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

Sugars repress alpha-amylase expression in germinating embryos and cell cultures of rice (Oryza sativa) through a sugar response complex (SRC) in alpha-amylase gene promoters and its interacting transcription factor MYBS1. The Snf1 protein kinase is required for the derepression of glucose-repressible genes in yeast. In this study, we explored the role of the yeast Snf1 ortholog in rice, SnRK1, in sugar signaling and plant growth. Rice embryo transient expression assays indicated that SnRK1A and SnRK1B act upstream and relieve glucose repression of MYBS1 and alphaAmy3 SRC promoters. Both SnRK1s contain N-terminal kinase domains serving as activators and C-terminal regulatory domains as dominant negative regulators of SRC. The accumulation and activity of SnRK1A was regulated by sugars posttranscriptionally, and SnRK1A relieved glucose repression specifically through the TA box in SRC. A transgenic RNA interference approach indicated that SnRK1A is also necessary for the activation of MYBS1 and alphaAmy3 expression under glucose starvation. Two mutants of SnRK1s, snrk1a and snrk1b, were obtained, and the functions of both SnRK1s were further studied. Our studies demonstrated that SnRK1A is an important intermediate in the sugar signaling cascade, functioning upstream from the interaction between MYBS1 and alphaAmy3 SRC and playing a key role in regulating seed germination and seedling growth in rice.