Pull the fuzes: Processing protein precursors to generate apoplastic danger signals for triggering plant immunity

Abstract

The apoplast is one of the first cellular compartments outside the plasma membrane that phytopathogenic microbes encounter in the early stages of plant tissue invasion. Plants have developed sophisticated surveillance mechanisms to sense the danger events at the cell surface and promptly activate immunity. However, a fine tuning of the activation of immune pathways is necessary to mount a robust and effective defense response. Several endogenous proteins and enzymes are synthesized as inactive precursors, and their post-translational processing emerges as a critical mechanism to trigger alarms in the apoplast. In this review, we focus on the precursors of the phytocytokines, cell wall remodeling enzymes and proteases. The physiological events that convert the inactive precursors into immunomodulatory active peptides or enzymes are described. This review also explores the functional synergies between phytocytokines, cell wall damage associated molecular patterns and remodeling, highlighting their role in boosting extracellular immunity and reinforcing defenses against pests.

Keywords: Apoplast; Immunity activation; cell wall; cell wall Remodeling Enzymes; phytocytokines; phytopathogenic microbes; plant immunity; post-translational processing; pro-enzymes; pro-peptides; proteases.