Evidence for a role for protein tyrosine phosphatase in the control of ion release from the guard cell vacuole in stomatal closure

Protein tyrosine phosphatases (PTPases) exist in plants, but their role in plant signaling processes is unknown. One of the most important signaling networks in plants concerns the regulation of stomatal aperture, by which closure of stomatal pores restricts water loss in dry conditions, a process essential for plant survival. Closure is achieved by reduction in guard cell volume as a consequence of net efflux of potassium salt at both plasmalemma and tonoplast. To test whether protein tyrosine phosphorylation has any role in guard cell signaling processes, the effects on stomatal aperture and on guard cell K(Rb) fluxes of a number of specific inhibitors of PTPases have been investigated. Stomatal closure induced by abscisic acid, high external Ca2+, hydrogen peroxide, and dark were all prevented by one such inhibitor, phenylarsine oxide, which added to closed stomata promoted reopening. Flux measurements with 86Rb+ identified the efflux across the tonoplast as the sensitive process, implying that protein tyrosine dephosphorylation must occur at or downstream of the Ca2+ signal responsible for triggering ion efflux from the vacuole. There was no inhibition of efflux at the plasmalemma. A second inhibitor of PTPases, 3,4 dephosphatin, gave very similar effects, inhibiting closure induced by abscisic acid, high external Ca2+, and dark, and promoting reopening if added to closed stomata. Again, the efflux of K(Rb) at the tonoplast was the sensitive process. These results provide clear evidence for the involvement of PTPases in a major signaling network in plants.