Deciphering the molecular pathways and codeχ-specificity of stomatal closure – from model plants to crops

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Stomata are tiny pores on the undersides of leaves that allow for CO2 uptake and transpiration but also provide a potential entry point for pathogenic microbes. Therefore, guard cells, which regulate the size of the stomatal aperture, respond to abiotic and biotic stresses, such as drought and pathogen infestation. Microbial plant pathogens that may enter via the stomata include bacteria, oomycetes, and fungi, which vary significantly in their pathogenicity to different plant species. Various external and internal signals must be integrated because although stomatal closure prevents pathogen entry and water loss, the plant's metabolism is also reduced by the lack of gas exchange. Adverse effects include increased photorespiration and tissue overheating due to a lack of transpiration. Nevertheless, applying a stomatal closure agonist that induces a uniform stomatal closure could combat pathogen infestations and protect plants during short drought periods. Artificial stomatal closure can be induced, for example, by chitosan. Chitosan is a derivative of chitin, a well-known microorganism-associated molecular pattern (MAMP), which is recognized by plants and induces an immune response. There are two advantages of chitosan over chitin. First, chitosan has good solubility in water and weak acids and can be used more efficiently in solution. Second, and more importantly, we found that the chitosan perception in guard cells differs from chitin perception, which might allow targeting stomatal closure with defined chitosans without triggering unnecessary plant defense mechanisms that reduce plant fitness. Although chitosan is already used in agriculture, the exact mechanisms of chitosan perception and subsequent signaling pathways are largely unknown. This makes it challenging to synthesize specific biologically active chitosans. The main objective of this project is to identify specific chitosans that can enhance plant resilience to drought and pathogen attacks. To achieve this, a comprehensive understanding of the chitosan perception mechanism, the specificity of chitosan receptors for different chitosan codes, and the underlying signaling pathways are required. This research will provide a deeper understanding of chitosan perception and the chitosan-mediated signaling pathways in plants and help in the development of new strategies for crop improvement.