A SNARE-Aquaporin complex in stomatal hydraulics

Stomata are pores that mediate gaseous exchange across the impermeable cuticle of plant leaves. They open for CO2 entry when photosynthesis depletes CO2 inside the leaf, and they close to reduce the transpiration of water vapour and prevent leaf drying when atmospheric humidity is low. Stomata are at the centre of a crisis in water availability and crop production that is beginning to unfold and can only escalate as the global demand, especially in agriculture, outstrips fresh water supplies. Thus stomata are an important target in efforts to enhance crop performance and efficiencies.

Stomata of most plants track the immediate demand for CO2 by photosynthesis, responding to CO2 within the leaf, opening in the light and closing in the dark. However, stomatal responses are slow by comparison with that of photosynthesis. Fluctuations in daylight, for example as clouds pass overhead, degrade photosynthesis and reduce water use efficiency (WUE=amount of carbon fixed in photosynthesis/amount of water transpired), principally because stomata generally lag behind changes in light.

Synthetic bioengineering has shown substantial gains in photosynthesis and WUE by accelerating the speed of stomatal response. We need now to understand how such gains might be achieved using the processes native to the stomata.

Stomatal movement is driven by solute and water transport across the membrane of the guard cells that surround the stomatal pore. Guard cells harbour ion channel proteins to facilitate solute flux and aquaporins to mediate water flux, and they rely on a traffic of membrane vesicles to adjust cell surface area during stomatal movements. Thus, coordination of these three processes is essential for stomatal responses.

From our previous work, we know that the dominant ion flux through K+ channels is coupled to membrane traffic by binding between subsets of channels and so-called SNARE proteins that facilitate vesicle traffic and are conserved across land plants. These interactions ensure solute flux and membrane traffic operate in 'lock-step' within guard cells. There is some evidence for a parallel coordination of water flux through aquaporins, but until now we have lacked an understanding of how this coordination might arise.

Plasma membrane (PIP) aquaporins are found across all angiosperms. Three PIPs contribute to water flux in guard cells of the model plant Arabidopsis although one, PIP2;1, dominates. We recently uncovered a selective interaction between all three PIPs and the SYP121 protein, one of two principal SNAREs at the plasma membrane.

These interactions depend on a cytosolic N-terminal region of SYP121 that is sequence-divergent, but functionally interchangable with other SNAREs and is widely recognised to regulate SNARE activity and vesicle traffic in all eukaryotes. Most exciting, we find that a chimeric SYP121 incorporating the same region of a non-interacting SNARE slows stomatal opening and closing when expressed in guard cells and suppresses WUE and growth when plants experience fluctuating daylight.

Our findings are the first direct evidence for SYP121-PIP binding in stomatal movements, and they point to the SNARE subdomain responsible for this action in vivo. SYP121 also binds guard cell K+ channels, coordinating vesicle traffic with K+ flux. Thus, SYP121-PIP binding suggests a SNARE nexus in stomatal regulation; it begs questions about the coordination of PIP and K+ channel binding; and it challenges established dogma about the roles of vesicle traffic in aquaporin hydraulics that impact on WUE and plant biomass gain.

We propose now to resolve the binding and function of SYP121 with the guard cell PIPs and to establish the consequences for the plant. This research is to understand the fundamental rules of life. Understanding this SNARE nexus nonetheless carries the promise of a potential target for future bioengineering to accelerate stomatal movements and enhance crop efficiencies.