Functional Characterization and in Silico Analysis of Phytoene Synthase Family Genes Responsible for Carotenoid Biosynthesis in Watermelon (Citrullus lanatus L.)

Abstract

Carotenoids are the main pigments in watermelon (Citrullus lanatus L.) fruit and contribute to its aesthetic and nutritional value. Phytoene synthase (PSY) is reported to be the first rate-limiting enzyme in carotenogenesis and controls the carotenoid flux. This study aimed to identify PSY genes responsible for carotenoid biosynthesis in the red-fleshed watermelon cultivar LSW-177. The PSY gene members ClPSY1, ClPSY2 and ClPSY3 were characterized and their catalytic activities were displayed in the heterologous complementation assay. The transcript levels of ClPSY genes at the different developmental stages of LSW-177 fruit and the promoter sequence of ClPSY1 were also analyzed. Transcription factors involved in regulating the ClPSY1 expression were scanned with previous RNA-seq data of the different stages during fruit ripening. Results showed that the PSY proteins from watermelon LSW-177 contained the conserved PSY domains and exhibited the ability to condense GGPP into phytoene in E. coli. ClPSY1 is the dominant carotenogenic gene during fruit ripening; and can be induced by light and hormones. Furthermore, Cla013914 and Cla007950 that, respectively encode the transcription factors WD40-like protein and bZIP, likely upregulate ClPSY1 during fruit ripening. In conclusion, ClPSY1 play a dominant role in carotenoid biosynthesis during watermelon fruit ripening and is regulated by complex light and hormone-responsive networks.