Conservation of the PBL-RBOH immune module in land plants

The rapid production of reactive oxygen species (ROS) is a key signaling output in plant immunity. In the angiosperm model species, Arabidopsis thaliana (hereafter Arabidopsis), recognition of non- or altered-self elicitor patterns by cell-surface immune receptors activates the receptor-like cytoplasmic kinases (RLCKs) of the AVRPPHB SUSCEPTIBLE 1 (PBS1)-like (PBL) family, particularly BOTRYTIS-INDUCED KINASE1 (BIK1)1–3. BIK1/PBLs in turn phosphorylate the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD) to induce apoplastic ROS production4,5. PBL and RBOH functions in plant immunity have been extensively characterized in flowering plants. Much less is known about the conservation of pattern-triggered ROS signaling pathways in non-flowering plants. In this study, we show that, in the liverwort Marchantia polymorpha (hereafter Marchantia), single members of RBOH and PBL families, namely MpRBOH1 and MpPBLa, are required for chitin-induced ROS production. MpPBLa directly interacts with and phosphorylates MpRBOH1 at specific, conserved sites within its cytosolic N-terminus and this phosphorylation is essential for chitin-induced MpRBOH1-mediated ROS production. Collectively, our work reveals the functional conservation of the PBL-RBOH module that controls pattern-triggered ROS production in land plants.