Effects of fusicoccin on ion fluxes in guard cells.

The pharmacology has been further investigated of the two transport systems mediating potassium (rubidium) (K(+)(Rb(+))) release from the guard cell vacuole, responsible, respectively, for the resting efflux and abscisic acid (ABA)-induced transient stimulation of efflux, and for the transient stimulation induced by hypotonic treatment. Here, the effects of fusicoccin and of butyrate-induced cytoplasmic acidification on (86)Rb efflux were measured in isolated guard cells of Commelina communis. Fusicoccin (10 microM) inhibited the resting efflux at the tonoplast and the ABA-induced transient, but had no effect on the hypotonic transient. All three processes were inhibited by cytoplasmic acidification. Fusicoccin did not inhibit efflux at the plasmalemma. As the hypotonic response is inhibited by cytoplasmic acidification but not by fusicoccin, the effect of fusicoccin on the resting efflux and ABA response must be direct, and not the result of fusicoccin-induced cytoplasmic acidification. The collected tonoplast efflux properties resemble those of TPC1 (two-pore channel) rather than TPK1 (two-pore K channel). The flux and TPC1 are both activated by Ca(2+), but inhibited by phenylarsine oxide and by cytoplasmic acidification. The flux is inhibited by fusicoccin. TPC1 is inhibited by 14-3-3 proteins and has the C-terminal sequence STSDT, a type III binding site for 14-3-3 proteins, of the kind involved in fusicoccin binding.