Comparative Transcriptome Analysis Identified Key Pathways and Genes Regulating Differentiated Stigma Color in Melon (Cucumis melo L.).

Stigma color is an important morphological trait in many flowering plants. Visual observations in different field experiments have shown that a green stigma in melons is more attractive to natural pollinators than a yellow one. In the current study, we evaluated the characterization of two contrasted melon lines (MR-1 with a green stigma and M4-7 with a yellow stigma). Endogenous quantification showed that the chlorophyll and carotenoid content in the MR-1 stigmas was higher compared to the M4-7 stigmas. The primary differences in the chloroplast ultrastructure at different developmental stages depicted that the stigmas of both melon lines were mainly enriched with granum, plastoglobulus, and starch grains. Further, comparative transcriptomic analysis was performed to identify the candidate pathways and genes regulating melon stigma color during key developmental stages (S1-S3). The obtained results indicated similar biological processes involved in the three stages, but major differences were observed in light reactions and chloroplast pathways. The weighted gene co-expression network analysis (WGCNA) of differentially expressed genes (DEGs) uncovered a "black" network module (655 out of 5302 genes), mainly corresponding to light reactions, light harvesting, the chlorophyll metabolic process, and the chlorophyll biosynthetic process, and exhibited a significant contribution to stigma color. Overall, the expression of five key genes of the chlorophyll synthesis pathway-CAO (MELO03C010624), CHLH (MELO03C007233), CRD (MELO03C026802), HEMA (MELO03C011113), POR (MELO03C016714)-were checked at different stages of stigma development in both melon lines using quantitative real time polymerase chain reaction (qRT-PCR). The results exhibited that the expression of these genes gradually increased during the stigma development of the MR-1 line but decreased in the M4-7 line at S2. In addition, the expression trends in different stages were the same as RNA-seq, indicating data accuracy. To sum up, our research reveals an in-depth molecular mechanism of stigma coloration and suggests that chlorophyll and related biological activity play an important role in differentiating melon stigma color.

Genetic Mapping and QTL Analysis of Stigma Color in Melon (Cucumis melo L.).

Melon is an important Cucurbitaceae crop. Field observations had shown that the green stigmas of melon are more attractive to pollinators than yellow stigmas. In this study, F2 and F2:3 populations obtained by crossing MR-1 (green stigma) and M4-7 (yellow stigma) were used for genetic analysis and mapping. A genetic map of 1,802.49 cm was constructed with 116 cleaved amplified polymorphism sequence (CAPS) markers. Two stable quantitative trait loci (QTLs) linked to the trait of stigma color were identified on chromosomes 2 (SC2.1) and 8 (SC8.1), respectively. An expanded F2 population was used to narrow down the confidence regions of SC2.1 and SC8.1. As a result, SC2.1 was further mapped to a 3.6 cm region between CAPS markers S2M3 and S2B1-3, explaining 9.40% phenotypic variation. SC8.1 was mapped to a 3.7-cm region between CAPS markers S8E7 and S8H-1, explaining 25.92% phenotypic variation. This study broadens our understanding of the mechanisms of stigma color regulation and will be of benefit to the breeding of melon.