Interaction between Rice MYBGA and the Gibberellin Response Element Controls Tissue-Specific Sugar Sensitivity of α-Amylase Genes

Abstract

Expression of a-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 a-amylase promoters, aAmy3 containing only SRC and aAmy8 containing overlapped SRC and GARC. aAmy3 was sugar-sensitive but GA-nonresponsive in both rice (Oryza sativa) embryos and endosperms, whereas aAmy8 was sugar-sensitive in embryos and GA-responsive in endosperms. Mutation of the GA response element (GARE) in the aAmy8 promoter impaired its GA response but enhanced sugar sensitivity, and insertion of GARE in the aAmy3 promoter rendered it GA-responsive but sugar-insensitive in endosperms. Expression of the GAREinteracting transcription factor MYBGA was induced by GA in endosperms, correlating with the endosperm-specific aAmy8 GA response. aAmy8 became sugar-sensitive in MYBGA knockout mutant endosperms, suggesting that the MYBGA-GARE interaction overrides the sugar sensitivity of aAmy8. In embryos overexpressing MYBGA, aAmy8 became sugar-insensitive, indicating that MYBGA affects sugar repression. a-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 a-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 a-amylase expression by sugar and GA signaling interference.

The SnRK1A Protein Kinase Plays a Key Role in Sugar Signaling during Germination and Seedling Growth of Rice

“…The callus-containing rice embryos were prepared and particlebombarded as described (Chen et al, 2006). The rice embryos were cobombarded with reporter, effector, and internal control plasmids at a ratio of 2:1:0.25.…”

“…In general, sugars upregulate genes involved in biosynthesis, transport, and storage of reserves and cell growth and downregulate those associated with photosynthesis, reserve mobilization, and response to stresses (Graham, 1996;Koch, 1996;Ho et al, 2001). Studies with Arabidopsis thaliana and rice (Oryza sativa) have revealed intimate interaction between sugar and plant hormones, environmental stimuli, and metabolic signaling pathways (Sheen et al, 1999;Gibson, 2000;Smeekens, 2000;Coruzzi and Zhou, 2001;Gazzarrini and McCourt, 2001;Rolland et al, 2002Rolland et al, , 2006Hong et al, 2003;Chen et al, 2006). Despite extensive efforts in the identification of mutants defective in sugar responses (Sheen et al, 1999;Rolland et al, 2002Rolland et al, , 2006, the identity of essential components in the sugar signaling pathway, the detailed mechanism underlying sugar-regulated transcription, and how they regulate plant growth and development remain mostly unclear.…”

“…In cereals, a-amylases are essential enzymes for the hydrolysis of starch to sugars to support germination and seedling development, with the expression of a-amylase genes activated by sugar depletion in embryos and induced by gibberellins in endosperms (Yu et al, 1992Perata et al, 1997;Chen et al, 2006). The expression of a-amylase in cultured rice suspension cells and rice and barley embryos is repressed by almost all metabolizable sugars (Yu et al, 1991;Perata et al, 1997;Umemura et al, 1998).…”

“…The expression of a-amylase in cultured rice suspension cells and rice and barley embryos is repressed by almost all metabolizable sugars (Yu et al, 1991;Perata et al, 1997;Umemura et al, 1998). aAmy3 and aAmy8 constitute two major rice a-amylases expressed under sugar-depleted conditions and are important for the catabolism of starch as a carbon source when sugar levels become too low to sustain respiration and growth in rice suspension cells and germinating embryos Yu, 1999;Chen et al, 2006). Sugar repression of a-amylase gene expression involves the control of transcription rate and mRNA stability (Chan et al, 1994;Sheu et al, 1994Sheu et al, , 1996Yu, 1998a, 1998b), and common regulatory elements in aAmy3 and aAmy8 sugar response complexes (SRCs) have been identified (Hwang et al, 1998;Lu et al, 1998;Toyofuku et al, 1998;Chen et al, 2006).…”

“…aAmy3 and aAmy8 constitute two major rice a-amylases expressed under sugar-depleted conditions and are important for the catabolism of starch as a carbon source when sugar levels become too low to sustain respiration and growth in rice suspension cells and germinating embryos Yu, 1999;Chen et al, 2006). Sugar repression of a-amylase gene expression involves the control of transcription rate and mRNA stability (Chan et al, 1994;Sheu et al, 1994Sheu et al, , 1996Yu, 1998a, 1998b), and common regulatory elements in aAmy3 and aAmy8 sugar response complexes (SRCs) have been identified (Hwang et al, 1998;Lu et al, 1998;Toyofuku et al, 1998;Chen et al, 2006). The 100-bp aAmy3 SRC contains three essential motifs, the GC box, the G box, and the TA box, for high-level promoter activity under sugar-depleted conditions (Lu et al, 1998).…”