Cytosolic alkalinization is a common and early messenger preceding the production of ROS and NO during stomatal closure by variable signals, including abscisic acid, methyl jasmonate and chitosan

Stomata are unique that they sense and respond to several internal and external stimuli, by modulating signaling components in guard cells. The levels of reactive oxygen species (ROS), nitric oxide (NO) and cytosolic calcium (Ca 2+ ) increase significantly during stomatal closure by not only plant hormones [such as abscisic acid (ABA) or methyl jasmonate (MJ)] but also elicitors (such as chitosan). We observed that cytosolic alkalinization preceded the production of ROS as well as NO during ABA induced stomatal closure. We therefore propose that besides ROS and NO, the cytosolic pH is an important secondary messenger during stomatal closure by ABA or MJ. We also noticed that there is either a cross talk or feedback regulation by cytosolic Ca 2+ and ROS (mostly H 2 O 2 ). Further experiments on the interactions between cytosolic pH, ROS, NO and Ca2+ would yield interesting results.