Sensitization of defense responses and activation of programmed cell death by a pathogen-induced receptor-like protein kinase in Arabidopsis

During the search for potential target genes of WRKY DNA-binding transcription factors, we have previously identified four pathogen-induced Arabidopsis genes (CRK5, CRK6, CRK10 and CRK11) encoding receptor-like protein kinases (RLKs) containing novel cysteine-rich repeats in their extracellular domains. In the present study, we transformed Arabidopsis plants with the RLK genes under control of the constitutive CaMV 35S promoter or a steroid-inducible Gal4 promoter. Expression of CRK5, but not the three other RLK genes, resulted in significant alterations in defense responses and leaf growth in transgenic plants. In transgenic plants harboring the 35S:: CRK5 construct, significantly elevated and constitutive expression of CRK5 correlated with enhanced leaf growth and increased resistance to the bacterial pathogen Pseudomonas syringae. The enhanced disease resistance in the transgenic plants was associated with more rapidly induced expression of the PR1 gene after pathogen infection. In transgenic plants transformed with CRK5 under control of the steroid-inducible promoter, expression of the transgene was induced at relatively high levels after the steroid application and this induced expression of CRK5 triggered hypersensitive response-like cell death. Induced CRK5 expression also activated cell death in the npr1, ndr1 and eds1 mutants and in the transgenic nahG plants that fail to accumulate salicylic acid. Thus, the novel RLK is capable of activating multiple distinct defense responses depending on the manner and/or the levels of its over-expression in transgenic plants.