Abscisic acid regulation of guard-cell K and anion channels in G - and RGS-deficient Arabidopsis lines

In mammals, basal currents through G protein-coupled inwardly rectifying K (GIRK) channels are repressed by G i/oGDP, and the channels are activated by direct binding of free G subunits released upon stimulation of G i/o-coupled receptors. However, essentially all information on G protein regulation of GIRK electrophysiology has been gained on the basis of coexpression studies in heterologous systems. A major advantage of the model organism, Arabidopsis thaliana, is the ease with which knockout mutants can be obtained. We evaluated plants harboring mutations in the sole Arabidopsis G (AtGPA1), G (AGB1), and Regulator of G protein Signaling (AtRGS1) genes for impacts on ion channel regulation. In guard cells, where K fluxes are integral to cellular regulation of stomatal apertures, inhibition of inward K (Kin) currents and stomatal opening by the phytohormone abscisic acid (ABA) was equally impaired in Atgpa1 and agb1 single mutants and the Atgpa1 agb1 double mutant. AGB1 overexpressing lines maintained a wild-type phenotype. The Atrgs1 mutation did not affect Kin current magnitude or ABA sensitivity, but Kin voltage-activation kinetics were altered. Thus, Arabidopsis cells differ from mammalian cells in that they uniquely use the G subunit or regulation of the heterotrimer to mediate Kin channel modulation after ligand perception. In contrast, outwardly rectifying (Kout) currents were unaltered in the mutants, and ABA activation of slow anion currents was conditionally disrupted in conjunction with cytosolic pH clamp. Our studies highlight unique aspects of ion channel regulation by heterotrimeric G proteins and relate these aspects to stomatal aperture control, a key determinant of plant biomass acquisition and drought tolerance.