Signaling module for the plant immune response elucidated.
Rising calcium concentrations inside the plant cell due to pathogen attack are recognized by calcium-dependent protein kinases (CDPKs) and translated into phosphorylation signals, which in turn trigger defense reactions. The calcium-dependent protein kinase CPK5 is a central node in the activation of the plant immune response. However, the CDPK signaling chain is linked to many additional molecular players. Researchers from the IPB and from China have now been able to elucidate interactions of such players with CPK5 and have recently published their results in the journal The Plant Cell.
One such factor, whose interaction with calcium-dependent protein kinases had been reported but not elucidated in detail, is the exocyst subunit EXO70B1. As part of the exocyst complex, it helps to recruit vesicles for exocytosis to the plasma membrane. exo70B1 mutants show symptoms of autoimmunity such as increased incidence of cell death and enhanced resistance to pathogens. It was already known that this mutant phenotype in exo70B1 only occurs when the calcium-dependent protein kinase CPK5 and another protein involved in defense, TN2, are present. In addition, plants overexpressing CPK5 exhibit a similar autoimmunity. These indications pointed to a close interaction between these factors and the researchers aimed to identify the molecular basis of this autoimmunity phenotype.
With the present study, the authors could first show that TN2 physically interacts with CPK5. This interaction appears to be necessary for the autoimmunity of exo70B1. The next step was to identify target proteins downstream of this CPK5-TN2 signaling complex that are phosphorylated by the CPK5 kinase. To this end, the researchers carried out a phosphoproteome analysis in which they compared the protein phosphorylation of wild-type and cpk5 mutant plants. Through this approach, they found another player, the transcription factor CAMTA3 from the calmodulin-binding transcription activator family. CAMTA3 was already known to act as a negative regulator in the immune response and to physically interact with CPK5 as well. Pathogen infection leads to CAMTA3 degradation and defense activation, while camta3 loss-of-function mutants also exhibit an autoimmunity phenotype.
The researchers from Halle have now been able to prove that CAMTA3 is a direct phosphorylation substrate of the kinase CPK5 and is phosphorylated at serine-964. They also confirmed that this modification contributes to CAMTA3 degradation and thus, this removal of a negative regulator releases defense genes for transcription and activates the defense response. By contrast, a more stable CAMTA3 isoform expressed in a gain-of-function camta3-3d mutant can suppress the exo70B1 autoimmunity, indicating that these two factors act in the same signaling pathway.
In summary, the researchers were able to describe a TN2-CPK5-CAMTA3 signaling module that contributes to exo70B1-mediated autoimmunity. They conclude that this pathway could also be relevant in the context of a pathogen attack since pathogen effectors can impair or even destabilize EXO70B1 - similar to the state found in the exo70B1 mutant.
Original publication: Liu N, Jiang X, Zhong G, Wang W, Hake K, Matschi S, Lederer S, Hoehenwarter W, Sun Q, Lee J, Romeis T, Tang D. CAMTA3 repressor destabilization triggers TIR domain protein TN2-mediated autoimmunity in the Arabidopsis exo70B1 mutant. Plant Cell. 2024 May 1;36(5):2021-2040. doi: 10.1093/plcell/koae036