The rise in pH of guard cells: An intriguing component of stomatal closure

Stomatal movements are associated with pH changes in guard cells. Several authors have demonstrated that cytosolic alkalinization preceded the rise in ROS or Ca2+of guard cells. In contrast, a few reports suggest that the increase in cytosolic pH follows the elevated ROS or Ca2+, suggesting that cytosolic pH rise may not always be an early event. The components, such as ROS, Ca2+, and Ca2+-dependent protein kinases, converge to modulate ion channels, promote ion efflux from guard cells and promote stomatal closure. We propose a hypothetical model to integrate the event of pH rise with other signalling components and explain the argument that cytosolic alkalization can occur downstream or upstream of ROS or Ca2+-rise. Changes in guard cell pH can occur when ATPases are modulated. Stomatal closure and guard cell pH rise are compromised in mutants deficient in vacuolar H+-ATPase (V-ATPase), pointing out the role of V-ATPase. My talk attempts to consider arguments for and against the significance of cytosolic alkalinization in guard cells. Stomatal guard cells are promising model systems for further research into this intriguing topic of cytosolic pH change during stomatal closure.