The Citrus Transcription Factor CsMADS6 Modulates Carotenoid Metabolism by Directly Regulating Carotenogenic Genes.

Although remarkable progress has been made toward understanding carotenoid biosynthesis, the mechanisms that regulate the transcription of carotenogenic genes remain poorly understood. Lycopene β-cyclases (LCYb) are critical enzymes located at the branch point of the carotenoid biosynthetic pathway. Here, we used the promoter sequence of as bait in a yeast one-hybrid screen for promoter-binding proteins from sweet orange (). This screen identified a MADS transcription factor, CsMADS6, that was coordinately expressed with fruit development and coloration. Acting as a nucleus-localized transcriptional activator, CsMADS6 directly bound the promoter of and activated its expression. Overexpression of in citrus calli increased carotenoid contents and induced the expression of and other carotenogenic genes, including (), (), and (). CsMADS6 up-regulated the expression of , , and by directly binding to their promoters, which suggested the multitargeted regulation of carotenoid metabolism by CsMADS6. In addition, the ectopic expression of in tomato () affected carotenoid contents and the expression of carotenogenic genes. The sepals of -overexpressing tomato lines exhibited dramatic changes in carotenoid profiles, accompanied by changes in plastid ultrastructure. Global transcriptome analysis of transgenic sepals revealed that CsMADS6 regulates a series of pathways that promote increases in flux through the carotenoid pathway. Overall, these findings establish that CsMADS6 directly regulates and other carotenogenic genes to coordinately and positively modulate carotenoid metabolism in plants, which may provide strategies to improve the nutritional quality of crops.