Pro-pectin methylesterases as zymogens for plant cell wall mediated immunity

Plants modulate the activity of immunity-related pectin methylesterase (IT-PME) 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. Pathogen recognition receptors such as wall-associated kinases (WAKs) and FERONIA preferentially bind 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 previously secreted via a conventional pathway. 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. Some of the recent advances in our work will be discussed together with current knowledge on the modification, perception, and signal transduction of pectin methylesterification in plant immunity.