold acclimation in the moss. Our results suggest that the mechanisms for osmotic acclimation mediated by carotenoid-derived synthesis of ABA are conserved in embryophytes and that acquisition of the mechanisms played a crucial role in terrestrial adaptation and colonization by land plant ancestors.

De novo transcriptome analysis of Liriodendron chinense petals and leaves by Illumina sequencing

Liriodendron chinense (Hemsl.) Sarg is an endangered species and occupies a pivotal position in phylogenetic studies of flowering plants, while its genomic resources are limited. In this study, we performed transcriptome sequencing for L. chinense petals and leaves using the Illumina paired-end sequencing technique. Approximately 17.02-Gb clean reads were obtained, and de novo assembly generated 87,841 unigenes, with an average length of 778 bp. Of these, there were 65,535 (74.61%) unigenes with significant similarity to publically available plant protein sequences. There were 3386 genes identified as significant differentially expressed between petals and leaves, among them 2969 (87.68%) were up-regulated and 417 (12.31%) down-regulated in petals. Metabolic pathway analysis revealed that 25 unigenes were predicted to be responsible for the biosynthesis of carotenoids, with 7 genes differentially expressed between these two tissues. This report is the first to identify genes associated with carotenoid biosynthesis in Liriodendron and represents a valuable resource for future genomic studies on the endangered species L. chinense.

Isohydric and anisohydric strategies of wheat genotypes under osmotic stress: biosynthesis and function of ABA in stress responses

Changes in water potential (ψw), stomatal conductance, abscisic acid (ABA) accumulation, expression of the major genes involved in ABA biosynthesis, activities of abscisic aldehyde oxidase (AO, EC 1.2.3.1) and antioxidant enzymes were studied in two wheat cultivars with contrasting acclimation strategies subjected to medium strength osmotic stress (-0.976MPa) induced by polyethylene glycol (PEG 6000). Because the biosynthetic pathway of ABA involves multiple gene products, the aim of this study was to unravel how these genes are regulated in isohydric and anisohydric wheat genotypes. In the root tissues of the isohydric cultivar, Triticum aestivum cv. Kobomugi, osmotic stress increased the transcript levels of 9-cis-epoxycarotenoid dioxygenase (NCED) gene, controlling the rate limiting step of ABA biosynthesis. Moreover, this cultivar exhibited a higher basal activity and a higher induction of aldehyde oxidase isoenzymes (AAO2-AAO3), responsible for converting ABAldehyde to ABA. It was found that the fast activation of the ABA biosynthesis in the roots generated an enhanced ABA pool in the shoot, which brought about a faster closure of the stomata upon increasing osmotic stress and, as a result, the plants could maintain ψw in the tissues close to the control level. In contrast, the anisohydric genotype, cv. GK Öthalom, exhibited a moderate induction of ABA biosynthesis in the roots, leading to the maintenance but no increase in the concentration of ABA on the basis of tissue water content in the leaves. Due to the slower response of their stomata to water deficit, the tissues of cv. GK Öthalom have to acclimate to much more negative water potentials during increasing osmotic stress. A decreased activity of superoxide dismutase (SOD) was found in the leaves and roots of both cultivars exposed to osmotic stress, but in the roots elevated activities of catalase (CAT), peroxidase (POX), glutathione reductase (GR) and glutathione transferase (GST) were detected in the isohydric cultivar, suggesting that this genotype was more successful in the elimination of reactive oxygen species caused by the stress conditions.

Carotenoids, versatile components of oxygenic photosynthesis

Carotenoids (CARs) are a group of pigments that perform several important physiological functions in all kingdoms of living organisms. CARs serve as protective agents, which are essential structural components of photosynthetic complexes and membranes, and they play an important role in the light harvesting mechanism of photosynthesizing plants and cyanobacteria. The protection against reactive oxygen species, realized by quenching of singlet oxygen and the excited states of photosensitizing molecules, as well as by the scavenging of free radicals, is one of the main biological functions of CARs. X-ray crystallographic localization of CARs revealed that they are present at functionally and structurally important sites of both the PSI and PSII reaction centers. Characterization of a CAR-less cyanobacterial mutant revealed that while the absence of CARs prevents the formation of PSII complexes, it does not abolish the assembly and function of PSI. CAR molecules assist in the formation of protein subunits of the photosynthetic complexes by gluing together their protein components. In addition to their aforementioned indispensable functions, CARs have a substantial role in the formation and maintenance of proper cellular architecture, and potentially also in the protection of the translational machinery under stress conditions.

