Interactions of Microbotryum violaceum (ustilago violacea) with its host plant Silene alba

Sex expression in Silene alba is determined genetically but can be changed in female plants upon infection with the heterobasidiomycete Microbotryum violaceum. This change is not caused by steroids, the classical plant growth regulators, nor by a diffusible morphogen produced and secreted by the fungus. Nor is the production of stamnes in genotypically female flowers caused by the transmission, incorporation and expression of a fungal plasmid since plants regenerated from diseased tissue of genotypically female S. alba did not yield stamen-producing flowers. Neither density gradient centrifugation nor agarose gel electrophoresis of endonuclease restricted DNA from M. violaceum revealed the presence of a plasmid. Southern blots of DNA from S. alba probed with labeled DNA of M. violaceum, however, indicated the presence of homologous, unique sequences absent in non-host plants. Since the same homologous sequences were identified in male and female S. alba, these DNA fragments are not homologous to the coding sequences for male sex expression in S. alba unless they represent genetic elements of the hypothetical gyndyioecious precursor. Two other aspects of the S. alba-M. violaceum interaction have yielded interesting results. M. violaceum grows as sporidia outside of the host, but as short hyphae in planta. The switch from sporidial to hyphal growth is mediated in vitro by hyphal growth factors (HGFs) isolated from aqueous host plant extracts as well as by -tocopherol. In addition to changing the fungal growth form. HGFs may serve as host recognition factors. Siderophore mutants of M. violaceum that accumulated less rhodotorulic acid than wild type also showed reduced or no pathogenicity, indicating that siderophores are an important factor in the host-pathogen interaction.