Comparative transcriptome analyses shed light on carotenoid production and plastid development in melon fruit

Carotenoids, such as beta-carotene, accumulate in chromoplasts of various fleshy fruits, awarding them with colors, aromas, and nutrients. The Orange (CmOr) gene controls beta-carotene accumulation in melon fruit by posttranslationally enhancing carotenogenesis and repressing beta-carotene turnover in chromoplasts. Carotenoid isomerase (CRTISO) isomerizes yellow prolycopene into red lycopene, a prerequisite for further metabolism into beta-carotene. We comparatively analyzed the developing fruit transcriptomes of orange-colored melon and its two isogenic EMS-induced mutants, low-beta (Cmor) and yofi (Cmcrtiso). The Cmor mutation in low-beta caused a major transcriptomic change in the mature fruit. In contrast, the Cmcrtiso mutation in yofi significantly changed the transcriptome only in early fruit developmental stages. These findings indicate that melon fruit transcriptome is primarily altered by changes in carotenoid metabolic flux and plastid conversion, but minimally by carotenoid composition in the ripe fruit. Clustering of the differentially expressed genes into functional groups revealed an association between fruit carotenoid metabolic flux with the maintenance of the photosynthetic apparatus in fruit chloroplasts. Moreover, large numbers of thylakoid localized photosynthetic genes were differentially expressed in low-beta. CmOR family proteins were found to physically interact with light-harvesting chlorophyll a-b binding proteins, suggesting a new role of CmOR for chloroplast maintenance in melon fruit. This study brings more insights into the cellular and metabolic processes associated with fruit carotenoid accumulation in melon fruit and reveals a new maintenance mechanism of the photosynthetic apparatus for plastid development.