Structural component which strengthens the primary cell wall. Predominantly expressed in the roots. Early expressed in the whole plant, but was restricted to lower stems, flower buds and roots in the mature plant (6 weeks old). By wounding and water stress; in response to plant hormones 2,4-D, BAP treatment; in response to L-Pro treatment. Repressed by salt stress. Extensins contain a characteristic repeat of the pentapeptide Ser-Pro(4). The proline residues are hydroxylated and then O-glycosylated (arabinosylation). Synthetised as soluble proteins which become insolubilised in the cell wall through the intermolecular cross-linking of Tyr on adjacent monomers. Isodityrosine (IDT) stabilizes and makes rigid the part of the polypeptide where IDT functional sites are present. Belongs to the extensin family. Incomplete cDNA clone.