Xylan biosynthesis in plants, simply complex

Xylans are major non-cellulosic polysaccharides in grasses and trees and represent the third most abundant biopolymer on earth (after cellulose and chitin). Xylans have important impacts on biofuel production because they are contributors to plant biomass recalcitrance, yet plants deficient in xylans synthesis grow abnormally. Therefore, deciphering the biochemical mechanisms of xylan biosynthesis will undoubtedly contribute to identifying ways to improve bioful yields from plant biomass. Arabidopsis irregular xylem (irx) mutants have shown that genes from GT43 and GT47 CAZy families encode proteins associated with xylan biosynthesis. These genes are duplicated, have overlapping expression patterns, and thus exhibit partial functional redundancy. However, genes from one pair are incapable of complementing mutations in the other, suggesting thattheir encoded proteins may function cooperatively in xylan synthase complexes (XSCs), and recent work in wheat supports the existence of such XSCs. More recent genetic studies in Arabidopsis suggest that xylan backbone elongation/synthesis can be uncoupled from side chains additions to a certain extent. However,what we still don't know is how xylan backbone synthesis is initiated and then elongated by XSCs? And how many different XSCs does a plant employ to make xylans? In this chapter, we will discuss what we know about xylan backbone initiation, elongation, and uncoupled substitution of the backbone. We will also discuss recent advances in the regulatory mechanisms of xylan synthesis.