Interaction between rice MYBGA and the gibberellin response element controls tissue-specific sugar sensitivity of α-amylase genes

Expression of alpha-amylase genes during cereal grain germination and seedling growth is regulated negatively by sugar in embryos and positively by gibberellin ( GA) in endosperm through the sugar response complex ( SRC) and the GA response complex ( GARC), respectively. We analyzed two alpha-amylase promoters, alpha Amy3 containing only SRC and alpha Amy8 containing overlapped SRC and GARC. alpha Amy3 was sugar-sensitive but GA- nonresponsive in both rice ( Oryza sativa) embryos and endosperms, whereas alpha Amy8 was sugar-sensitive in embryos and GA-responsive in endosperms. Mutation of the GA response element ( GARE) in the alpha Amy8 promoter impaired its GA response but enhanced sugar sensitivity, and insertion of GARE in the alpha Amy3 promoter rendered it GA-responsive but sugar-insensitive in endosperms. Expression of the GARE-interacting transcription factor MYBGA was induced by GA in endosperms, correlating with the endosperm-specific alpha Amy8 GA response. alpha Amy8 became sugar-sensitive in MYBGA knockout mutant endosperms, suggesting that the MYBGA-GARE interaction overrides the sugar sensitivity of alpha Amy8. In embryos overexpressing MYBGA, aAmy8 became sugar-insensitive, indicating that MYBGA affects sugar repression. alpha-Amylase promoters active in endosperms contain GARE, whereas those active in embryos may or may not contain GARE, confirming that the GARE and GA-induced MYBGA interaction prevents sugar feedback repression of endosperm alpha-amylase genes. We demonstrate that the MYBGA-GARE interaction affects sugar feedback control in balanced energy production during seedling growth and provide insight into the control mechanisms of tissue-specific regulation of alpha-amylase expression by sugar and GA signaling interference.