Activation of hypersensitive cell death by pathogen-induced receptor-like protein kinases from Arabidopsis

In Arabidopsis, there is a family of receptor-like protein kinases (RLKs) containing novel cysteine-rich repeats in their extracellular domains. Genes encoding many of these cysteine-rich RLKs (CRKs) are induced by pathogen infection, suggesting a possible role in plant defense responses. We have previously generated Arabidopsis plants expressing four pathogen-regulated CRK genes (CRK5, 6, 10 and 11) under control of a steroid-inducible promoter and found that induced expression of CRK5, but not the other three CRK genes, triggered hypersensitive response-like cell death in transgenic plants. In the present study, we have analyzed the structural relationship of the CRK family and identified three CRKs (CRK4, 19 and 20) that are structurally closely related to CRK5. Genes encoding these three CRKs are all induced by salicylic acid and pathogen infection. Furthermore, induced expression of CRK4, 19 and 20 all activates rapid cell death in transgenic plants. Thus, the activity of inducing rapid cell death is shared by these structurally closely related CRKs. We have also performed yeast two-hybrid screens and identified proteins that interact with the kinase domains of CRKs. One of the identified CRK-interacting proteins is the kinase-associated type 2C protein phospohatase known to interact with a number of other RLKs through its kinase-interacting FHA domain. Other CRK-interacting proteins include a second protein with a FHA domain and another type 2C protein phosphatase. Interactions of CRKs with these three proteins in vivo were demonstrated through co-immunoprecipitation. These CRK-interacting proteins may play roles in the regulation and signaling of CRKs.