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Innate sensing of glycan in plant roots

The ability to distinguish self from non-self is a fundamental aspect of any immune system. Plants rely on the innate ability of each cell to recognize invaders and are capable of perceiving and responding to pathogenic or beneficial microbes via a multilayered, highly specific and effective immune system comprised of surveillance of non-self, damaged self and altered self as danger signals (Rovenich et al., 2016). To protect and defend against microbial invaders, but also to start the symbiotic programme, plants need to detect the presence of microbes and to discriminate among them by means of pattern-recognition receptors that respond to specific microbe-associated molecular patterns (MAMPs). MAMPs are usually slowly evolving components of microbes with crucial biological functions that are not present in the host and are normally exposed and thus accessible for the plant cell-surface receptor proteins. Cell-surface glycans and glycan-binding proteins such as lectins and lectin-like receptors regulate many of the signalling processes throughout host-microbe interactions and immune responses. Surface exposed glycans and glycan-binding proteins reside at the key interface between host and microbe and are well-characterized factors that orchestrate host immunity following microbial exposure in animal systems. Advances in functional glycomics revolutionized analysis of glycan-binding proteins / ligands interactions with significant implications in understanding the role of glycan-binding proteins in animal-pathogen and pathogen-pathogen interactions. On the other hand, glycans and their interactions with other biomolecules in plant-fungal systems are still largely unexplored. It can be hypothesized that plants employ glycan-mediated recognition in order to combat invading fungi or to establish symbiosis and that fungi use glycan-mediated communication and glycan-binding proteins to facilitate or circumvent recognition. This hypothesis is supported by recently published evidence, which remains fragmented and mainly limited to chitin and chitooligosaccharides and their derivatives (Fesel & Zuccaro, 2016). The availability of a large number of fungal and plant genomes as a tool to identify putative fungal glycan-binding proteins and respective plant receptors can now help study the role played by cell-surface sugars and their binding proteins in plant-fungal interactions. The overall scientific objective of this research topic in my group is to elucidate the biological functions of cell-surface glycans at the plant-fungus interface and the corresponding glycan-binding proteins and receptors, focusing primarily on glycan-mediated plant root immune activation and suppression (Wawra et al., 2016; 2019; Wanke et al., 2020; 2023; Chandrasekar et al., 2022). A special focus is given to innate sensing of β-glucans as recent evidence in my group suggests the existence of fungal glycan-binding effectors deregulating β-glucan–triggered immunity in plants (Wawra et al., 2016; 2019) and palnt receptors and enzymes for compatibility durign symbiosis (Wanke et al., 2023; Kelly et al., 2023). Understanding the genetic basis of glycan-mediated recognition and communication will help develop crop varieties that are more resistant to biotic stresses and disease epidemics and improve plant health and productivity. We expect this research area to lead to both application-specific (e.g. improvement of crop production) and technology-developing (e.g. glycan-specific probes) innovative approaches.

Model for the production and function of the conserved fungal EPS-derived β-1,3;1,6-glucan decasaccharide. The fungal-responsive GH17 family member HvBGLUII is found in the apoplast of barley roots and acts on β-1,3-glucan. Digestion of linear β-1,3-glucan (laminariheptaose) with HvBGLUII enhances ROS accumulation in barley roots, corroborating its role as a host-defense enzyme with a function in β-glucan perception. To counteract the activity of HvBGLUII, plant-colonizing fungi produce a β-1,3;1,6-glucan-rich EPS matrix. The activity of HvBGLUII on the EPS matrix releases a conserved β-1,3;1,6-glucan decasaccharide (β-GD) which is resilient to further digestion by GH17 family members. The β-GD acts as a carbohydrate-class effector by scavenging reactive oxygen species and enhancing fungal colonization. Lectins containing WSC domains are enriched in the outer EPS matrix and lectins containing LsyM domains are enriched in the CW of S. indica. Graphical illustration was designed with the BioRender online tool. (Chandrasekar et al., 2022).