Putative NAD(P)-binding Rossmann fold protein is involved in chitosan-induced peroxidase activity and lipoxygenase expression in Physcomitrella

Oxidative burst, the rapid production of high levels of reactive oxygen species (ROS) in response to external stimuli, is an early defense reaction against pathogens. The fungal elicitor chitosan causes an oxidative burst in the moss Physcomitrella (new species name: Physcomitrium patens) mainly due to the peroxidase enzyme Prx34. To better understand the chitosan responses in Physcomitrella, we conducted a screen of part of a P. patens mutant collection to isolate plants with less peroxidase activity than wild-type plants after chitosan treatment. We isolated a P. patens mutant that affected the gene encoding NAD(P)-binding Rossmann fold protein (hereafter, Rossmann fold protein). Three Rossmann fold protein-knockout (KO) plants (named Rossmann fold KO-lines) were generated and used to assess extracellular peroxidase activity and expression of defense-responsive genes including alternative oxidase (AOX), lipoxygenase (LOX), NADPH-oxidase (NOX) and peroxidase (Prx34) in response to chitosan treatment. Extracellular (apoplastic) peroxidase activity was significantly lower in Rossmann fold KO-lines than in wild-type plants after chitosan treatments. Expression of the LOX gene in Rossmann-fold KO-plants was significantly lower before and after chitosan treatment when compared to WT. Peroxidase activity assays together with gene expression analyses suggest that the Rossmann-fold protein might be an important component of the signaling pathway leading to oxidative burst and basal expression of the LOX gene in Physcomitrella.