Environmental Biology Group, Centre for Environmental Sciences, Hasselt University, Agoralaan, Diepenbeek, Belgium.
ISME J. 2013 Oct;7(10):2010-22. doi: 10.1038/ismej.2013.91. Epub 2013 Jun 20.
The majority of nitrogen in forest soils is found in organic matter-protein complexes. Ectomycorrhizal fungi (EMF) are thought to have a key role in decomposing and mobilizing nitrogen from such complexes. However, little is known about the mechanisms governing these processes, how they are regulated by the carbon in the host plant and the availability of more easily available forms of nitrogen sources. Here we used spectroscopic analyses and transcriptome profiling to examine how the presence or absence of glucose and/or ammonium regulates decomposition of litter material and nitrogen mobilization by the ectomycorrhizal fungus Paxillus involutus. We found that the assimilation of nitrogen and the decomposition of the litter material are triggered by the addition of glucose. Glucose addition also resulted in upregulation of the expression of genes encoding enzymes involved in oxidative degradation of polysaccharides and polyphenols, peptidases, nitrogen transporters and enzymes in pathways of the nitrogen and carbon metabolism. In contrast, the addition of ammonium to organic matter had relatively minor effects on the expression of transcripts and the decomposition of litter material, occurring only when glucose was present. On the basis of spectroscopic analyses, three major types of chemical modifications of the litter material were observed, each correlated with the expression of specific sets of genes encoding extracellular enzymes. Our data suggest that the expression of the decomposition and nitrogen assimilation processes of EMF can be tightly regulated by the host carbon supply and that the availability of inorganic nitrogen as such has limited effects on saprotrophic activities.
森林土壤中的大部分氮存在于有机物质-蛋白质复合物中。外生菌根真菌（EMF）被认为在外生菌根真菌从这些复合物中分解和移动氮方面起着关键作用。然而，对于控制这些过程的机制、它们如何被宿主植物中的碳以及更容易获得的氮源形式的可用性调节，我们知之甚少。在这里，我们使用光谱分析和转录组谱分析来研究葡萄糖和/或铵的存在或不存在如何调节外生菌根真菌 Paxillus involutus 对凋落物材料的分解和氮的移动。我们发现，氮的同化和凋落物材料的分解是由葡萄糖的添加引发的。葡萄糖的添加还导致参与多糖和多酚氧化降解、肽酶、氮转运蛋白和氮碳代谢途径中酶的编码基因的表达上调。相比之下，向有机物质中添加铵对转录本的表达和凋落物材料的分解只有在葡萄糖存在的情况下才会产生相对较小的影响。基于光谱分析，观察到凋落物材料的三种主要化学修饰类型，每种类型都与编码细胞外酶的特定基因集的表达相关。我们的数据表明，外生菌根真菌的分解和氮同化过程的表达可以通过宿主碳供应进行严格调节，而无机氮的可用性对腐生活性的影响有限。
