Utilizing the Model Legume Medicago truncatula to Dissect Legume-Infecting Fusarium oxysporum Pathogenomics

Fusarium wilt disease caused by the root-infecting fungal pathogen Fusarium oxysporum affects over 100 different plant species and is a major constraint to global production of grain legumes such as chickpea, where in Australia it represents a significant biosecurity threat. To gain molecular insight into the F. oxysporum-legume interaction, we developed a model pathosystem using the model legume Medicago truncatula and its corresponding pathogenic forma specialis, F. oxysporum f. sp. medicaginis (Fom-5190a). We also supplement our findings with knowledge acquired from other model systems such as the Arabidopsis thaliana Brassica infecting isolate Fo5176. Focusing on the identification of pathogenicity gene content in Fom-5190a, we leveraged knowledge of ‘core’ and ‘dispensable’ chromosomes from other model Fusarium pathogens where dispensable chromosomes and their role in host pathogenicity are well defined. Combining the predicted dispensome with hallmark fungal effector features and high-coverage in planta RNA-sequencing to detect up-regulated gene expression early in host infection, we developed a robust method for the prediction of candidate F. oxysporum effectors. When applied to Fom-5190a, we shortlisted a priority set that included novel effector candidates and orthologues of the F. oxysporum f. sp. lycopersici (tomato-infecting) SECRETED IN XYLEM (SIX) proteins. Four of the effector candidates shared closest identity to proteins from two other legume infecting formae speciales, with two differing by only one amino acid, suggestive of conserved roles that may be important for legume pathogenicity. Recognition of conserved effectors facilitates their functional characterisation and role in inciting disease on economically important legume hosts.