A limiting source of organic nitrogen induces specific transcriptional responses in the extraradical structures of the endomycorrhizal fungus Glomus intraradices

The molecular bases of organic nitrogen (N) metabolism in arbuscular mycorrhizal (AM) fungi remain so far largely unexplored. To isolate genes responsive to low versus high organic N concentrations, the techniques of suppressive subtractive hybridization (SSH) and reverse Northern dot blot were performed on extraradical structures of the AM fungus Glomus intraradices grown on carrot hairy roots. This approach allowed the identification of 32 up-regulated and 2 down-regulated genes following a 48-h treatment with 2 mu M of an amino acid pool (leucine, alanine, asparagine, lysine, tyrosine). The expression profile of eight genes was further confirmed by semi-quantitative and real-time RT-PCR. The majority of the sequences showed no significant similarity to proteins in databases. The other responsive genes code for putative glyoxal oxidases, transcription factors, a subunit of the 20S proteasome, a protein kinase and a Ras protein. This novel set of data indicates that G. intraradices extraradical structures perceive organic N limitation in the surrounding environment leading to a response at transcriptional level and supports the role of N as signalling molecule in AM fungi.