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

T he 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. Hosy et al. identified an insertional T-DNA disruption mutant in the Arabidopsis guard cell-expressed outwardrectifying 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 (8). Hosy et al. further generated a dominant negative GORK mutant by inserting the positively charged amino acid arginine into the K selectivity filter domain of the GORK channel. The T-DNA disruption mutant, gork-1, showed no measurable K out channel activity in guard cells and dominant negative mutant lines showed 10% of native K channel activity (4). Stomatal movement analyses in response to abscisic acid and darkness showed that the gork-1 disruption mutant caused a reduced stomatal closing response, providing genetic evidence for the function of K out channels (4). Furthermore, gork dominant negative mutants showed slight slowing of stomatal closure compared with WT responses. K channel current activities in guard cells are approximately an order of magnitude larger than physiological K f luxes during stomatal movements (6), which may account for the relatively limited effect of partial dominant negative gork repression. Gas exchange and plant water loss analyses also show that gork-1 disruption reduces the rate of stomatal closing and increases water loss of leaves and whole plants. The gork-1 mutant also shows slightly enhanced light-induced stomatal opening, which can be explained by the functions of K channels in balancing K efflux and K influx in guard cells (4).