Phosphatase Regulation of Plant K+ Channels

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9316319 Assmann Application of Ca++ to plants prevents stomatal opening. One effect of Ca++ is to inactivate the inward K+ channels that mediate K+ uptake. Evidence has recently been obtained that this activation results from Ca++ activation of a phosphatase analogous to animal protein phosphatase 2B. The proposed whole-cell patch clamp experiments will test whether the protein phosphatase 2B acts " downstream" of abscisic acid and/or activated G-protein and/or inositoltriphosphate in a signal transduction pathway. Parallel biochemical studies utilizing guard cell protoplasts and guard cell protein extracts will determine whether abscisic acid, Ca++ and protein phosphatase 2B cause dephosphorylation of the same protein, as would be predicted if protein phosphatase 2B is a second mesenger for the abscisic acid response. Microinjection experiments on intact guard cells will address the possibility that protein phosphatase 2B is, alternatively or in addition, a second messenger for other environmental signals that prevent stomatal opening. %%% Stomatal guard cells are vital control points in the regulation of photosynthetic CO2 uptake and transpirational water loss. Stomatal appertures are regulated by osmotic swelling and shrinking of guard cells, driven by ion fluxes across the guard-cell plasma membrane. Several reports implicate Ca++ as one of the second messengers for abscisic acid, a plant hormone that prevents stomatal opening and also reduces inward K+ current. In this proposal complementary electrophysiological, cell biological and biochemical experiments are outlined to test the hypothesis that environmental signals which regulate stomatal opening act on inward K+ channels via a phosphatase 2B homologue. Results from these studies will provide insight into the second messenger systems utilized by plants to translate environmental signals into alterations of ion channel activity. Results will also be of interest to environmental physiologis ts who would like to understand how the external and internal environments affects stomatal apertures and thereby impacts plant productivity. ***