Roles of Pectin Methylesterases and Pectin Methylesterase Inhibitors in Plant Physiology

Pectin, an enriched component in primary cell walls and middle lamellae, is an essential polysaccharide in all higher plants. Homogalacturonans (HGs), a major form of pectin, are synthesized and methyl-esterified by enzymes localized in the Golgi apparatus and transported into the cell wall. Depending on cell type, the degree and pattern of pectin methylesterification are strictly regulated by cell wall-localized pectin methylesterases (PMEs) which are governed by multiple pectin methylesterase inhibitors (PMEIs) in vivo. Pectin methylesterases (PMEs, EC 3.1.1.11), belonging to carbohydrate esterase family 8, cleave the ester bond between a galacturonic acid and an methyl group and the resulting change in pectin methylesterification status from which clearly impacts diverse plant developmental processes and stress responses. PMEs play major roles in modification of pectin properties, such as the stiffening by forming Ca2+-pectate cross-link complexes or loosening by triggering cell wall degrading enzymes (CWDE)s to break out cell wall components including pectin and its interactiors. The action of PMEIs results in opposite consequences to PMEs on pectin properties. Optimal pectin methylesterification status in each cell type is determined by the balance between PME activity and post-translational PME inhibition by PMEIs.