Arabidopsis CaM Binding Protein CBP60g Contributes to MAMP-Induced SA Accumulation and Is Involved in Disease Resistance against Pseudomonas syringae

Abstract

Salicylic acid (SA)-induced defense responses are important factors during effector triggered immunity and microbe-associated molecular pattern (MAMP)-induced immunity in plants. This article presents evidence that a member of the Arabidopsis CBP60 gene family, CBP60g, contributes to MAMP-triggered SA accumulation. CBP60g is inducible by both pathogen and MAMP treatments. Pseudomonas syringae growth is enhanced in cbp60g mutants. Expression profiles of a cbp60g mutant after MAMP treatment are similar to those of sid2 and pad4, suggesting a defect in SA signaling. Accordingly, cbp60g mutants accumulate less SA when treated with the MAMP flg22 or a P. syringae hrcC strain that activates MAMP signaling. MAMP-induced production of reactive oxygen species and callose deposition are unaffected in cbp60g mutants. CBP60g is a calmodulin-binding protein with a calmodulin-binding domain located near the N-terminus. Calmodulin binding is dependent on Ca²⁺. Mutations in CBP60g that abolish calmodulin binding prevent complementation of the SA production and bacterial growth defects of cbp60g mutants, indicating that calmodulin binding is essential for the function of CBP60g in defense signaling. These studies show that CBP60g constitutes a Ca²⁺ link between MAMP recognition and SA accumulation that is important for resistance to P. syringae. Plants respond to attack by microbial pathogens through activation of a battery of defense responses. This activation is controlled by a complex signaling network. Disease resistance depends on rapid activation of plant defense responses. Improved understanding of the signaling network may lead to development of crops with improved disease resistance. Here, we used the model plant Arabidopsis thaliana to study activation of defense responses after infection by a bacterial pathogen, Pseudomonas syringae. We found that a gene not previously known to function in defense signaling, CBP60g, is needed for resistance. By studying plants with mutations in this gene, we found that CBP60g contributes to the increases in levels of the important signaling molecule, salicylic acid, that occur after pathogen recognition. We also found that the CBP60g protein binds calmodulin, a protein that mediates calcium regulation of protein function. Calmodulin binding was necessary for the function of CBP60g in disease resistance. We conclude that CBP60g is a protein that mediates calmodulin-dependent activation of salicylic acid signaling in response to pathogen recognition.