Inward potassium channel in guard cells as a target for polyamine regulation of stomatal movements

A number of studies show that environmental stress conditions such as droug ht, high salt, and air pollutants increase polyamine levels in plant cells. However, little is understood about the physiological function of elevated polyamine levels. We report here that polyamines regulate the voltage-depe ndent inward K+ channel in the plasma membrane of guard cells and modulate stomatal aperture, a plant "sensor" to environmental changes. All natural p olyamines, including spermidine, spermine, cadaverine, and putrescine, stro ngly inhibited opening and induced closure of stomata. Whole-cell patch-cla mp analysis showed that intracellular application of polyamines inhibited t he inward K+ current across the plasma membrane of guard cells. Single-chan nel recording analysis indicated that polyamine regulation of the K+ channe l requires unknown cytoplasmic factors. Ln an effort to identify the target channel at the molecular level, we found that spermidine inhibited the inw ard K+ current carried by KAT1 channel that was functionally expressed in a plant cell model. These findings suggest that polyamines target KAT1-like inward K+ channels in guard cells and modulate stomatal movements, providin g a link between stress conditions, polyamine levels, and stomatal regulati on.