Chara — a living sister to the land plants with pivotal enzymic toolkit for mannan and xylan remodelling

Abstract

Land‐plant transglycosylases ‘cut‐and‐paste’ cell‐wall polysaccharides by endo‐transglycosylation (transglycanases) and exo‐transglycosylation (transglycosidases). Such enzymes may remodel the wall, adjusting extensibility and adhesion. Charophytes have cell‐wall polysaccharides that broadly resemble, but appreciably differ from land‐plants'. We investigated whether Chara vulgaris has wall‐restructuring enzymes mirroring those of land‐plants.

Wall enzymes extracted from Chara were assayed in vitro for transglycosylase activities on various donor substrates — β‐(1→4)‐glucan‐based [xyloglucan and mixed‐linkage glucans (MLGs)], β‐(1→4)‐xylans and β‐(1→4)‐mannans — plus related acceptor substrates (tritium‐labelled oligosaccharides, XXXGol, Xyl ₆‐ol and Man ₆‐ol), thus 12 donor:acceptor permutations. Also, fluorescent oligosaccharides were incubated in situ with Chara, revealing endogenous enzyme action on endogenous (potentially novel) polysaccharides.

Chara enzymes acted on the glucan‐based polysaccharides with [ ³H]XXXGol as acceptor substrate, demonstrating ‘glucan:glucan‐type’ transglucanases. Such activities were unexpected because Chara lacks biochemically detectable xyloglucan and MLG. With xylans as donor and [ ³H]Xyl ₆‐ol (but not [ ³H]Man ₆‐ol) as acceptor, high trans‐β‐xylanase activity was detected. With mannans as donor and either [ ³H]Man ₆‐ol or [ ³H]Xyl ₆‐ol as acceptor, we detected high levels of both mannan:mannan homo‐trans‐β‐mannanase and mannan:xylan hetero‐trans‐β‐mannanase activity, showing that Chara can not only ‘cut/paste’ these hemicelluloses by homo‐transglycosylation but also hetero‐transglycosylate them, forming mannan→xylan (but not xylan→mannan) hybrid hemicelluloses. In in‐situ assays, Chara walls attached endogenous polysaccharides to exogenous sulphorhodamine‐labelled Man ₆‐ol, indicating transglycanase (possibly trans‐mannanase) action on endogenous polysaccharides.

In conclusion, cell‐wall transglycosylases, comparable to but different from those of land‐plants, pre‐dated the divergence of the Charophyceae from its sister clade (Coleochaetophyceae/Zygnematophyceae/land‐plants). Thus, the ability to ‘cut/paste’ wall polysaccharides is an evolutionarily ancient streptophytic trait.