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Abstract
Ca^sup 2+^ and calmodulin (CaM), a key Ca^sup 2+^ sensor in all eukaryotes, have been implicated in defense responses in plants. To elucidate the role of Ca^sup 2+^ and CaM in defense signaling, we used ^sup 35^S-labeled CaM to screen expression libraries prepared from tissues that were either treated with an elicitor derived from Phytophthora megasperma or infected with Pseudomonas syringae pv. tabaci. Nineteen cDNAs that encode the same protein, pathogen-induced CaM-binding protein (PICBP), were isolated. The PICBP fusion proteins bound ^sup 35^S-CaM, horseradish peroxidase-labeled CaM and CaM-Sepharose in the presence of Ca^sup 2+^ whereas EGTA, a Ca^sup 2+^ chelator, abolished binding, confirming that PICBP binds CaM in a Ca^sup 2+^-dependent manner. Using a series of bacterially expressed truncated versions of PICBP, four CaM-binding domains, with a potential CaM-binding consensus sequence of WSNLKKVILLKRFVKSL, were identified. The deduced PICBP protein sequence is rich in leucine residues and contains three classes of repeats. The PICBP gene is differentially expressed in tissues with the highest expression in stem. The expression of PICBP in Arabidopsis was induced in response to avirulent Pseudomonas syringae pv. tomato carrying avrRpm1. Furthermore, PICBP is constitutively expressed in the Arabidopsis accelerated cell death2-2 mutant. The expression of PICBP in bean leaves was also induced after inoculation with avirulent and non-pathogenic bacterial strains. In addition, the hrp1 mutant of Pseudomonas syringae pv. tabaci and inducers of plant defense such as salicylic acid, hydrogen peroxide and a fungal elicitor induced PICBP expression in bean. Our data suggest a role for PICBP in Ca^sup 2+^-mediated defense signaling and cell-death. Furthermore, PICBP is the first identified CBP in eukaryotes with four Ca^sup 2+^-dependent CaM-binding domains.[PUBLICATION ABSTRACT]