Plant roots respond to environmental stresses or the exogenous plant hormone abscisic acid (ABA) by undergoing marked physiological and morphological changes. We show here that PERK4, a gene that encodes a member of the Arabidopsis thaliana proline-rich extensin-like receptor kinase family, plays an important role in ABA responses. Mutation of PERK4 by T-DNA insertion decreased sensitivity to ABA with respect to seed germination, seedling growth and primary root tip growth. The effect on root growth was due to enhanced cell elongation rather than cell division. The cytosolic free calcium concentration and Ca(2+) channel currents were lower in perk4 root cells than in wild-type cells in the presence of ABA. Root growth was similar in wild-type and perk4 plants after the application of a Ca(2+) channel blocker. PERK4 localised to the plasma membrane, and was shown to be an ABA- and Ca(2+)-activated protein kinase. Our data suggest that the receptor-like kinase encoded by PERK4 functions at an early stage of ABA signalling to inhibit root cell elongation by perturbing Ca(2+) homeostasis.