Comparative transcriptome analysis implied a ZEP paralog was a key gene involved in carotenoid accumulation in yellow-fleshed sweetpotato

The mechanisms of carotenoid accumulation in yellow-fleshed sweetpotato cultivars are unclear. In this study, we compared the transcriptome profiles of a yellow-fleshed cultivar, Beniharuka (BH) and two of its spontaneous white-fleshed mutants (WH2 and WH3) to reveal the genes involved in yellow flesh. As a result of RNA sequencing, a total of 185 differentially expressed genes (DEGs) were commonly detected in WH2 and WH3 compared to BH. Of these genes, 85 DEGs and 100 DEGs were commonly upregulated and downregulated in WH2 and WH3 compared to BH, respectively. g1103.t1, a paralog of zeaxanthin epoxidase (ZEP), was only DEG common to WH2 and WH3 among 38 genes considered to be involved in carotenoid biosynthesis in storage roots. The expression level of g1103.t1 was also considerably lower in five white-fleshed cultivars than in five yellow-fleshed cultivars. Analysis of carotenoid composition in the storage roots showed that the epoxidised carotenoids were drastically reduced in both WH2 and WH3. Therefore, we propose that the ZEP paralog, g1103.t1, may be involved in carotenoid accumulation through the epoxidation of β-carotene and β-cryptoxanthin in sweetpotato.

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 a 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 reported. 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 condition, parameters measured were not significantly different between transgenic and wild-type (WT) plants except for non-photochemical quenching value and chlorophyll a content, while difference existed in 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 stoma 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, wherase non-photochemical quenching (NPQ), net photosynthetic rate (Pn), stomata conductance (Gs) and transpiration rate (Tr) of MsZEP-overexpression tobacco were significantly higher than WT plants. However, no significant difference was existed 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 stomata density under low light.