Abstract
A gene controlling golden flesh trait in watermelon was discovered and fine mapped to a 39.08 Kb region on chromosome 1 through a forward genetic strategy, and Cla97C01G008760 (annotated as phytoene synthase protein, ClPsy1 ) was recognized as the most likely candidate gene. Vitamin A deficiency is a worldwide public nutrition problem, and β-carotene is the precursor for vitamin A synthesis. Watermelon with golden flesh (gf, which occurs due to an accumulated abundance of β-carotene) is an important germplasm resource. In this study, a genetic analysis of segregated gf gene populations indicated that gf was controlled by a single recessive gene. BSA-seq (Bulked segregation analysis) and an initial linkage analysis placed the gf locus in a 290-Kb region on watermelon chromosome 1. Further fine mapping in a large population including over 1000 F2 plants narrowed this region to 39.08Kb harboring two genes, Cla97C01G008760 and Cla97C01G008770, which encode phytoene synthase (ClPsy1) and GATA zinc finger domain-containing protein, respectively. Gene sequence alignment and expression analysis between parental lines revealed Cla97C01G008760 as the best possible candidate gene for the gf trait. Nonsynonymous SNP mutations in the first exon of ClPsy1 between parental lines co-segregated with the gf trait only among individuals in the genetic population and were not related to flesh color in natural watermelon panels. Promoter sequence analysis of 26 watermelon accessions revealed two SNPs in the cis-acting element sequences corresponding to MYB and MYC2 transcription factors. RNA-seq data and qRT-PCR verification showed that two MYBs exhibited expression trends similar to that of ClPsy1 in the parental lines and may regulate the ClPsy1 expression. Further research findings indicate that the gf trait is determined not only by ClPsy1 but also by ClLCYB, ClCRTISO and ClNCED7, which play important roles in watermelon β-carotene accumulation.
Highlights
Watermelon (Citrullus lanatus L.) is a commercial Cucurbitaceae crop that is cultivated worldwide and consumed fresh
Based on the flesh color associated with the two pigments, we speculated that the high accumulation of β-carotene in PI 192938 may be the main reason for the gf trait (Fig. 1a)
Based on the above results, we conclude that the gf trait in watermelon flesh is controlled by a inherited gene and that pale yellow flesh is partly dominant to the gf color
Summary
Watermelon (Citrullus lanatus L.) is a commercial Cucurbitaceae crop that is cultivated worldwide and consumed fresh. Carotenoids are the main pigment responsible for formation of the watermelon flesh color. Violaxanthin and lutein are the two main pigments responsible for the formation of canary and pale yellow flesh, and violaxanthin is significantly more abundant in canary yellow than in pale yellow watermelon (Yuan et al, 2015 and Fang et al, 2020). Carotenoids, which form part of the human diet, serve as precursors of vitamin A and substrates for many nutrients and exhibit antioxidant activity. These compounds are indispensable for health, humans are incapable of de novo carotenoid synthesis (Rodriguez-Concepcion et al, 2018). The World Health Organization (WHO) estimates that approximately 190 million preschool children are deficient in vitamin A, and vitamin A deficiency is one of the major public health problems in the world. βCarotene is the precursor for vitamin A synthesis, and the consumption of horticulture crops with high levels of β-carotene (golden flesh), which constitute an important germplasm resource, is an effective strategy for preventing vitamin A deficiency
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