Calcium-dependent protein kinases play a key role in plant defense against pathogens and pests

As a class of Ca^2 + -dependent serine/threonine protein kinases, calcium-dependent protein kinases (CDPKs) play crucial roles in regulating plant growth, development, and stress responses. CDPKs are widely distributed across various subcellular compartments. Upon perception of pathogens including bacteria, fungi, and viruses or herbivorous insects, plants trigger intracellular calcium signal oscillations. CDPKs sense these signals through their unique kinase domains and EF-hand calcium-binding domains, leading to precise activation. They subsequently regulate downstream immune responses through mechanisms such as autophosphorylation, protein-protein interactions, and dynamic subcellular localization. Research demonstrates that CDPKs coordinate plant pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) through multiple pathways: phosphorylating and regulating respiratory burst oxidase homologs (RBOHs) (a type of NADPH oxidase) to generate reactive oxygen species (ROS); activating the expression of pathogenesis-related (PR) proteins and other defense-related genes; and participating in jasmonic acid (JA), ethylene (ET), and salicylic acid (SA) phytohormone signaling pathways. Additionally, CDPKs regulate the synthesis of secondary metabolites such as phytoalexins and volatile organic compounds, to directly or indirectly inhibit pathogen proliferation and insect feeding. However, the specific molecular mechanisms underlying some of these processes remain to be further elucidated. To date, there has been no systematic review summarizing the biological functions of CDPKs in plant defense against pathogens and herbivores. Therefore, this study systematically reviews the discovery history and structural characteristics of plant CDPKs as well as research progress regarding their roles in defending against bacteria, fungi, oomycetes, viruses, and herbivores. It also discusses future research trends, laying a theoretical foundation for the genetic improvement of crop stress resistance and providing potential candidate gene resources.