Structural component which strengthens the primary cell wall (PubMed:11475326). Forms dendritic structures indicating a propensity for self-assembly through tyrosine cross-linking (PubMed:18256186, PubMed:21415277). Forms intermolecular cross-links exclusively by pulcherosine (three Tyr) (PubMed:18256186). Scaffold formation requires an unobstructed C-terminus of EXT3 (PubMed:18256186). Required for the correct positioning of the cell plate during cytokinesis in cells of the developing embryo (PubMed:12034904). Extensins contain a characteristic repeat of the pentapeptide Ser-Pro(4). For this particular extensin, a typical repeat of Ser-Pro(3) is found (PubMed:11475326). 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) (PubMed:11475326). Detected in cells throughout the embryo, including the suspensor, from the 32-cell stage, with expression increasing up to the torpedo stage, followed by a decrease at the bent-cotyledon stage, with little or no expression detectable in the nearly mature cotyledon stage of development (PubMed:12034904). By wounding, water and cold stresses; in response to plant hormones 2,4-D, BAP, GA3, and ABA treatment; in response to L-Ser, Hyp and L-Pro treatment. Slightly repressed by salt stress. The proline residues of the Ser-Pro(3) repeats are hydroxylated and then O-glycosylated (arabinosylation) by HPAT1, HPAT2 and HPAT3 (PubMed:24036508). Around 20% of Hyp units are in the nonglycosylated form (PubMed:18256186). The Ser residues are O-galactosylated (Ref.9, PubMed:25944827). The lack of Ser-O-galactosylation does not affect Hyp-O-arabinosylation, but both types of O-glycosylation are central for the functionality of the protein (PubMed:25944827). Correct Hyp-O-arabinosylation appears to be responsible for generating a bend on the EXT3 backbone around a YVY motif, which may represent a better scenario for Tyr intramolecular cross-links (isodityrosine type) (PubMed:25944827). 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. Embryo lethality. Germination-defective agravitropic seedlings with severely defective root, shoot, and hypocotyl and a vitreous appearance throughout (PubMed:12034904). Defective Cell Walls (PubMed:18256186). Belongs to the extensin family. Chimeric cDNA. Its C-terminal part is derived from the gene At2g20230. Truncated cDNA resulting of the removal of two cryptic introns in the genomic sequence.