Inhibition of guard-cell inward K+ channels by abscisic acid: links and gaps in the signal transduction chain.

Abscisic acid (ABA) affects a number of ion transport mechanisms in guard cells. One effect of ABA is to inhibit inward K+ channels, an effect that can contribute to the inhibition of stomatal opening. One model of the signal transduction cascade mediating this response involves ABA-activation of a G-protein, which in turn activates phospholipase C, resulting in production of inositol 1,4,5-trisphosphate (IP3), elevation of cytosolic free Ca2+ levels (Cai), and activation of a Ca(2+)-dependent phosphatase (protein phosphatase 2B; PP2B) which mediates channel inhibition. A review of the literature reveals that several of the links in this putative signal transduction chain have been established. The G-protein activator, GTP gamma S, inhibits inward K+ currents. Exogenous IP3 inhibits inward K+ currents. Exogenous IP3 elevates Cai. Elevated Cai inhibits inward K+ currents, and the inhibition by ABA of inward K+ currents does require Ca2+. Exogenous PP2B inhibits inward K+ currents, and an endogenous Ca(2+)-dependent phosphatase activity is present in guard cells. However, significant gaps in the chain remain. There is no direct evidence that ABA activates a G-protein. The presence or absence of a G-protein-activated phospholipase C in guard cells has not been investigated. Elevation of Cai by ABA is variable, and the reasons for this variability remain to be established. It is not known whether or not activation of a guard-cell PP2B homolog is the exclusive mechanism by which Ca2+ inhibits the channels.