Abstract
The G-box (CACGTG) and H-box (CCTACC) cis elements function in the activation of phenylpropanoid biosynthetic genes involved in the elaboration of lignin precursors, phytoalexins and the secondary signal salicylic acid as early responses to pathogen attack. We have isolated a soybean cDNA encoding a novel bZIP protein, G/HBF-1, which binds to both the G-box and adjacent H-box in the proximal region of the chalcone synthase chs15 promoter. While G/HBF-1 transcript and protein levels do not increase during the induction of phenylpropanoid biosynthetic genes, G/HBF-1 is phosphorylated rapidly in elicited soybean cells, almost exclusively on serine residues. Using recombinant G/HBF-1 as a substrate, we identified a cytosolic protein-serine kinase that is rapidly and transiently stimulated in cells elicited with either glutathione or an avirulent strain of the soybean pathogen Pseudomonas syringae pv. glycinea. Phosphorylation of G/HBF-1 in vitro enhances binding to the chs15 promoter and we conclude that stimulation of G/HBF-1 kinase activity and G/HBF-1 phosphorylation are terminal events in a signal pathway for activation of early transcription-dependent plant defense responses.