A calcium-dependent protein kinase regulates the defense response in Citrus sinensis.

Abstract

Citrus Huanglongbing (HLB), which is caused by Candidatus Liberibacter asiaticus (CLas), is one of the most destructive citrus diseases worldwide, and defense-related Citrus sinensis gene resources remain largely unexplored. Calcium signaling plays an important role in diverse biological processes. In plants, a few calcium-dependent protein kinases (CDPKs/CPKs) have been shown to contribute to defense against pathogenic microbes. The genome of Citrus sinensis encodes dozens of CPKs. In this study, the role of CsCPKs in Citrus sinensis defense was investigated. Silencing of CsCPK6 compromised the induction of defense-related genes in Citrus sinensis. Expression of a constitutively active form of CsCPK6 (CsCPK6CA) triggers the activation of defense-related genes in Citrus sinensis. Complementation of CsCPK6 rescued the defense-related gene induction in an Arabidopsis thaliana cpk4/11 mutant, indicating that CsCPK6 carries CPK activity and is capable of functioning as a CPK in Arabidopsis. Moreover, an effector derived from CLas inhibits defense induced by the expression of CsCPK6CA and autophosphorylation of CsCPK6, which suggests the involvement of CsCPK6 and calcium signaling in defense. These results support the positive role of CsCPK6 in Citrus sinensis defense against CLas, and the auto-inhibitory regulation of CsCPK6 provides a potential genome-editing target for improving the defense of Citrus sinensis.