The plant immune system consists of two levels of defense based on the combined action of receptors with varying specificities for the recognition of pathogens and subsequent activation of biochemical responses. The first level of defense depends on transmembrane receptors for the recognition of extracellular molecules. The second level is formed by intracellular receptors called NLRs (nucleotide binding - leucine-rich repeat) that recognize pathogen molecules inside plant cells. These molecules, in the absence of their cognate receptors, ae virulence effectors that contribute to the establishment of the infection. In phytopathogenic bacteria of the genus Xanthomonas, effectors from the TAL (transcription activator-like) family play an important role in host colonization and adaptation. Such effectors are able to bind to the plant DNA at specific sites and activate the transcription of genes that will facilitate infection. Recently, our laboratory found that at least one effector of the bacterium Xanthomonas citri subsp. citri 306 (Xcc306), pathogenic in citrus, is recognized by the immune system of the non-host plant Solanum lycopersicum (tomato). Since this plant carries an NLR protein (called Bs4) capable of recognizing TAL effectors, this project will test whether the Bs4 receptor has any role in the tomato immune response against Xcc306. For this, the four TAL effectors found in Xcc306 will be cloned and transferred to X. euvesicatoria 85-10, which is naturally pathogenic in tomato. Bacterial inoculation in tomatoes will then be performed to test whether any of the Xcc306 TALs effectors confers avirulence to X. euvesicatoria 85-10. The same experiment will be carried out with the wild tomato S. pennelli, which does not have a functional Bs4 receptor. Thus, we hope to genetically determine the possible role of the Bs4 receptor in the recognition of Xcc306 TAL effectors. This work will support the understanding of the resistance mechanisms of non-host plants against Xcc306, which may lead to the identification of an immune receptor with the potential to recognize this bacterium.
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