Carotenoid Accumulation and Carotenogenic Gene Expression during Fruit Development in Novel Interspecific Inbred Squash Lines and Their Parents

Carotenoid levels and composition during squash fruit development were compared in Cucurbita moschata, Cucurbita maxima, and two lines of their interspecific inbred lines, namely, Maxchatal and Maxchata2. Eight genes associated with carotenoid biosynthesis were analyzed by quantitative RT-PCR. The two squash species and their interspecific inbred lines exhibited different qualitative and quantitative carotenoid profiles and regulatory mechanisms. C. moschata had the lowest total carotenoid content and mainly accumulated a-carotene and beta-carotene, as expected in a fruit with pale-orange flesh. Low carotenoid content in this species was probably due to the comparatively low expression of all genes investigated, especially PSYI gene, compared to the other squashes. The predominant carotenoids in C. maxima were violaxanthin and lutein, which produced a corresponding yellow flesh color in mature fruit. The relationship between the expression of the CHYB and ZEP genes may result in almost equal concentrations of violaxanthin and lutein in C. maxima at fruit ripening. In contrast, their interspecific inbred lines principally accumulated lutein and beta-carotene, leading to orange flesh color. The PSYI gene exhibited higher expression levels at earlier stages of fruit development in the Maxchata lines, potentially triggering the increased carotenoid accumulation seen in these fruits. Likewise, the higher transcription level of CHYB gene observed in the two interspecific inbred lines might be correlated with high lutein in these hybrids. However, this study could not explain the observed beta-carotene accumulation on the basis of gene expression.