Synthetic Glycan Ligands for Plant Immune Receptors

In today`s societies with ever-growing populations, the demand for food, energy, and raw materials is increasing dramatically, creating an urgent need to increase crop productivity. Improving crops for food, fiber, and biofuel production would greatly benefit from a more detailed understanding of the plant immune system. Plants recognize and respond to attacks by pathogens using a part of the immune system that is based on the recognition of their own or foreign molecular patterns by so-called pattern recognition receptors (PRRs). These are, for example, receptor-like kinases (RLKs) that transmit danger signals into the plant upon contact with matching binding partners. Despite the large number of RLKs in plants and the great abundance of glycans in the cell walls of plants, bacteria, and fungi, only a handful of glycans have been found to elicit plant immune responses, and for only two of them have the corresponding receptors been described. We recently identified two new glycan -RLK pairs by robotically printing glycans on glass slides and studying their interactions with RLKs (glycan arrays). The structurally highly complex polysaccharides rhamnogalacturonan-I (RG-I) and galactomannan (GM) from the plant cell wall were able to induce an immune response in living plants. Our goal is to establish RG-I and GM as novel plant molecular patterns for activating innate immunity in plants and to determine the precise chemical structures recognized by their cognate RLKs. This will be accomplished by the chemical synthesis of various fragments of RG-I and GM polysaccharides, which will subsequently enable glycan array-based characterization of the RLKs. After validation with further biophysical methods, the identified oligosaccharides will be evaluated for their potential to stimulate plant immune defense responses. The unique approach of combining synthetic carbohydrate chemistry and glycan arrays with plant immunity research will enable the elucidation of more advanced molecular structures with maximum capacity to trigger immune responses. The knowledge gained will enable the development of preparations from glycan molecules to boost the plant immune system, with the idea of avoiding the need for conventional pesticides in the future.