Common mechanisms regulating expression of rice aleurone genes that contribute to the primary response for gibberellin

Abstract

During germination of cereal seeds, aleurone cells respond to gibberellins (GA) by synthesizing and secreting hydrolytic enzymes that mobilize the reserved nutrients. It has been shown that products of early GA response genes, like a transcription factor GAMyb, act as key molecules leading to this regulation. Pivotal roles of GAMyb on expression of hydrolase genes have been well documented, whereas regulation of GAMyb expression itself remains obscure. In order to understand virtual mechanisms of the GA-mediated expression of genes, it is important to know how GA control expression of early GA response genes. Using an aleurone transient expression system of rice (Oryza sativa L.), we examined GA responsive domains of early GA response genes in the aleurone, such as GAMyb and OsDof3. The upstream promoter could not confer GA response. Extensive analyses revealed the presence of enhancer-like activities in a large first intron. In Arabidopsis, intron enhancers have been identified in MADS-box homeotic genes, AGAMOUS (AG) and FLOWERING LOCUS C (FLC), in which large introns should not only confer proper gene expressions, but also associate with gene silencing by covalent modifications of both DNA and histone. These evidences prompt us to assign that chromatin-based control might be important for initial GA action. Based on this assumption, we have identified DNA methylation of the GAMyb locus in germinated rice seeds.