Abstract
Cell wall matrices are complex composites mainly of polysaccharides, phenolics (monomers and polymers), and protein. We are beginning to understand the synthesis of these major wall components individually, but still have a poor understanding of how cell walls are assembled into complex matrices. Valuable insight has been gained by examining intact components to understand the individual elements that make up plant cell walls. Grasses are a prominent group within the plant kingdom, not only for their important roles in global agriculture, but also for the complexity of their cell walls. Ferulate incorporation into grass cell wall matrices (C3 and C4 types) leads to a cross-linked matrix that plays a prominent role in the structure and utilization of grass biomass compared to dicot species. Incorporation of p-coumarates as part of the lignin structure also adds to the complexity of grass cell walls. Feruoylation results in a wall with individual hemicellulosic polysaccharides (arabinoxylans) covalently linked to each other and to lignin. Evidence strongly suggests that ferulates not only cross-link arabinoxylans, but may be important factors in lignification of the cell wall. Therefore, the distribution of ferulates on arabinoxylans could provide a means of structuring regions of the matrix with the incorporation of lignin and have a significant impact upon localized cell wall organization. The role of other phenolics in cell wall formation such as p-coumarates (which can have concentrations higher than ferulates) remains unknown. It is possible that p-coumarates assist in the formation of lignin, especially syringyl rich lignin. The uniqueness of the grass cell wall compared to dicot sepcies may not be so much in the gross composition of the wall, but how the distinctive individual components are organized into a functional wall matrix. These features are discussed and working models are provided to illustrate how changing the organization of feruoylation and p-coumaroylation could lead to differing cell wall properties.
Highlights
Annual and perennial grasses play a vital role in agriculture by providing feedstuffs for animals in the forms of fresh forage and preserved forage
Recent work in Arabidopsis has indicated that the cytoplasmic localized UDP-Xylose synthases (UXS) are responsible for providing more substrate for xylan biosynthesis than the Golgi localized family members (Kuang et al, 2016). These results suggest that transport of UDP- α-DGlcA, UDP- α-D-Xyl, and UDP- β-L-Araf into the Golgi are all needed for GAX biosynthesis and could function as control points for GAX biosynthesis
It is clear that the molar ratios of Ara:Xyl and ferulic acid (FA):Xyl change during development within grasses (Carpita, 1984a; Gibeaut and Carpita, 1991; Obel et al, 2002; Hatfield et al, 2008b; Rancour et al, 2012), is this achieved during biosynthesis or postsynthesis processing? Are different α-1,3-Araf T and α-1,2-Araf T enzymes with altered processivity expressed developmentally or in a tissue-specific manner? Alternatively, do glycosidase activities, such as α-arabinofuranosidase, analogous to those observed in dicot plant systems (Goujon et al, 2003; Montes et al, 2008) have a role in grasses? these questions remain to be answered they point to areas of research that can be addressed
Summary
Annual and perennial grasses play a vital role in agriculture by providing feedstuffs for animals in the forms of fresh forage (grazing) and preserved forage (silage and hay). Unlike ferulates that do become readily cross-linked into the growing lignin polymer, pCA remains bound within the cell wall matrix by this single covalent linkage, i.e., esterified to SA residues. This property of pCA raises two possible scenarios for its role in grass lignin (1) to act as a termination molecule for a developing lignin polymer and (2) to contribute to enhancing the linear or less reticulated nature of syringyl type lignin found in grasses. The p-hydroxycinnamates, ferulic acid (FA), and p-coumaric acid (pCA), have been shown to be ester or ether linked to lignin in grasses (Figure 3)
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