Knockout of the guard cell K + out channel and stomatal movements

The central roles of potassium channels in regulating membrane potential and controlling action potential repolarization are well documented (1). In plants an additional important function of potassium channels in mediating long-term potassium transport during cell movements, turgor changes, and tropisms has been proposed. Two guard cells surround each stomatal pore in leaves and control the opening and closing of their central pore via increases in their solute content during stomatal opening (2) and decreases in solute content during stomatal closing (3). In this issue of PNAS, Hosy et al. (4) provide direct genetic evidence that outward rectifying potassium (K+out) channels in guard cells contribute to stomatal closing in leaves. Guard cells accumulate potassium (K+), which results in stomatal opening (2) and release K+, which results in stomatal closing (3). Ion channel characterizations in guard cells, and motor cells that control turgor-driven leaf movements, led to the model that K+ channels can contribute to the underlying long-term K+ influx (5, 6) and K+ efflux (5–7). Biophysical, cell biological, and second messenger regulation analyses by several groups have supported this model. Genetic evidence shows that outward rectifying potassium channels in guard cells contribute to stomatal closing in leaves. Hosy et al. identified an insertional T-DNA disruption mutant in the Arabidopsis guard cell-expressed outward-rectifying K+out channel gene, GORK. Heterologous expression in Xenopus oocytes has previously shown that the GORK cDNA encodes an outward rectifying K+ channel activity with properties similar to those described in guard cells and GORK is expressed in guard cells …