The plant immune system perceives a wide range of carbohydrates (sugars) derived from plant cell walls and microorganisms through extracellular domains of pattern recognition receptors, which activate pattern recognition-dependent immunity (PTI). Various oligosaccharides have been described, such as those derived from mixed glucans and cellulose ꟷtwo of the main polysaccharides of plant cell wallsꟷ, which are capable of inducing ITP and activating resistance against pathogens in different plant species. However, scientific knowledge about the perception of these oligosaccharides by the plant immunity system and the molecular signaling mechanisms involved has been scarce. The only well-characterized mechanism of oligosaccharide perception by plants is that of chitin oligosaccharides, which is a polysaccharide of the cell walls of fungi, and is carried out by plant receptors with extracellular domains of the LysM type.
Knowledge of how plants perceive oligosaccharides and activate immunity has received a notable boost thanks to recent discoveries by researchers at the Center for Plant Biotechnology and Genomics (CBGP), a joint center of the Polytechnic University of Madrid (UPM) and the National Institute of Agricultural and Food Research and Technology, attached to the Higher Council for Scientific Research (CSIC) in Spain. The research group led by Antonio Molina ꟷin collaboration with Julia Santiago’s group from the University of Lausanne in Switzerlandꟷhas isolated and characterized a group of mutants of Arabidopsis thaliana which they have named igps (for its acronym in English for “impaired in glycan perception”, “unable to perceive glycans”) that are defective in the activation of PTI by the oligosaccharides MLG43 and CEL3, but not by oligosaccharides derived from chitin (CHI6). These results suggested that the mechanism of perception of MLG43 and CEL3 was new and distinct from that of chitin oligosaccharide (CHI6) perception. The identification of the igp1-igp4 plant mutations allowed us to corroborate this hypothesis since the researchers demonstrated that these plants are altered in three receptor kinases (RK) of a new family of proteins with LRR-MAL extracellular domains that have not been identified. previously implicated in plant immunity.
Three-dimensional model of the structure of the IGP1/CORK1 receptor with its extracellular domain (LRR-MAL, in blue and yellow respectively), the transmembrane domain (purple) and the kinase domain (green). The structure of cellotriose (CEL3) that binds IGP1/CORK1 is depicted. Recognition of CEL3 by the receptor activates plant immunity (PTI) and resistance to pathogens. With “PM” the plasmatic membrane is indicated. (Image: María Garrido and Luis Fernández-Pacios (CBGP / UPM / INIA / CSIC)
The data obtained in the study suggest that the LRR-MAL-RKs proteins identified are part of a new system for oligosaccharide recognition and activation of the immune response in plants different from that of the LysM-PRR proteins (chitin receptors). This now characterized immunity mechanism is activated when a pathogen tries to colonize a plant and secreting enzymes to degrade cell wall polymers (for example, cellulases or other hydrolases) releases oligosaccharides from these polymers (cellulose and MLG) that act as signaling molecules that alert cell wall damage that are recognized by the identified IGP receptors, which activate plant immunity.
In the opinion of the researcher Antonio Molina: “The discovery of this new family of carbohydrate receptors, which is the second described in plants, opens the possibility of developing biological products based on cellulose oligosaccharides that stimulate plant immunity and protection against pathogens.” . These new products based on sugars of natural origin can replace chemically synthesized phytosanitary products and thus contribute to a more sustainable agriculture”. “In addition,” he adds, “this technology will contribute to a circular economy because the cellulose and mixed glucans present in plant waste could be used to develop these new products; that is, we could protect the crops with waste or remains of biomass from the crops themselves.”
The research team exposes the technical details of their latest findings in this regard in the academic journal The Plant Journal, under the title “Arabidopsis immune responses triggered by cellulose- and mixed-linked glucan-derived oligosaccharides require a group of leucine-rich repeat malectin receptor kinases.
This novel technology is currently being developed by the CBGP group within the framework of a license agreement for the exploitation of a patent with an international company. (Source: UPM)