Most plant cell wall models depict a matrix of three types of polysaccharides and, in most cells with secondary walls, the polyphenolic lignin. The polysaccharides are cellulose, and the two families of matrix polysaccharides, hemicellulose and pectin. However, with the identification of the cell wall proteoglycan, arabinoxylan-pectin-arabinogalactan protein 1 (APAP1), that contains both hemicellulosic and pectic glycans, such wall models have come into question. Furthermore, the phenotypes of biomass feedstock modified in the synthesis of relatively minor wall components, such as pectin in grasses and in woody tissues, indicate that current wall models are insufficient to account for the effects that such changes have on plant cell wall structure and wall properties. Pectin is the most structurally complex family of the cell wall polysaccharides. It is comprised of the three pectic glycans homogalacturon (HG), rhamnogalacturonan I (RG-I) and rhamnogalacturonan II (RG-II). The most abundant pectic glycan is HG, accounts for roughly 65% of pectin. HG is a partially methyl-esterified and acetylated homopolymer of alpha-1,4-linked galacturonic acid. It is synthesized by the GAUT gene family of proven and putative HG alpha-1,4-galacturonosyltransferases (HG:GalATs), with 15 members in Arabidopsis. During efforts to generate biomass feedstock with reduced recalcitrance for biofuel production, we discovered that modification of GAUT gene expression in woody and grass feedstock leads to reduced biomass recalcitrance and enhanced biofuel production by microbes, as well as improving plant agronomic performance. The result of efforts to identify the specific matrix polysaccharide polymer(s) effected and the mechanism(s) for the improved wall properties has led us to hypothesize that specific GAUTs synthesize specific HG glycans that function in unique polysaccharides and proteoglycans with unique roles in the wall. We propose that the wall is not merely a matrix of three classes of polysaccharides, but rather a matrix of polysaccharides and matrix-glycan containing wall proteoglycans that contribute to cell wall physical, biochemical and agronomic properties. We put forward that an in depth understanding of (1) the specific polymeric structures, (2) their mechanism of synthesis, and (3) their chemical, biophysical and biological properties are critical to enable the most effective use of biomass as a source of energy and bioproducts in a sustainable bioeconomy.

New views of plant cell wall non-cellulosic polysaccharide and proteoglycan structure from studies of pectin biosynthetic enzymes and plants modified in their expression

253rd National Meeting (2017)

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