The Monothiol Glutaredoxin Grx4 Influences Iron Homeostasis and Virulence in Ustilago maydis.

The corn smut fungus, Ustilago maydis, is an excellent model for studying biotrophic plant-pathogen interactions, including nutritional adaptation to the host environment. Iron acquisition during host colonization is a key aspect of microbial pathogenesis yet less is known about this process for fungal pathogens of plants. Monothiol glutaredoxins are central regulators of key cellular functions in fungi, including iron homeostasis, cell wall integrity, and redox status via interactions with transcription factors, iron-sulfur clusters, and glutathione. In this study, the roles of the monothiol glutaredoxin Grx4 in the biology of U. maydis were investigated by constructing strains expressing a conditional allele of grx4 under the control of the arabinose-inducible, glucose-repressible promoter Pcrg1. The use of conditional expression was necessary because Grx4 appeared to be essential for U. maydis. Transcriptome and genetic analyses with strains depleted in Grx4 revealed that the protein participates in the regulation of iron acquisition functions and is necessary for the ability of U. maydis to cause disease on maize seedlings. Taken together, this study supports the growing appreciation of monothiol glutaredoxins as key regulators of virulence-related phenotypes in pathogenic fungi.

Regulation of the fungal secretome.

The ability of countless representatives of the Kingdom Fungi to adapt to and proliferate in diverse environments is facilitated by regulation of their secretomes to respond to changes in environmental conditions and to mediate interactions with other organisms. Secretome changes often fulfill common functions of nutrient acquisition, facilitation of host/symbiont interactions, cell wall modification, and optimization of the enzyme suite to adapt to new environmental resources. In this review, we expand on our recent work on signaling and the secretome in the pathogenic fungus Cryptococcus neoformans to consider a range of selected examples of regulation of fungal secretomes. These examples include the impact of carbon source and aspects of the response to plant and animal hosts. Additionally, the influence of key protein kinases (e.g., Pka1, Snf1) and transcription factors (e.g., Rim101/PacC) is highlighted to illustrate some underlying regulatory factors influencing the secretome. Although there is a wealth of information about fungal secretomes from both experimentation and genome sequence mining, there are also major gaps in our knowledge about the complete composition of fungal secretomes and mechanisms of dynamic change. For example, a more comprehensive understanding of the composition and regulation of the secretome will require consideration of the emerging roles of unconventional secretion and extracellular vesicles in delivering proteins outside the cell. Overall, changes in the secretome are well documented in diverse fungi and the underlying mechanisms are currently under investigation; however, there remain unknown steps in the regulation of secretory pathways and gaps in understanding the regulation of unconventional secretion, which warrant further research.

Mitospore stages of Disciotis, Gyromitra and Morchella in the inland Pacific Northwest USA.

Colonies of Costantinella species growing on soil, moss and woody debris in the autumn in the inland Pacific Northwest USA were established in culture. Five different mitospore taxa were distinguished based on colony color, presence or absence of setae and internal transcribed spacer region (ITS) rDNA amplicon size. Sequence data from the largest and second largest subunits of RNA polymerase II, translation elongation factor 1-α, D1 and D2 domains of nuclear large subunit rDNA and ITS were used to connect each of the distinct mitospore taxa to corresponding vernal-fruiting Pezizales, including Disciotis cf. venosa, Gyromitra cf. esculenta and three species of Morchella. Both meiospore and mitospore stages of Morchella brunnea and M. populiphila collected in spring and autumn within a meter of each other at two urban sites had identical multilocus haplotypes, providing evidence connecting the two stages of the life cycle. Among other Morchella mitospore stages collected, some had identical haplotypes to previously sampled meiospore stages, while others were distinct, possibly representing undescribed species. Mitospore isolates with sequences assigning them to Disciotis or Gyromitra had different haplotypes from meiospore stages occurring in the same area. Meiospore stages of Disciotis and Gyromitra sampled as part of the study were also genetically distinct from European collections of D. venosa and G. esculenta, indicating more diversity is present in these taxa than is reflected in the current taxonomy. The widespread occurrence of mitospore stages of these fungi suggests that the life cycles of morels, false morels and allied taxa are more complex than previously recognized.