| Siderophores are small molecular mass compounds that are produced and secreted by many microorganisms when iron-starved. Siderophores chelate iron and make it available for uptake. I investigated the role of extracellular siderophore production in the obligate plant pathogen Microbotryum violaceum. Siderophore-mediated iron-uptake has been implicated in the virulence of several bacterial pathogens of animals as well as that of the plant pathogen Erwinia chrysanthemi. The extracellular production of the siderophore rhodotorulic acid was assayed in low-iron grown cultures of 23 strains of M. violaceum collected from locations in North America and Europe. All 23 strains did produce the hydroxamate siderophore rhodotorulic acid, a finding consistent with the hypothesis that siderophore production plays an important role at some point in the fungal lifecycle.; In analogy to the demonstrated function of siderophores in bacterial pathogenesis of animals, I hypothesized that siderophore production in M. violaceum was critical for pathogenesis. I therefore produced and characterized siderophore mutants and tested their pathogenicity. Four siderophore mutant strains were produced from wild type strain 1.C425. Crosses of these mutant lines however showed aberrant segregation patterns for the two markers that were followed, i.e., recovery of three meiotic products from a single teliospore was common. This is likely a result of the same phenomenon that has been reported by other researchers where crosses between different strains show abnormal meioses and unexpected progeny types. Mutagenesis was then carried out in strain 2.A2, which has been well characterized genetically, and a null mutant that secreted no detectable rhodotorulic acid was isolated. Segregation of this mutant and the wild type siderophore phenotypes occurred in a 1:1 ratio, indicating a monogenic basis; tetrad analysis determined that the locus was situated near the centromere (2.6-5 cM). Isolates resulting from a cross between the null mutant and the wild type were used to test pathogenicity. Normal levels of pathogenicity in the host plant Silene latifolia were shown by isolates carrying the null mutation, indicating that extracellular siderophore production may not play a critical role in the in planta growth of this pathogen. Other stages of the fungal lifecycle where extracellular siderophore production may be important include haploid sporidial saprophytic growth and teliospore germination. |
