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. GDPβS (100 to 500 μM) increases inward K⁺ current, whereas GTPγS (500 μM) has the opposite effect. The control nucleotides ADPβS and ATPγS (500 μM) do not affect K⁺ current. Reduction of inward current by GTPγS 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.