Regulation of genes involved in nitrogen utilization on different C/N ratios and nitrogen sources in the model ectomycorrhizal fungus Hebeloma cylindrosporum
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Nitrogen (N) utilization by ectomycorrhizal fungi is an essential aspect of their ecosystem function. N deposition changes both the N pools and the carbon/nitrogen (C/N) ratio of the substrates where ectomycorrhizal fungi are found, and it is important to understand how these changes affect the N forms used by ectomycorrhizal fungi. To overcome the difficulties of studying ectomycorrhizal fungi in situ, we investigated all known N genes in the model fungus, Hebeloma cylindrosporum in a culture study. In addition to studying the regulation of all known N utilization genes, we aimed to understand whether there are gene clusters that undergo similar regulation. Lastly we studied how C/N ratio, N transporter type, and N source affected relative gene expression levels. We grew the D2 strain of H. cylindrosporum on a range of inorganic and organic N sources under low, medium, and high C/N ratios. We found three gene clusters that were regulated in a similar pattern. Lastly, we found C/N ratio, N source and N transporter type all affected gene expression levels. Relative expression levels were highest on the high C/N ratio, BSA and diLeucine N sources, and inorganic N transporters were always expressed at higher levels than organic N transporters. These results suggest that inorganic N sources may always the default preference for H. cylindrosporum, regardless of both the N sources and the C/N ratio of the substrate.
KeywordsNitrogen transporters Inorganic nitrogen Organic nitrogen Gene expression Carbon/nitrogen ratio
This research was funded in part by grants from Deutsche Forschungsgemeinschaft (DFG WI1994/2-1 and 2-2), Agence Nationale de la Recherche Française (ANR blanche TRANSLMUT) and Burgundy Region to DW. We thank the Fulbright Organization for providing funding to MA to live in Germany while this research took place. We also thank Cynthia Chang for her help with statistical analyses.
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