A novel histone methyltransferase gene CgSDG40 positively regulates carotenoid biosynthesis during citrus fruit ripening

The flesh color of pummelo (Citrus maxima) fruits is highly diverse and largely depends on the level of carotenoids, which are beneficial to human health. It is vital to investigate the regulatory network of carotenoid biosynthesis to improve the carotenoid content in pummelo. However, the molecular mechanism underlying carotenoid accumulation in pummelo is not fully understood. In this study, we identified a novel histone methyltransferase gene, CgSDG40, involved in carotenoid regulation by analyzing the flesh transcriptome of typical white-fleshed pummelo, red-fleshed pummelo and extreme-colored F1 hybrids from a segregated pummelo population. Expression of CgSDG40 corresponded to flesh color change and was highly coexpressed with CgPSY1. Interestingly, CgSDG40 and CgPSY1 are located physically adjacent to each other on the chromosome in opposite directions, sharing a partially overlapping promoter region. Subcellular localization analysis indicated that CgSDG40 localizes to the nucleus. Overexpression of CgSDG40 significantly increased the total carotenoid content in citrus calli relative to that in wild type. In addition, expression of CgPSY1 was significantly activated in overexpression lines relative to wild type. Taken together, our findings reveal a novel histone methyltransferase regulator, CgSDG40, involved in the regulation of carotenoid biosynthesis in citrus and provide new strategies for molecular design breeding and genetic improvement of fruit color and nutritional quality.