Understanding cell wall re-modelling during the symbiotic interaction between the ectomycorrhizal fungus Tuber melanosporum and Corylus avellana

It has been reported that the symbiotic ectomycorrhizal fungi have a small set of genes coding for secreted enzymes putatively involved in the degradation of plant cell wall polysaccharides [1, 2]. Within the context of the Tuber melanosporum genome sequencing project, genes coding for putative plant cell wall degrading enzymes (CDWEs) have been identified and manually annotated; several of them were found to be up-regulated during the symbiosis, suggesting a role in plant cell wall degradation and facilitation of the fungal progression through the pectin-rich middle lamella, when the fungus develops inside plant tissues. Gene expression data (microarray, RNAseq, qRT-PCR) on T. melanosporum and Corylus avellana ectomycorrhizae (ECM) have shown that two fungal putative endoglucanases (TmelCMC3 and TmelEG), a pectate lyase B (TmelPLB), a GH28 polygalacturonase (TmelPGN1) and a rhamnogalacturonase (TmelRghA) were up-regulated. qRT-PCR experiments have demonstrated that, in agreement with the RNAseq data, a gene encoding a rhamnogalacturonan acetylesterase (TmelRgaE) is also highly up-regulated in ECM, although the array data did not indicate any up- regulation. In addition, we have employed glycan microarray technology [3] to analyse the impact of fungal colonization on plant cell wall composition in the ectomycorrhizae, compared with uncolonized halzenut roots. Preliminary data suggest that cell walls are affected by the presence of the fungus and the observed changes are consistent with gene expression data, which have shown an up-regulation of enzymes involved in pectin degradation. To support glycoarray data, in situ immunolabelling experiments are ongoing by using monoclonal antibodies with specificity for plant cell-wall components.