Hormonal regulation of guard cell ion transport

Much of our understanding of transporters in guard cells has come from efforts to dissect the effects of hormones on stomatal function. Arguably the largest contribution has come from the study of ABA-induced stomatal closure. Classically, ABA has also received the most attention as a phytohormone regulator of stomatal apertures (see review by Raschke 

Promotion of stomatal closure ABA-induced membrane depolarization 

In order to produce the K efflux associated with stomatal closure, several different transporters must be tightly regulated. The fastest recorded response to ABA, occurring on a time-scale in the range of approx. 10-206 s, is a depolarization of the plasma membrane 

ABA is known to decrease rates of H extrusion from other tissue types [59,60], and reports from several laboratories indicate that it reduces acidification of the external medium from Vida guard cells [55,61,62]. Most recently, Shimazaki s laboratory [63] performed a detailed analysis of blue light-stimulated extrusion from guard cells and demonstrated that ABA could inhibit pumping. Such H extrusion is also inhibited by 

Secondly, an inward flux of Ca could contribute to depolarization. Although the flux of Ca required to fully account for the observed changes in V , would be great enough to be cytotoxic, Ca influx could still contribute a portion of the depolarization. There is some electrophysiological evidence for an ABA-stimulated Ca influx, but this does not appear to be an obligate ABA response, as the current attributed to Ca influx was detected in only approximately one-third of the cells treated with ABA [65], 

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