Update on Xylan Biosynthesis in Grasses Xylan Biosynthesis: News from the Grass

Plant cell wall polysaccharides are critical components of numerous products in our everyday life. Because of this, modification of their constituent components offers unique opportunities for product improvement and economic advancement. Xylans are one of these polysaccharides that have a variety of applications that affect our well-being. For example, xylans are important functional ingredients in baked products. They affect the quality of cereal flours for breadmaking and themechanical properties of dough. They also impact brewing properties of grains (Vinkx and Delcour, 1996). Xyl, the main constituent of xylans, can be converted into important value-added products such as xylitol, used as a natural food sweetener, a dental cavities reducer, and a sugar substitute for diabetics; in 1997, its market was estimated to approximately $125 million (Saha and Bothast, 1997). Xylans are also important for the livestock industry, as they are critical factors for silage digestibility (Vinkx and Delcour, 1996). Xylans are major constituents in the nonnutritional constituent of feed (mostly cereal grain by-products) in monogastric animals (poultry; Choct and Annison, 1990). Thus, any small change in the xylan content of grains that go into poultry and swine feed could reflect billions of dollars of savings through an improvement of the food conversion ratio (Donohue and Cunningham, 2009). Given the functional and economical importance of these polymers, their biosynthesis has attracted and puzzled plant cell wall biologists for decades. Only in the last 5 years has significant progress been made in identifying candidates for the glycosyltransferase (GT) genes involved in the biosynthetic process in dicots and monocots. The focus of this Update, which should complement the extensive review on xylan biosynthesis in dicots by York andO’Neill (2008), is the recent progress made on the biosynthesis of xylans in grasses. Xylans are one of the major hemicelluloses in secondary cell walls of dicots and all walls of grasses. Grasses have typical type II walls that are rich in glucurono(arabino)xylans (GAXs) and b-(1,3/4)glucan (also called mixed-linkage glucan [MLG]; Bacic et al., 1988; Ebringerova et al., 2005). Therefore, grasses have been increasingly used as models to investigate xylans and MLG biosynthesis. Progress has been made in understanding the biosynthesis of many hemicelluloses (including MLG); however, investigating the biosynthetic mechanism of xylans at the biochemical and molecular levels has proven to be more challenging. Now, with the genome sequences of rice (Oryza sativa) and Brachypodium distachyon available, it is an exciting time to work on xylan biosynthesis. Only recently, through an extensive bioinformatics approach (Mitchell et al., 2007), the identity of some GTs has been postulated as candidates for xylan biosynthetic enzymes in grasses. Despite this major advance, the biosynthetic mechanism has yet to be fully characterized. Currently, we cannot delve deeper into the bioinformatics classification and predict the exact biochemical function of putative GTs without isolation of the proteins involved and functional enzymatic assays.