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 developmental processes in plants. Calcium-permeable channels activated by reactive oxygen 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 I_Ca Ca²⁺ channels requires the presence of NAD(P)H in the cytosol. The protein phosphatase 2C (PP2C) mutant abi1-1 disrupted ABA activation of I_Ca channels. Moreover, in abi1-1, ABA did not induce ROS production. Consistent with these findings, in abi1-1, H₂O₂ activation of I_Ca channels and H₂O₂-induced stomatal closing were not disrupted, suggesting that abi1-1 impairs ABA signaling between ABA reception and ROS production. The abi2-1 mutation, which lies in a distinct PP2C gene, also disrupted ABA activation of I_Ca. However, in contrast to abi1-1, abi2-1 impaired both H₂O₂ activation of I_Ca and H₂O₂-induced stomatal closing. Furthermore, ABA elicited 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, I_Ca Ca²⁺ channel activation followed by stomatal closing.