Characterization of Phosphate Permease in Fusarium Oxysporum F. SP. Vasinfectum: Growth, Virulence and Secondary Metabolism

Microbial development depends on the effective sensing of environmental signals and appropriate acquisition of nutrients and other molecules critical for the organism’s growth. Levels of nutrients and other molecules within the cell, and those outside of the cell, are usually relayed to the nucleus via signal transduction pathways. Inorganic phosphate (Pi) is one of the most important macronutrients in fungal cells and its intracellular levels are tightly regulated via the phosphate signaling transduction pathway (PHO). A Tfo1 transposon insertion has been found in a phosphate permease gene (pho84), a constituent of the PHO pathway, of highly virulent California race 4 isolates of the cotton pathogen Fusarium oxysporum f. sp. vasinfectum (Fov). Strains where the insertion is present are more pathogenic, even in the absence of nematodes, than those lacking the insertion. Because pho84 not only function as a phosphate transporter but also as a receptor for signal transduction, we hypothesized that disruption of the phosphate permease gene in Fov may alter its pathogenicity, fungal development and fitness. We have generated Agrobacterium-mediated knock-out mutants of the gene in four Fov isolates displaying varying levels of virulence in soil drenching pathogenicity assays: CA9 (Highly virulent), MD308 (Highly virulent), MD312 (moderately virulent) and NRRL25433 (non-virulent). Three knock-out mutants per isolates were confirmed for the targeted gene replacement with a hygromycin resistance gene. Pathogenicity assays with the wild type progenitor and mutant isolates show no difference in plant weight or disease rating. We are currently characterizing the role of the phosphate permease in fungal growth, competition with other soil microbes, and production of secondary metabolites (heptaketides and fusaric acid) involved in fungal competence and pathogenicity.