LandWall: Understanding the remodelling and recruitment of internal and external hemicelluloses in the cell walls of land-conquering Zygnematophyceae and the liverwort Marchantia

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The cells of most green algae and land plants are surrounded by a primary cell wall, which is a load bearing yet extendable matrix composed of various polysaccharides and protects algae from environmental stress such as wate scarcity. A major fraction of these cell wall polysaccharides are hemicelluloses, which can be enzymatically cut and re-connected to another hemicellulose molecules nearby. Such reactions are catalysed by cell wall-bound transglycanases, which occur abundantly in both the Zygnematophyceae and their sister lineage, the land plants. Interestingly, transglycanases are less abundant in early diverging Charophytes (e.g. Klebsormidiophyceae) and Chlorophyta. Hemicellulose transglycosylation is considered to play a key role in the cell wall metabolism of uprightly growing land plants, however, its functions in algae are elusive, even though gene families encoding the responsible enzymes strongly expanded in land-conquering algae, suggesting that they have played important roles in preparing algae and their cell walls for terrestrialization. Moreover, the major substrate of transglycanses and most abundant hemicellulose in land plants – xyloglucan – evolved in Zygnematophyceae. Our recent data indicate that these algae can secrete xyloglucan and other hemicelluloses in surprisingly large amounts into the environment and – vice versa – recruit external polysaccharides back into their cell walls via transglycosylation. We believe that this unexplored hemicellulose secretion and recruitment of polysaccharides helped algae to produce biocrusts that most likely served as their primary habitats during terrestrialization and can be still found worldwide in harsh environments. In order to explore the role of polysaccharide secretion and recruitment, we will devise a series of cultivation experiments and analyses the external polysaccharide footprint of Zygnema and Mesotaenium grown on different substates by state-of-the-art glycobiology techniques. We will compare the secretion pattern with the basal land plant Marchantia, which is known to release xyloglucan from rhizoids. This will be complemented by evaluation compositional and ultrastructural consequences for the cell wall and by tracking polysaccharide secretion and recruitment on a cellular level using new in vivo click chemistry-based imaging approaches. Tracking will consider cross-species exchange of polysaccharides. Finally, transcriptomic changes of relevant carbohydrate-active enzymes will be monitored in response to different external polysaccharides and desiccation stress. We anticipate that our in-depth investigation of polysaccharide remodelling and recruitment will provide novel insights into the role of the cell wall during terrestrialization.