The immunity regulator BAK1 contributes to resistance against diverse RNA viruses

The plant's innate immune system detects potential biotic threats through recognition of microbe-associated molecular patterns (MAMPs), or danger-associated molecular patterns (DAMPs), by pattern recognition receptors (PRRs). A central regulator of pattern-triggered immunity (PTI) is the BRI1 associated kinase 1 (BAK1) which undergoes complex formation with PRRs upon ligand binding. Although viral patterns inducing PTI are well known from animal systems, nothing similar has been reported for plants. Antiviral defense in plants is rather thought to be mediated by posttranscriptional gene silencing of viral RNA, or through effector-triggered immunity, i.e. recognition of virus-specific "effectors" by resistance proteins. Nevertheless, infection by compatible viruses can also lead to the induction of defense gene expression, indicating that plants may also recognize viruses through PTI. Here we show that PTI, or at least the presence of the regulator BAK1, is important for antiviral defense of Arabidopsis plants. Arabidopsis bak1 mutants show increased susceptibility to three different RNA viruses during compatible interactions. Furthermore, crude viral extracts, but not purified virions, induce several PTI marker responses in a BAK1-dependent manner. Overall, we conclude that BAK1-dependent PTI contributes to antiviral resistance in plants.