The Calmodulin-Binding Protein IQM1 Interacts with CATALASE2 to Affect Pathogen Defense

Calmodulin (CaM) regulates plant disease responses through its downstream calmodulin-binding proteins (CaMBPs) often by affecting the biosynthesis or signaling of phytohormones, such as jasmonic acid (JA) and salicylic acid. However, how these CaMBPs mediate plant hormones and other stress resistance-related signaling remains largely unknown. In this study, we conducted analyses in Arabidopsis (Arabidopsis thaliana) on the functions of AtIQM1 (IQ-Motif Containing Protein1), a Ca2+-independent CaMBP, in JA biosynthesis and defense against the necrotrophic pathogen Botrytis cinerea using molecular, biochemical, and genetic analyses. IQM1 directly interacted with and promoted CATALASE2 (CAT2) expression and CAT2 enzyme activity and indirectly increased the activity of the JA biosynthetic enzymes ACX2 and ACX3 through CAT2, thereby positively regulating JA content and B. cinerea resistance. In addition, in vitro assays showed that in the presence of CaM5, IQM1 further enhanced the activity of CAT2, suggesting that CaM5 may affect the activity of CAT2 by combining with IQM1 in the absence of Ca2+. Our data indicate that IQM1 is a key regulatory factor in signaling of plant disease responses mediated by JA. The study also provides new insights that CaMBP may play a critical role in the cross talk of multiple signaling pathways in the context of plant defense processes.