A novel family of SOS2 (salt overly sensitive 2)-like protein kinase genes (designated PKSes) have been recently identified in Arabidopsis. The biochemical characteristics as well as in vivo roles of most of the PKSes are unclear at present. In this work, we isolated and characterized one of the PKSes, PKS18. PKS18 was expressed in leaves of mature Arabidopsis plants. The glutathione S-transferase (GST)-PKS18 fusion protein was inactive by itself in substrate phosphorylation. An activation loop Thr(169) to Asp mutation, however, highly activated this kinase in vitro (designated PKS18T/D). Kinase activity of the PKS18T/D preferred Mn(2+) to Mg(2+). The activated kinase showed a substrate specificity, and high catalytic efficiency for a peptide substrate p3 and for ATP. Interestingly, PKS18T/D transgenic plants were hypersensitive to the phytohormone abscisic acid (ABA) in seed germination and seedling growth, whereas silencing the kinase gene by RNA interference (RNAi) conferred ABA-insensitivity, indicating the involvement of PKS18 in plant ABA signaling.