Cell wall mechanics and growth control in plants: the role of pectins revisited

How is the extensibility of growing plant cell walls regulated? In the past, most studies have focused on the role of the cellulose/xyloglucan network and the enigmatic wall-loosening agents expansins. Here we review first how in the closest relatives of the land plants, the Charophycean algae, cell wall synthesis is coupled to cell wall extensibility by a chemical Ca(2+)-exchange mechanism between Ca(2+)-pectate complexes. We next discuss evidence for the existence in terrestrial plants of a similar "primitive" Ca(2+)-pectate-based growth control mechanism in parallel to the more recent, land plant-specific, expansin-dependent process.