How Calcium Signals Early Warning

Liu et al. uncover a direct link between calcium signaling and a plant immune receptor

Plants have evolved a sophisticated system to defend against pathogen attack that involves recognition of an invading pathogen and activation of the immune system. An increase in calcium concentration in the cell interior is one of the earliest responses of plants exposed to a pathogen attack, and rapid calcium changes are required to the activate the plant immune system. Calcium-dependent protein kinases are calcium sensors and signal transmitters, and are key players in plant immune signaling.

The exocyst is a protein complex involved in trafficking of proteins and other molecules around to various locations inside cells. In the model plant Arabidopsis thaliana, a loss-of-function mutation in one of proteins of this complex leads to autoimmunity, in which the immune system is always activated, instead of being induced only after pathogen infection. This autoimmunity phenotype also depends on the presence of a protein called TN2, which is an atypical immune receptor of the  TIR-NBS protein family.

Liu et al. wanted to know how TN2 contributes to plant immunity and what other proteins or components might be involved in TN2 signaling. They conducted a genetic screen in Arabidopsis and identified a specific calicum-dependent kinase named CPK5 that interacts with TN2 and is required for exocyst-mediated plant immune responses.

These data provide the first identification of a direct link between an immune receptor and a signaling component of the calcium regulatory network that is required in the early immune response and for the onset of plant defense. Future challenges include determining the precise biochemical activities and interactions of CPK5, TN2, and exocyst proteins in defense signal initiation and propagation during the development of plant immunity.

Liu, N., Hake, K., Wang, W., Zhao, T., Romeis, T., and Tang, D. (2107). CALCIUM-DEPENDENT PROTEIN KINASE5 Associates with the Truncated NLR Protein TIR-NBS2 to Contribute to exo70B1-Mediated Immunity. Plant Cell 29: 746-759.