O-Acetylation of Plant Cell Wall Polysaccharides

Abstract

Plant cell walls are composed of structurally diverse polymers, many of which are O-acetylated. How plants O-acetylate wall polymers and what its function is remained elusive until recently, when two protein families were identified in the model plant Arabidopsis that are involved in the O-acetylation of wall polysaccharides – the reduced wall acetylation (RWA) and the trichome birefringence-like (TBL) proteins. This review discusses the role of these two protein families in polysaccharide O-acetylation and outlines the differences and similarities of polymer acetylation mechanisms in plants, fungi, bacteria, and mammals. Members of the TBL protein family had been shown to impact pathogen resistance, freezing tolerance, and cellulose biosynthesis. The connection of TBLs to polysaccharide O-acetylation thus gives crucial leads into the biological function of wall polymer O-acetylation. From a biotechnological point understanding the O-acetylation mechanism is important as acetyl-substituents inhibit the enzymatic degradation of wall polymers and released acetate can be a potent inhibitor in microbial fermentations, thus impacting the economic viability of, e.g., lignocellulosic based biofuel production.

The Role of the Plant-Specific ALTERED XYLOGLUCAN9 Protein in Arabidopsis Cell Wall PolysaccharideO-Acetylation

“…The TBL proteins have been reported previously to be putative polysaccharide acetyltransferases acting in the Golgi apparatus, with specificity for particular acceptor polysaccharides and positions Gille and Pauly, 2012;Xiong et al, 2013;Yuan et al, 2013). In the case of TBL29/ESK1, xylan O-acetyltransferase activity has been demonstrated in vitro (Urbanowicz et al, 2014).…”

“…These nonplant organisms contain acetylation machinery that has homology to the RWA and TBL proteins, sometimes as parts of a single polypeptide (Gille and Pauly, 2012). Additional components of the O-acetylation machinery such as AXY9 in plants have not been reported in mammalian or fungal systems.…”

“…While not part of the TBL family, AXY9 does share limited sequence homology with certain parts of these proteins, including a putative GDS active site and a DXXH motif (Supplemental Fig. S8; Akoh et al, 2004;Bischoff et al, 2010b;Gille and Pauly, 2012;Moynihan and Clarke, 2014b). According to the Arabidopsis eFP browser, the AXY9 gene is expressed throughout all examined developmental stages, with moderately elevated expression observed in mature pollen, seeds, and stems (Winter et al, 2007).…”

“…These properties are likely to be important for appropriate function in planta. Recently identified Arabidopsis (Arabidopsis thaliana) mutants with polysaccharide O-acetylation deficiencies (reduced wall acetylation [rwa] and trichome birefringence-like [tbl]; Gille and Pauly, 2012) have allowed for testing of the in vivo role of this substituent. The ALTERED XYLOGLUCAN4 (AXY4 [TBL27]) gene from the TBL family was identified in a forward genetic screen of Arabidopsis and is believed to code for a XyG acetyltransferase .…”

Two Trichome Birefringence-Like Proteins Mediate Xylan Acetylation, Which Is Essential for Leaf Blight Resistance in Rice

“…To achieve these functions, plants often incorporate acetyl substituents into cell wall polymers (Gille and Pauly, 2012). Hence, many cell wall polymers, including pectin, several kinds of hemicellulose, and lignin, are O-acetylated (Gille and Pauly, 2012;Pawar et al, 2013).…”