Plant Cell http://doi.org/bj2h (2016)

The basal land plant model Physcomitrella patens is a powerful tool for understanding the evolution of plant biological processes. Simon Bressendorff and colleagues from Copenhagen University, Denmark, studied innate immunity pathways in the non-vascular moss, and found them to be surprisingly similar to those in angiosperms, confirming that the ability to detect and fight pathogens efficiently evolved very early in plants.

Recognition of generic non-self patterns such as bacterial flagellin or fungal chitin by membrane receptors induces a cascade of molecular events often mediated by protein kinases, leading to immunity. The authors showed that chitin, unlike flagellin, activates moss MAP kinases. They identify a homologue of the Arabidopsis chitin receptor CERK1, which is essential for this response and other chitin-induced phenotypes. From a slightly simplified repertoire of kinases, they also implicated redundant MEKK1, MKK1 and MPK4 homologues, similar to one branch of Arabidopsis defence pathways. They found that moss mutated in one MPK4 is less resistant to necrotrophic fungi, confirming its role in immunity.

The role of other MAP kinases in moss remains to be elucidated. But, by discovering strong similarities (and also significant specificity differences), this work provides evidence that fully formed immunity pathways were already present before the separation between bryophytes and angiosperms 500 million years ago.