Two Arabidopsis proteins synthesize acetylated xylan in vitro.

Xylan is the third most abundant glycopolymer on earth after cellulose and chitin. As a major component of wood, grain and forage, this natural biopolymer has far-reaching impacts on human life. This highly acetylated cell wall polysaccharide is a vital component of the plant cell wall, which functions as a molecular scaffold, providing plants with mechanical strength and flexibility. Mutations that impair synthesis of the xylan backbone give rise to plants that fail to grow normally because of collapsed xylem cells in the vascular system. Phenotypic analysis of these mutants has implicated many proteins in xylan biosynthesis; however, the enzymes directly responsible for elongation and acetylation of the xylan backbone have not been unambiguously identified. Here we provide direct biochemical evidence that two Arabidopsis thaliana proteins, IRREGULAR XYLEM 10-L (IRX10-L) and ESKIMO1/TRICOME BIREFRINGENCE 29 (ESK1/TBL29), catalyze these respective processes in vitro. By identifying the elusive xylan synthase and establishing ESK1/TBL29 as the archetypal plant polysaccharide O-acetyltransferase, we have resolved two long-standing questions in plant cell wall biochemistry. These findings shed light on integral steps in the molecular pathways used by plants to synthesize a major component of the world's biomass and expand our toolkit for producing glycopolymers with valuable properties.

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