Functional interaction of the SNARE protein NtSyp121 in Ca²⁺ channel gating, Ca²⁺ transients and ABA signalling of stomatal guard cells

There is now growing evidence that membrane vesicle trafficking proteins, especially of the superfamily of SNAREs, are critical for cellular signalling in plants.Work from this laboratory first demonstrated that a soluble, inhibitory (dominant-negative) fragment of the SNARE NtSyp121 blocked K⁺ and Cl^- channel responses to the stress-related hormone abscisic acid (ABA), but left open a question about functional impacts on signal intermediates, especially on Ca²⁺-mediated signalling events. Here,we report one mode of action for the SNARE mediated directly through alterations in Ca²⁺ channel gating and its consequent effects on cytosolic-free [Ca²⁺] ([Ca²⁺]_i) elevation. We find that expressing the same inhibitory fragment of NtSyp121 blocks ABA-evoked stomatal closure, but only partially suppresses stomatal closure in the presence of the NO donor, SNAP, which promotes [Ca²⁺]_i elevation independently of the plasma membrane Ca²⁺ channels. Consistent with these observations, Ca²⁺ channel gating at the plasma membrane is altered by the SNARE fragment in a manner effective in reducing the potential for triggering a rise in [Ca²⁺]_i, and we show directly that its expression in vivo leads to a pronounced suppression of evoked [Ca ²⁺]_i transients. These observations offer primary evidence for the functional coupling of the SNARE with Ca²⁺ channels at the plant cell plasma membrane and, because [Ca²⁺]_i plays a key role in the control of K⁺ and Cl^- channel currents in guard cells, they underscore an important mechanism for SNARE integration with ion channel regulation during stomatal closure.