Leucine-rich repeat receptor-like kinases (LRR-RLKs) are ubiquitous in higher plants and act as receptors of extracellular signals to trigger multiple physiological processes. However, the functions of the majority of LRR-RLKs remain largely unknown, especially in tomato (Solanum lycopersicum L.). Here, we found that MRK1 (Multiple resistance-associated kinase 1), encoding a novel tomato LRR-RLK, was significantly induced by temperature stresses and bacterial pathogen attacks. Knocking out MRK1 impaired tolerance to both cold and heat stress, accompanied by decreased transcript levels of the master regulators C-repeat binding factor 1 (CBF1) and Heat shock transcription factor a-1a (HsfA1a), respectively. In addition, mrk1 mutants were hypersensitive to Pseudomonas syringae pv. tomato DC3000 and Ralstonia solanacearum and showed compromised pattern-triggered immunity (PTI) responses, as evidenced by decreased production of reactive oxygen species and reduced upregulation of PTI marker genes. Moreover, bimolecular fluorescence complementation, split-luciferase assays, and co-immunoprecipitation supported the formation of a complex of MRK1, FLS2, and Somatic embryogenesis receptor kinase (SERK3A/SERK3B) in a ligand-independent manner. This work demonstrates that tomato MRK1 is a novel positive regulator of multiple stress responses and may be a potential breeding target for improving crop stress resistance.

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