RAPID STIMULATION OF A SOYBEAN PROTEIN-SERINE KINASE THAT PHOSPHORYLATES A NOVEL BZIP DNA-BINDING PROTEIN, G HBF-1, DURING THE INDUCTION OFEARLY TRANSCRIPTION-DEPENDENT DEFENSES/

The G-box (CACGTG) and H-box (CCTACC) cis elements function in the act ivation of phenylpropanoid biosynthetic genes involved in the elaborat ion of lignin precursors, phytoalexins and the secondary signal salicy lic acid as early responses to pathogen attack. We have isolated a soy bean 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 sy nthase chs15 promoter, While G/HBF-1 transcript and protein levels do not increase during the induction of phenylpropanoid biosynthetic gene s, G/HBF-1 is phosphorylated rapidly in elicited soybean cells, almost exclusively on serine residues. Using recombinant G/HBF-1 as a substr ate, we identified a cytosolic protein-serine kinase that is rapidly a nd 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 acti vity and G/HBF-1 phosphorylation are terminal events in a signal pathw ay for activation of early transcription-dependent plant defense respo nses.