A protein scaffold essential for K+ transport and stomatal control

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This proposal builds on the discovery in this laboratory of an unusual SNARE protein complex that is essential for K+ channel regulation in Arabidopsis, and of our recent finding of its impact on K+ uptake by the roots and plant growth. Because the SNARE, SYP121, is already known to have roles in pathogen defence and cell volume responses meidated by the plant hormone abscisic acid (ABA), these findings offer the first substantive evidence that physically links the protein complex to actions in abiotic stress, mineral nutrition, water use efficiency (defined as the amount of dry matter produced per unit of water transpired through stomata) and cell expansion in the plant. 'Closing the circle' of K+ channel control, its contributions to K+ assimilation, water flux and cell volume control in the growing plant is expected to yield substantive insights in future efforts towards 'hardening' and development of water-effcient crops. In general, eukaryotic cells engage SNARE proteins as part of a well-defined mechanism for traffic of membrane vesicles, proteins and soluble cargo between compartments within the cell. Work from this laboratory first identified genes encoding two plasma membrane SNAREs from tobacco and Arabidopsis, and demonstrated their association with the ABA signalling cascade leading stomata to close and reduce water loss from the plant. Much is known about these cellular signals as well as the mechanisms they engage in regulating transmembrane ion flux. By contrast, we have little knowledge of the mechanisms coordinating ion transport with cellular volume and membrane surface in guard cells. Indeed, how plant cells regulate transport of osmotically-active solutes (especially of K+) in parallel with cell volume, whether reversible as in guard cells or during irreversible expansive growth, remains a matter of considerable debate. Knowing how transport is coordinated with changes in cell surface area will be of great fundament importance to understanding cell volume control in plants generally, and is likely to have practical relevance in our ability to manipulate the growth and homeostasis of plants, for example in altering stomatal controls to improve water use efficiency of crops. Our working hypothesis is that SNARE-K+ channel interaction serves as a molecular governor, analogous to the mechanical invention of James Watt, to coordinate channel-mediated uptake of the osmotically-active K+ ion with cell expansion We want to explore the molecular mechanics of the interaction, its implications for such a governor model in cell volume control and, equally, we want to examine its potential for applications in agriculture. Our immediate goals are (1) to explore the dynamics of the protein partners and their interactions, (2) to analyse their impact on ion transport and its coordination with membrane traffic, and (3) to assess their importance for stomatal function and water use by plants under stress.