Figure 3.

Overview of precursor maturation, perception, and function in plant immunity.

During various pathogen infections, pro-proteins are processed by proteases and released into the apoplast, where they are recognized by specific cell surface sensors, resulting in the activation of plant defense responses. MAPKs may phosphorylate transcription factors (TFs), which control the expression of PTI-related genes (in blue are the typical PTI responses: ROS burst, increased cytosolic Ca²⁺ influx, callose deposition, and CW strengthening) as well as SA-, JA-, and/or ethylene (ET)-responsive genes that regulate immunity. The precursor forms of the CW remodeling enzymes (pro-PME and pro-ARA-I) can be processed to favor the production of oligosaccharides, which are elicitors of plant immunity. Oligogalacturonides (OGs) can be released by the combined action of polygalacturonases (PGs), polygalacturonase-inhibiting proteins (PGIPs), and PMEs. Processed PMEs can also perform pectin de-methylesterification. WAK1, RFO1, RLP44, and FERONIA can interact with de-methylesterified pectin. WAK1 senses OGs and FERONIA can sense condensed OG-RALFs to modulate plant defense responses. Oligorhamnogalacturonides (ORhams) can potentially be released by debranching enzymes such as the bifunctional α-L-arabinofuranosidase/β-D-xylosidases (ARA-I). The precursor/protease pairs and the putative or demonstrated subcellular compartment of maturation are shown. Green bars and pink and blue lines represent cellulose, pectin, and hemicellulose, respectively. Pink dots and diamond symbols represent methylester groups and Ca²⁺, respectively. Dashed arrows indicate pathways that lack direct evidence. RBOHD, respiratory burst oxidase homolog protein D; AHA1/2, Arabidopsis H⁺-ATPase isoforms 1 and 2.