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

大多数绿色藻类和陆地植物的细胞被初生细胞壁包围，初生细胞壁是由各种多糖组成的承重但可延伸的基质，并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，其可以被酶促切割并重新连接到附近的另一半纤维素分子。这种反应是由细胞壁结合的转聚糖酶催化的，这种酶大量存在于接合藻纲和它们的姐妹谱系陆地植物中。有趣的是，转聚糖酶在早期分歧轮藻（例如克雷伯藻纲）和绿藻门中不太丰富。半纤维素转糖基化被认为在直立生长的陆生植物的细胞壁代谢中起关键作用，然而，其在藻类中的功能是难以捉摸的，即使编码负责酶的基因家族在陆地征服藻类中强烈扩展，表明它们在准备藻类及其细胞壁用于陆地化中发挥了重要作用。此外，转聚糖的主要底物和陆地植物中最丰富的半纤维素-木葡聚糖-在接合藻中进化。我们最近的数据表明，这些藻类可以分泌大量的木葡聚糖和其他半纤维素到环境中，反之亦然，通过转糖基化将外部多糖重新吸收到它们的细胞壁中。我们认为，这种未经探索的半纤维素分泌和多糖的补充有助于藻类产生生物结壳，这些生物结壳最有可能在陆地化期间作为其主要栖息地，并且仍然可以在世界各地的恶劣环境中找到。为了探索多糖分泌和募集的作用，我们将设计一系列培养实验，并通过最先进的糖生物学技术分析在不同基质上生长的接合藻和中带藻的外部多糖足迹。我们将比较分泌模式与基底陆地植物地钱，这是已知的释放木葡聚糖从假根。这将通过评价细胞壁的组成和超微结构结果以及通过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分泌和募集来补充。追踪将考虑多糖的跨物种交换。最后，相关的碳水化合物活性酶的转录组的变化将监测响应于不同的外部多糖和干燥胁迫。我们预计，我们对多糖重塑和招募的深入研究将为细胞壁在terrestrialization过程中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.