Pectin methylesterase activity is required for RALF1 peptide signalling output

Abstract The extracellular matrix plays an integrative role in cellular responses in plants, but its contribution to the signalling of extracellular peptides largely remains unknown. RAPID ALKALINIZATION FACTOR (RALF) are extracellular peptide hormones that play pivotal roles in various physiological processes. Here, we unveil a crucial connection between the demethylation machinery of the cell wall component pectin and RALF1 activity. Pectin is a polysaccharide homogalacturonan, contributing to the structural integrity of the cell wall. Our data illustrate that the pharmacological and genetic interference with PECTIN METHYL ESTERASEs (PMEs) abolishes RALF1 perception. Similarly, the application of free demethylated oligogalacturonides in the medium also reduces RALF1 activity. Accordingly, we conclude that RALF1 requires demethylated pectin to signal at the cell surface. We propose that demethylated and hence negatively charged pectin acts as a signalling scaffold for the positively charged RALF peptides, defining its output signalling. This mode of RALF peptide hormone signalling could shed light on a broader plant developmental context, linking extracellular matrix dynamics to hormone-mediated responses.