EVIDENCE FOR G-PROTEIN REGULATION OF INWARD K+ CHANNEL CURRENT IN GUARD-CELLS OF FAVA-BEAN

Recent reports have shown that GTP-binding proteins (G-proteins) are present in plants but have given limited indication as to their site of action. G-proteins in animal cells transduce extracellular signals into intracellular or membrane-mediated events, including the regulation of ion channels. Using whole-cell patch clamp, we provide evidence that a G-protein in guard cells of fava bean regulates the magnitude (and not the kinetics) of inward current through K+-selective ion channels in the plasma membrane. GDP-beta-S (100 to 500-mu-M) increases inward K+ current, whereas GTP-gamma-S (500-mu-M) has the opposite effect. The control nucleotides ADP-beta-S and ATP-gamma-S (500-mu-M) do not affect K+ current. Reduction of inward current by GTP-gamma-S is eliminated in the presence of the Ca2+ chelator, BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N',-tetraacetic acid) (5 mM). When applied intracellularly, the G-protein regulators, cholera toxin and pertussis toxin, both decrease inward K+ current. The entry of K+ (and anions) into guard cells increases their turgor, opening stomatal pores in the leaf epidermis that allow gas exchange with the environment. Our data suggest the involvement of a G-protein in the inhibition of K+ uptake and stomatal opening. Changes in stomatal aperture, vital to both photosynthesis and plant water status, reflect guard-cell responsiveness to a variety of known environmental signals. The results presented here indicate that, in plants as well as animals, ion channel regulation by environmental stimuli may be mediated by G-proteins.