title: An Abscisic Acid-Activated and Calcium-Independent Protein Kinase from Guard Cells of Fava Bean.
abstract: Abscisic acid (ABA) regulation of stomatal aperture is known to involve both Ca2+-dependent and Ca2+-independent signal transduction pathways. Electrophysiological studies suggest that protein phosphorylation is involved in ABA action in guard cells. Using biochemical approaches, we identified an ABA-activated and Ca2+- independent protein kinase (AAPK) from guard cell protoplasts of fava bean. Autophosphorylation of AAPK was rapidly (~1 min) activated by ABA in a Ca2+- independent manner. ABA-activated autophosphorylation of AAPK occurred on serine but not on tyrosine residues and appeared to be guard cell specific. AAPK phosphorylated histone type III-S on serine and threonine residues, and its activity toward histone type III-S was markedly stimulated in ABA-treated guard cell protoplasts. Our results suggest that AAPK may play an important role in the Ca2+-independent ABA signaling pathways of guard cells.
authors.affiliation: Department of Biology and Plant Physiology Program, Pennsylvania State University, University Park, Pennsylvania 16802.
authors.forename: J.
authors.initials: J
authors.lastname: Li
authors.forename: S. M.
authors.initials: SM
authors.lastname: Assmann