Vesicular mechanics regulating stomatal latency

Stomata are microscopic pores that mediate gas exchange across the impermeable cuticle of plant leaves. Stomata open to allow CO2 entry for photosynthesis, and they close to prevent water loss via transpiration and leaf drying when atmospheric humidity is low. The guard cells that drive stomatal opening and closing generally respond slowly to environmental change - for example, fluctuations in daylight as clouds pass overhead - leading to periods when photosynthesis is limited by CO2 availability and when water is lost without commensurate gains in carbon assimilation.

Reducing the impacts of this stomatal hysteresis has proven possible by enhancing guard cell ion transport that facilitates stomatal movements.

Stomata also exhibit use-dependent, 'memory-like' latencies in responsiveness with recurrent environmental challenge. A use-dependent latency, or 'carbon memory', slows stomatal kinetics with repeated fluctuations in response to light and CO2, and a related phenomenon, dubbed 'programmed closure', slows stomatal recovery following exposures to the water-stress hormone ABA.

As these memory-like phenomena can degrade both longer-term carbon assimilation and the efficiency of water use by the plant, understanding their mechanics and regulation is certain to inform efforts towards greater crop yields.

From past research we know that membrane vesicle traffic makes a major contribution to latencies in stomatal responsiveness. Notably, with ABA vesicle traffic removes the KAT1 K+ channel out of the cell membrane and later recycles the channel to the membrane for K+ uptake and stomatal re-opening. This cycling of KAT1 was shown to require the vesicle trafficking protein SYP121.

Our studies indicate a similar cycling process with recurrent changes in light and CO2. What we do not know is how these cycling events are initiated and regulated.

Recently, we discovered a protein with similarities to a component of the BORC complex that in mammals is thought to regulate vesicle trafficking, neuronal transmission, and insulin secretion. The plant BORC1-like protein, BLP1, is strongly expressed in guard cells; it binds trafficking SNARE SYP121 in an ABA-dependent manner; and it alters K+ channel activity in association with SYP121.

Furthermore, mutants of blp1 and of two putative BLP1 partners exhibit cumulative use-dependent changes in stomatal responsiveness to light and CO2 with consequences for carbon assimilation and plant water use. These findings point to hitherto unexpected mechanism that controls vesicle trafficking and stomatal latencies. Most important, our findings point to a trafficking mechanism at the centre of memory-like behaviours affecting how plants respond to environmental change.

We intend to resolve the mechanics of vesicle traffic regulation and cycling that underpin the latencies regulating stomatal memory-like behaviours. We want (1) to examine how BLP1 and its putative BORC complex partners regulate SNARE complex assembly and SYP121-associated interactions, (2) to determine the impact on the KAT1 channel as a marker for environmentally sensitive traffic and activity, and (3) to characterise the actions of BLP1 and its partners on stomata, biomass gain, and plant water use efficiencies.

The research is for fundamental knowledge directed to uncovering the 'rules of life'. Nonetheless, it holds immediate relevance for crop improvement. As the global demand, especially in agriculture, outstrips fresh water supplies, the knowledge gained from this work will help inform strategies to increase crop performance and efficiencies for mitigating the crisis in water availability and crop production.