Abscisic acid activation of plasma membrane Ca2+ channels in guard cells requires cytosolic NAD(P)H and is differentially disrupted upstream and downstream of reactive oxygen species production in abi1-1 and abi2-1 protein phosphatase 2C mutants

The hormone abscisic acid (ABA) regulates stress responses and developmenta l processes in plants. Calcium-permeable channels activated by reactive oxy gen species (ROS) have been shown recently to function in the ABA signaling network in Arabidopsis guard cells. Here, we report that ABA activation of these 1(ca) Ca2+ channels requires the presence of NAD(P)H in the cytosol. The protein phosphatase 2C (PP2C) mutant abi1-1 disrupted ABA activation o f 1(ca). channels. Moreover, in ab1-1, ABA did not induce ROS production. C onsistent with these findings, in abi1-1, H2O2 activation of 1(ca). channel s and H2O2-induced stomatal closing were not disrupted, suggesting that abi 1-1 impairs ABA signaling between ABA reception and ROS production. The abi 2-1 mutation, which lies in a distinct PP2C gene, also disrupted ABA activa tion of 1(ca). However, in contrast to abi1-1, abi2-1 impaired both H2O2 ac tivation of 1(ca) and H2O2-induced stomatal closing. Furthermore, ABA elici ted ROS production in abi2-1. These data suggest a model with the following sequence of events in early ABA signal transduction. ABA, abi1-1, NAD(P)H- dependent ROS production, abi2-1, 1(ca) Ca2+ channel activation followed by stomatal closing.