A novel gene, AAPK, is disclosed. Loss of function of the protein encoded by AAPK is associated with reduced sensitivity to abscisic acid-induced stomatal closure in plants. Also disclosed are transgenic plants and mutants having altered sensitivity to abscisic acid-mediated transpiration and other desirable agronomic features. The regulation of transpiration provided by the present invention is different from that of previously described mechanisms to control transpiration in plants.