Investigation of the Regulation Mechanisms of Ion Channels in Arabidopsis Guard Cells

Stomata is the main gateway for plants to intake CO₂ for photosynthesis and release water and O₂ into atmosphere. It has been well-known that diverse ion channels in the plasma membrane of guard cells play important roles for stomatal movements, including K⁺ channels, anion channels, and Ca²⁺ channels. The activation of inward K⁺ currents (K⁺_in) is required for stomatal opening, the outward K⁺ channels and anion channels mediate the efflux of osmotic ions to drive stomatal closure, and inward Ca²⁺ channels function as a cytosolic Ca²⁺ signaling generator by mediating extracellular Ca²⁺ influx in guard cells although the identities of ABA-activated Ca²⁺ channels are still unknown. Drought stress closes stomata to prevent water loss, and light induces stomatal opening to facilitate plant growth and developments. But how the diverse plasma membrane ion channels are precisely regulated differently and simultaneously to ensure a single direction for stomatal movements, closure or opening, is still largely unknown. Recently, we found that the mutations of S-type anion channel SLAC1 led to a strong inhibition of inward K⁺ channel activity in Arabidopsis guard cells by approximately 80%, and light-induced stomatal opening was slowed down compare to Columbia wild type. Interestingly, we found that the decrease of [Ca²⁺]_cyt from 2 mM to <10 nM restored the activity of K⁺_in partially in slac1 mutants. This research indicated that anion channel SLAC1, inward K⁺ channels and inward Ca²⁺ channels function together coordinatedly, and are “connected” to each other functionally by an unknown mechanism for the regulation of stomatal movements. To unravel the unknown molecular mechanism, we tried to isolate ABA-activated Ca²⁺ channels in the plasma membrane of Arabidopsis guard cells, and found that CNGC5 and CNGC6 function as cGMP-activated Ca²⁺ channels, but not as ABA-activated Ca²⁺ channels, indicating that further work is needed to identify the ABA-activated Ca²⁺ channels in the plasma membrane of plant guard cells.