Plant cells often use cell surface receptors to sense environmental changes and then transduce external signals via activated signaling pathways to trigger adaptive responses. In Arabidopsis, the receptor-like protein kinase (RLK) gene family contains more than 600 members, and some of these are induced by pathogen infection, Suggesting a possible role in plant defense responses. We previously characterized an S-locus RLK (CBRLK1) at the biochemical level. In this study, we examined the physiological function of CBRLK1 in defense responses. CBRLK1 mutant and CBRLK1-overexpressing transgenic plants showed enhanced and reduced resistance against a virulent bacterial pathogen, respectively. The altered pathogen resistances of the mutant and overexpressing transgenic plants were associated with increased and reduced induction of the pathogenesis-related gene PR1, respectively. These results suggest that CBRLK1 plays a negative role in the disease resistance signaling pathway in Arabidopsis. (C) 2009 Elsevier Inc. All rights reserved.