Hemicellulose‐remodelling transglycanase activities from charophytes: Towards the evolution of the land‐plant cell wall

Transglycanases remodel cell-wall polymers, having critical impact on many physiological processes. Unlike xyloglucan endotransglucosylase (XET) activity, widely studied in land-plants, very little is known about charophyte wall-modifying enzymes — information that would promote our understanding of the ‘primordial’ wall, revealing how the wall matrix is re-modelled in the closest living algal relatives of land-plants, and what changed during terrestrialisation. We conducted various in-vitro assays for wall-remodelling transglycosylases, monitoring either (a) polysaccharide-to-[3H]oligosaccharide transglycosylation or (b) non-radioactive oligosaccharide-to-oligosaccharide transglycosylation. We screened a wide collection of enzyme extracts from charophytes (and early-diverging land-plants for comparison) and discovered several homo- and hetero-transglycanase activities. In contrast to most land-plants, charophytes possess high trans-β-1,4-mannanase activity, suggesting that land-plants’ algal ancestors prioritised mannan remodelling. Trans-β-1,4-xylanase activity was also found, most abundantly in Chara, Nitella and Klebsormidium. Exo-acting transglycosidase activities (trans-β-1,4-xylosidase and trans-β-1,4-mannosidase) were also detected. In addition, charophytes exhibited homo- and hetero-trans-β-glucanase activities [XET, mixed-linkage-glucan:xyloglucan endotransglucosylase, and cellulose:xyloglucan endotransglucosylase] despite the paucity or lack of land-plant-like xyloglucan and mixed-linkage-glucan (MLG) as potential donor substrates in their cell walls. However, trans-α-xylosidase activity (which remodels xyloglucan in angiosperms) was absent in charophytes and early-diverging land-plants. Transglycanase action was also found in situ, acting on endogenous algal polysaccharides as donor-substrates and fluorescent xyloglucan-oligosaccharides as acceptor-substrates. We conclude that trans-β-mannanase and trans-β-xylanase activities are present and thus may play key roles in charophyte walls (most of which possess little or no xyloglucan and MLG, but often contain abundant β-mannans and β-xylans), comparable to the roles of XET in xyloglucan-rich land-plants.