Abstract
Phytohormone abscisic acid (ABA) promotes stomatal closure to adapt plants to drought. Recent studies revealed that protein phosphorylations have an important role in ABA signaling, but a few phosphoproteins have been characterized in detail. In this study, we investigated ABA-induced protein phosphorylations in Vicia guard cell protoplasts using 14-3-3 protein, as a probe. We found that a 61 kDa protein bound to 14-3-3 protein tightly in response to physiological concentration of ABA within a few minutes. This reaction was found exclusively in guard cells, and inhibited by the protein kinase inhibiter, K-252a. Furthermore, reactive oxygen species (ROS), such as H2O2, as secondary messengers of ABA signaling in guard cells, and inhibitors of ROS production had no effect on this reaction. Thus ABA-induced phosphorylation of 61 kDa protein may participate in the upstream of ROS production or in the parallel pathway of ABA signaling in guard cells.
