A Pseudomonas aeruginosa-secreted protease modulates host intrinsic immune responses, but how?

Recently, we found that the Pseudomonas aeruginosa type II secreted protease IV functions as a unique Arabidopsis innate immunity elicitor. The protease IV-activated pathway involves G protein signaling and raises the question of how protease elicitation leads to the activation of G protein-mediated signaling, because plants do not appear to have metazoan-like G protein-coupled receptors. Importantly, our data suggest that Arabidopsis has evolved a mechanism to detect the proteolytic activity of a pathogen-encoded protease, supporting the host-pathogen arms race model. In the case of opportunistic multi-host pathogens like P. aeruginosa, however, it is not plausible that P. aeruginosa is simultaneously co-evolving in a gene-for-gene manner with all of its potential hosts, which include plants, nematodes, insects, and mammals. This prompts us to ask what is the driving force for co-evolution of defense response in Arabidopsis and pathogenesis of P. aeruginosa, which might not have been subject to iterative cycles of evolutionary selections.