BAK1 and BKK1 in Arabidopsis thaliana confer reduced susceptibility to turnip crinkle virus

BAK1 and BKK1 are receptor-like protein kinases (RLKs) involved in brassinosteroid signal transduction and plant resistance to bacteria and fungi. Here we report that loss-of-function mutants of BAK1 or BKK1 in Col-0 showed enhanced susceptibility to Turnip crinkle virus (TCV) infection. Cell death and chlorosis occurred much earlier in bak1-4 and bkk1-1 mutants than in wild-type plants, suggesting an important role of BAK1 and BKK1 in controlling cell death associated with TCV infection. The bak1-4 and bkk1-1 mutants showed the elevated transcription levels of pathogenesis genes, such as PR1, PR2, PR5, PDF1.2. Enhanced expression of these genes, however, failed to resist the transport of TCV virus. At 9 days postinoculation, TCV levels were the highest in the systemic leaves of bak1-4, but were undetected in BAK1 or BKK1 overexpressing plants. These results signify that both BAK1 and BKK1 are important components in controlling TCV infection in Arabidopsis plants.