Functional Characterization of a Glycosyltransferase from the Moss Physcomitrella patens Involved in the Biosynthesis of a Novel Cell Wall Arabinoglucan

Abstract

Mixed-linkage (1,3;1,4)-β-glucan (MLG), an abundant cell wall polysaccharide in the Poaceae, has been detected in ascomycetes, algae, and seedless vascular plants, but not in eudicots. Although MLG has not been reported in bryophytes, a predicted glycosyltransferase from the moss (Pp3c12_24670) is similar to a bona fide ascomycete MLG synthase. We tested whether encodes an MLG synthase by expressing it in wild tobacco () and testing for release of diagnostic oligosaccharides from the cell walls by either lichenase or (1,4)-β-glucan endohydrolase. Lichenase, an MLG-specific endohydrolase, showed no activity against cell walls from transformed , but (1,4)-β-glucan endohydrolase released oligosaccharides that were distinct from oligosaccharides released from MLG by this enzyme. Further analysis revealed that these oligosaccharides were derived from a novel unbranched, unsubstituted arabinoglucan (AGlc) polysaccharide. We identified sequences similar to the AGlc synthase from algae, bryophytes, lycophytes, and monilophytes, raising the possibility that other early divergent plants synthesize AGlc. Similarity of AGlc synthase to MLG synthases from ascomycetes, but not those from Poaceae, suggests that AGlc and MLG have a common evolutionary history that includes loss in seed plants, followed by a more recent independent origin of MLG within the monocots.

Evolution of Cell Wall Polymers in Tip-Growing Land Plant Gametophytes: Composition, Distribution, Functional Aspects and Their Remodeling

“…P. patens produces a Glycosyl Transferase (GT) ( Pp3c124670 ) similar to MLG synthases found in ascomycetes (Roberts et al, 2018). This protein appears to be involved in the synthesis of a new type of cell wall component: an unbranched glucan punctuated by Ara.…”

“…Xylan-type hemicellulose has been described as xylans, arabinoxylans (AX) and as represented in the figure: glucuronoarabinoxylans (GAXs). Arabinoglucans were recently described in the moss P. patens (Roberts et al, 2018). Structures under braces are only found in monocots.…”

“…This new arabinoglucan (Figure 2) has not been described before. However, some similar GT genes present in algae, bryophytes, lycophytes and monilophytes make it possible to consider that a similar polymer might exist in other species (Roberts et al, 2018). Even though arabinoglucan is quite similar in structure to MLG, it is unlikely that this P. patens GT has led to the synthesis of MLG in monocots due to the important differences with MLG synthases.…”

“…Even though arabinoglucan is quite similar in structure to MLG, it is unlikely that this P. patens GT has led to the synthesis of MLG in monocots due to the important differences with MLG synthases. Thus, the appearance of MLG in monocots (Poales) would be from an independant origin (Roberts et al, 2018).…”

A New Polysaccharide with a Long Evolutionary History

“…For the first time in a very long time, a new polysaccharide is reported in plants. Roberts et al (2018) discovered an arabinoglucan in the moss Physcomitrella patens. This discovery came about not as a result of biochemical characterization of the moss cell wall, but rather as a targeted study of the gene Pp3c12_ 24670 by transient expression analysis in Nicotiana benthamiana and reduced or enhanced expression in the moss, identifying not only the polymer, but also its synthase.…”