Molecular characterisation and the light-dark regulation of carotenoid biosynthesis in sprouts of tartary buckwheat (Fagopyrum tataricum Gaertn.)

Seven partial-length cDNAs and 1 full-length cDNA that were involved in carotenoid biosynthesis and 2 partial-length cDNAs that encoded carotenoid cleavage dioxygenases were first isolated and characterised in 2 tartary buckwheat cultivars (Fagopyrum tataricum Gaertn.), Hokkai T8 and Hokkai T10. They were constitutively expressed at high levels in the leaves and flowers, where carotenoids are mostly distributed. During the seed development of tartary buckwheat, an inverse correlation between transcription level of carotenoid cleavage dioxygenase and carotenoid content was observed. The light-grown sprouts exhibited higher levels of expression of carotenoid biosynthetic genes in T10 and carotenoid content in both T8 and T10 compared to the dark-grown sprouts. The predominant carotenoids in tartary buckwheat were lutein and β-carotene, and very abundant amounts of these carotenoids were found in light-grown sprouts. This study might broaden our understanding of the molecular mechanisms involved in carotenoid biosynthesis and indicates targets for increasing the production of carotenoids in tartary buckwheat.

Efficient heterologous transformation of Chlamydomonas reinhardtii npq2 mutant with the zeaxanthin epoxidase gene isolated and characterized from Chlorella zofingiensis

In the violaxanthin cycle, the violaxanthin de-epoxidase and zeaxanthin epoxidase catalyze the inter-conversion between violaxanthin and zeaxanthin in both plants and green algae. The zeaxanthin epoxidase gene from the green microalga Chlorella zofingiensis (Czzep) has been isolated. This gene encodes a polypeptide of 596 amino acids. A single copy of Czzep has been found in the C. zofingiensis genome by Southern blot analysis. qPCR analysis has shown that transcript levels of Czzep were increased after zeaxanthin formation under high light conditions. The functionality of Czzep gene by heterologous genetic complementation in the Chlamydomonas mutant npq2, which lacks zeaxanthin epoxidase (ZEP) activity and accumulates zeaxanthin in all conditions, was analyzed. The Czzep gene was adequately inserted in the pSI105 vector and expressed in npq2. The positive transformants were able to efficiently convert zeaxanthin into violaxanthin, as well as to restore their maximum quantum efficiency of the PSII (Fv/Fm). These results show that Chlamydomonas can be an efficient tool for heterologous expression and metabolic engineering for biotechnological applications.

Identification of alleles of carotenoid pathway genes important for zeaxanthin accumulation in potato tubers

We have investigated the genetics and molecular biology of orange flesh colour in potato (Solanum tuberosum L.). To this end the natural diversity in three genes of the carotenoid pathway was assessed by SNP analyses. Association analysis was performed between SNP haplotypes and flesh colour phenotypes in diploid and tetraploid potato genotypes. We observed that among eleven beta-carotene hydroxylase 2 (Chy2) alleles only one dominant allele has a major effect, changing white into yellow flesh colour. In contrast, none of the lycopene epsilon cyclase (Lcye) alleles seemed to have a large effect on flesh colour. Analysis of zeaxanthin epoxidase (Zep) alleles showed that all (diploid) genotypes with orange tuber flesh were homozygous for one specific Zep allele. This Zep allele showed a reduced level of expression. The complete genomic sequence of the recessive Zep allele, including the promoter, was determined, and compared with the sequence of other Zep alleles. The most striking difference was the presence of a non-LTR retrotransposon sequence in intron 1 of the recessive Zep allele, which was absent in all other Zep alleles investigated. We hypothesise that the presence of this large sequence in intron 1 caused the lower expression level, resulting in reduced Zep activity and accumulation of zeaxanthin. Only genotypes combining presence of the dominant Chy2 allele with homozygosity for the recessive Zep allele produced orange-fleshed tubers that accumulated large amounts of zeaxanthin.