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

大多数绿藻和陆地植物的细胞被初级细胞壁包围，初级细胞壁是由多种多糖组成的承重但可扩展的基质，保护藻类免受环境胁迫，如缺水。这些细胞壁多糖的主要部分是半纤维素，可以用酶切割并重新连接到附近的另一个半纤维素分子上。这种反应是由细胞壁结合的转葡聚糖酶催化的，这种转葡聚糖酶在Zygnematophyceae及其姊妹植物陆生植物中都大量存在。有趣的是，转聚糖酶在早期分叉轮藻(如Klebsormidiophyceae)和绿藻中含量较低。半纤维素糖基化被认为在直立生长的陆地植物的细胞壁代谢中起着关键作用，然而，它在藻类中的功能尚不清楚，尽管编码相关酶的基因家族在陆地征服的藻类中得到了强烈的扩展，这表明它们在为陆地化准备藻类及其细胞壁方面发挥了重要作用。此外，陆地植物中最丰富的半纤维素和转糖的主要底物--木葡聚糖--都是在Zygnematophyceae进化的。我们最近的数据表明，这些藻类可以惊人地大量分泌木葡聚糖和其他半纤维素进入环境，反之亦然，通过转糖基化将外部多糖重新招募到它们的细胞壁中。我们认为，这种未被探索的半纤维素分泌和多糖类的补充帮助藻类产生了生物壳，这些生物壳很可能在陆地化期间作为它们的主要栖息地，并且在恶劣的环境中仍然可以在世界各地找到。为了探索多糖的分泌和募集的作用，我们将设计一系列的培养实验，并利用最新的糖生物学技术分析生长在不同底物上的Zygnema和Mesotaum的外部多糖足迹。我们将比较这种分泌模式与基地植物地钱草，后者已知能从根状植物中释放木葡聚糖。此外，还将评估细胞壁的成分和超微结构，并使用新的基于体内点击化学的成像方法在细胞水平上跟踪多糖的分泌和募集。跟踪将考虑多糖的跨物种交换。最后，将监测相关碳水化合物活性酶的转录变化，以响应不同的外部多糖和干燥胁迫。我们预计，我们对多糖重塑和募集的深入研究将为陆地化过程中细胞壁的作用提供新的见解。

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.