Calcium-regulated signalling network meets atypical NLR immune receptor: Control of CPK5-dependent plant defence initiation and immune signal propagation through TN2 and EXO70B1

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An increase in the cytoplasmic calcium concentration is one of the earliest intracellular responses of plants exposed to a pathogen attack and rapid calcium changes are prerequisite to the activation of the immune system in both, PTI and ETI. Calcium-dependent protein kinases (CPKs) are calcium sensors and signal transmitters, and are important key players in plant immune signalling. EXO70B1 is a subunit of the exocyst complex and loss-of-function of EXO70B1 leads to autoimmunity. Enhanced signalling by CPK5 in CPK5-OE plant lines leads to enhanced defence responses and increased pathogen resistance similar to those observed in the exo70B1 mutant. Remarkably, the defence phenotypes in both scenarios require a truncated nucleotide binding domain and leucine-rich repeat (NLR) protein TIR-NBS2 (TN2). In a forward genetic screen, the Tang group found that CPK5, but not other CPK isoforms, is required for exo70B1-activated immune responses, indicating that CPK5 plays a unique role in plant immunity. TN2 interacts with the N-terminal part of CPK5 encompassing the variable and protein kinase domain. The Romeis group could show that upon ectopic expression in exo70B1 protoplasts, CPK5 displays enhanced constitutive protein kinase activity, which is not detectable in the tn2 background. These data provide the first identification of a direct link between an atypical immune receptor and a signalling component of the calcium-regulatory network that is required in the early immune response and for the onset of defence. This joint proposal addresses the mechanism, the mutual requirement, and the functions of CPK5, EXO70B1 and TN2 in defence signal initiation and propagation during PTI and ETI. The current understanding in the field allows the projection of two models for the TN2-mediated regulation of CPK5 and EXO70B1: (i) CPK5 can be considered a guardee of TN2 with a respective full-length NLR protein still to be identified, and (ii) TN2-activated immunity and increased immunity in exo70B1 is induced by a positive feedback loop between CPK5 and TN2 through the stabilization of active CPK5 by TN2. Our research aims to distinguish between these two scenarios.