Shaping susceptibility The role of SIX8 and TOPLESS in Fusarium wilt disease

The Fusarium oxysporum (Fo) species complex causes Fusarium wilt disease in a wide range of economically important crops, including tomato, cotton, banana, spinach, lettuce, melon, and ornamentals such as chrysanthemum and tulip. Current management strategies are often environmentally harmful, expensive, and unsustainable. A viable alternative is to the use of genetically resistant plant varieties. This thesis identifies members of the small TOPLESS (TPL) gene family as key Susceptibility (S) genes for Fo infection. Two specific TPL proteins directly interact with the Fo-secreted effector protein, SECRETED IN XYLEM 8 (SIX8). CRISPR/Cas9-mediated knockout of these TPL genes in both tomato and Arabidopsis thaliana conferred near-complete resistance to Fo, confirming their role as S genes. The SIX8 gene is present in 14 Fo isolates with distinct host specificities, while TPL genes are conserved across all land plants, suggesting that this resistance strategy could be broadly applicable across different crop species. TPL proteins function as transcriptional co-repressors, leading us to hypothesize that SIX8 promotes disease by manipulating host transcription. To investigate this, we generated A. thaliana lines expressing SIX8 and discovered that the immune receptor SNC1 monitors both TPL and TPL-RELATED 1 (TPR1). Furthermore, expression of SIX8 in a snc1 null mutant background, followed by transcriptome profiling revealed biological pathways that are potentially targeted by the pathogen during infection.