Role of the Extensin Superfamily in Primary Cell Wall Architecture

Nearly two centuries of progress have established the major components of the plant cell wall, a composite that includes interpenetrating networks of cellulose (Payen, 1838; Schulze, 1891), microfibrils (Frey-Wyssling et al., 1948; Preston et al., 1948), pectin (Braconnot, 1825) and lignin (Payen, 1838). However, only over the last five decades has a relatively minor hydroxyproline-rich structural glycoprotein component emerged with essential roles in building and maintaining the growing primary cell wall. Here we highlight unique advances of each decade from the initial discovery of hydroxyproline (Hyp) in cell walls to the current definition of extensins as self-assembling amphiphiles that generate scaffolding networks, and where acid-base interaction - extensin pectate - may template assembly of the pectic matrix. Subsequent polymerization toughens up the wall as networks resisting both microbial and mechanical stress. At each stage we explore hypotheses arising from synthesis of emerging data with focus on structure. This review celebrates the 50th birthday of extensin.