Uncovering New Genetic Tools and Mechanisms in Mediating Carotenoid Accumulation via Investigation of Melon CmOr Gene

Carotenoids are responsible for the orange flesh color found in various melon fruit. Our recent studies defined CmOr as the single gene that controls fruit flesh color and identified CmOr as the previously described gf locus in melon. CmOr was found to co-segregate with fruit flesh color and present with two haplotypes in a broad germplasm collection, one being associated with orange flesh and the second being associated with either white or green flesh. A single “golden” SNP that alters the evolutionarily highly-conserved arginine¹⁰⁸ (Arg) to histidine (His) in CmOR governs increased b-carotene accumulation visualized as orange fruit flesh melon. Functional characterization of CmOr alleles of orange and non-orange fruits in transgenic Arabidopsis plants confirms the “golden” SNP role in controlling the inheritance of fruit color in melon. Moreover, we found that alteration of the nucleotides to change the conserved Arg into both histidine and alanine in Or homologs of other plant species also promote high levels of carotenoid accumulation, demonstrating the effectiveness of the “golden” SNP and its critical role in promoting carotenoid accumulation. The OR protein was discovered to possess dual functions in regulating both PSY for carotenoid biosynthesis and chromoplast differentiation to enhance storage sink strength. The metabolic and cellular processes associated with CmOr allelic variation and genes that participate in these processes were revealed via an F₃ family bulked segregant RNA-Seq (BSR-Seq) analysis. These findings broaden our fundamental understanding of mechanisms underlying carotenoid accumulation in plants, and provide effective genetic tools for developing agricultural products with improved b-carotene content.