The xylem warriors Unveiling the role of pathogenesis-related proteins in tomato resistance to Fusarium wilt

Vascular wilt, caused by Fusarium oxysporum (Fo), poses a serious threat to crops worldwide. Current control methods are often ineffective and environmentally harmful, emphasizing the need for new strategies. Genetic resistance is one of the most effective approaches for managing Fusarium wilt. However, resistance (R) genes have been identified in only a few crops, and pathogens evolve to overcome R-gene-mediated resistance. This thesis explores the underlying mechanisms of I-1, I-2, I-3, and I-7 gene-based immunity against Fo f. sp. lycopersici (Fol) in tomatoes. I-genes encode immune receptors that initiate defense responses in roots, which are then transmitted to the shoots via the xylem vessels. In the stems of resistant plants, the pathogen remains confined to the xylem and does not spread to surrounding tissues, even though it continues to secrete effector proteins. Comparative proteomic analyses of xylem sap from resistant tomato lines revealed distinct profiles of pathogenesis-related (PR) proteins associated with each R-gene. Notably, a shared set of four PR proteins emerged as potentially associated with Fusarium wilt resistance. Characterization of two PR-5 proteins, accumulating in the xylem sap during resistance, demonstrated their antifungal activity against Fol in vitro. CRISPR/Cas9-mediated knockout of either PR-5 isoform in tomatoes did not affect Fol resistance, while overexpression of one of the PR-5 isoforms conferred reduced susceptibility to Fol. Several other PR proteins identified in xylem sap during resistance belong to families with known antifungal activity. Therefore, a pipeline was developed to characterize the antifungal activity of PR proteins against Fo. This thesis advances our understanding of the molecular defenses in tomatoes against Fol and outlines potential directions for future research.