Abstract text: Plant cell wall dynamic during development is significantly influenced by pectin remodeling. Interactions between pectin methylesterases (PMEs) and pectin methylesterase inhibitors (PMEIs) control the degree methylesterification of homogalacturonan-type pectins (HG). Although the regulation of these protein families in early land plants remains poorly understood, they have been extensively studied in angiosperms. The genome of the moss Physcomitrium patens encodes 46 PME isoforms but only one PMEI, providing a simplified system to study this interaction during plant development.

To determine the developmental roles of PpPMEI, CRISPR–Cas9 was used to generate independent knockout lines. When grown on BCD medium, pppmei mutants consistently produced smaller colonies than the wild type, whereas differences were less pronounced on BCDAT medium, possibly due to accelerated protonemal development on BCD. Several recombinant PMEs for P. patens were produced, and PpPMEI was expressed and purified. Enzymatic assays confirmed PME catalytic activity. However, gel diffusion experiments showed that while commercial citrus PME was inhibited, moss PMEI didn’t inhibit the tested P. patens PMEs.

These results suggest that noncanonical mechanisms, such as regulatory functions of the PME pro-part, may contribute to PME regulation in P. patens.