Protein precursors for effective plant immunity: the case of pectin methylesterases

Plants exploit an immunity-related pectin methylesterase (IT-PME) activity against microbes (Del Corpo et al., 2020). This process could induce the formation of "egg box" structures, resulting in stiffening of the cell wall. IT-PME activity may also promote the production/sensing of damage-associated molecular patterns such as oligogalacturonides and methanol, promoting defensive priming in plants. PME activity can also favour the binding of pathogen recognition receptors such as wall-associated kinases (WAKs), RESISTANCE TO FUSARIUM OXYSPORUM 1 (RFO1) and FERONIA to de-methylesterified pectins. A precise on/off mechanisms regulating PME activity seem necessary for an efficient defense response without compromising growth (Lionetti et al., 2017; Coculo et al., 2022). We revealed that Arabidopsis PMEs involved in plant defense are pro-proteins. We identified two subtilases (SBT), SBT3.3 and SBT3.5, as modulators of IT-PME activity in Arabidopsis immunity to Botrytis. SBT3.3 follows an unconventional secretion pathway to reach the CW where they can activate Pro-PMEs secreted through a conventional protein secretion. The Pro region shares structural similarities with functionally characterized PME inhibitors, mediators of maintenance of CW integrity. Our research reveals the activation of PRO-PMEs as a resistance factor against pathogens. Modification, perception, and signal transduction of pectin methylesterification in plant immunity will be discussed.