Molecular studies on a large multi-gene family of polygalacturonases in two different phytophthora species

The plant cell wall is a structural barrier to pathogens, composed of a network of polysaccharides such as cellulose, hemicellulose and pectin. The majority of pathogenic microorganisms produce cell wall degrading enzymes (CWDEs) that are essential for the invasion process. Among the different CWDEs, polygalacturonases (PGs) play a critical role since their action on pectin makes other cell wall components more accessible to other CWDEs and causes tissue maceration. PGIPs (polygalacturonase-inhibiting proteins) are plant cell wall proteins that specifically modulate the activity of the PGs, and hamper the invasion process by limiting the host tissue colonization. The PG†"PGIP interaction retards pectin hydrolysis and favors oligogalacturonide (OGs) accumulation and leading to plant defense activation. This work wants to contribute to study the role of the PGs in P. nicotianae and P. capsici, among the most dangerous pathogens for many plant species: Specific points of this thesis are: 1) Identification of the whole set of the PGs from well-known oomycetes, which present different lifestyles. 2) Comparison of large PG families found in the oomycetes species using phylogenetic analysis for tracking evolutionary relationships. 3) Analysis of amino acid sequences on identified PGs to detect domains and/or amino acids involved in PG-PGIP interaction. 4) Characterization of PGs from P. nicotianae and P. capsici. 5) Construction of P. capsici mutants for investigate the role of PGs in the pathogenesis, using different approach of reverse genetics. The results from this thesis enhances the hypothesis that the multiplicity of PGs may give flexibility to the pathogen, with each enzyme having its own unique properties to contribute to the performance of all the enzymes to successfully colonize plants.