AtCaM4 interacts with a Sec14-like protein, PATL1, to regulate freezing tolerance in Arabidopsis in a CBF-independent manner.

Calmodulin (CaM), a multifunctional Ca2+ sensor, mediates multiple reactions involved in regulation of plant growth and responses to environmental stress. In this study, we found that AtCaM4 plays a negative role in freezing tolerance in Arabidopsis. The deletion of AtCaM4 resulted in enhanced freezing tolerance in cam4 mutant plants. Although AtCaM4 and AtCaM1 were cold-induced isoforms, cam4/cam1Ri double-mutant and cam4 single-mutant plants exhibited similar improvements in freezing tolerance, indicating that AtCaM4 plays major role. Furthermore, we found that AtCaM4 may influence freezing tolerance in a C-repeat binding factor (CBF)-independent manner as cold-induced expression patterns of CBFs did not change in the cam4/cam1Ri mutant. In addition, among the cold-responsive (COR) genes detected, KIN1, COR15b, and COR8.6 exhibited clearly enhanced expression over the long term in cam4/cam1Ri mutant plants exposed to cold stress. Using immunoprecipitation and mass spectrometry, we identified multiple candidate AtCaM4-interacting proteins. Co-immunoprecipitation assays confirmed the interaction of AtCaM4 with PATL1 in vivo and a phenotype analysis showed that patl1 mutant plants exhibited enhanced freezing tolerance. Thus, we conclude that AtCaM4 negatively regulates freezing tolerance in Arabidopsis by interacting with the novel CaM-binding protein PATL1.