A homologue of the cell cycle check point TOR2 from Saccharomyces cerevisiae exists in the arbuscular mycorrrhizal fungus Glomus mosseae

A homologue of the gene TOR2 from Saccharomyces cere isiae has been found in the arbuscular mycorrhizal (AM) fungus Glomus mosseae (BEG 12) during a differential RNA display experiment. Further downstream sequence was obtained by a nested-PCR approach. Eight introns were found in 2.6 kb sequence. The fragment encodes a putative protein with high homology (53% identity) to the C terminus of S. cere isiae TOR2 and its homologues in Schizosaccharomyces pombe and humans. The gene was named GmTOR2. The expression of the gene was studied by reverse transcriptase-polymerase chain reaction and it was found to be expressed at a relatively high level during all the different life cycle stages of the AM fungus. TOR2 is known to be involved in the control of the cell cycle in S. cere isiae and the organization of the actin cytoskeleton in response to nutrients. The anti-inflammatory drug rapamycin, known to interfere with the role of TOR2 controlling the arrest of the cell cycle in G(1) but not with its signalling to the actin cytoskeleton, was found to decrease hyphal growth of G. mosseae sporocarps but not to affect spore germination. This result confirms that DNA replication is not needed for germination but during the presymbiotic growth. The immunostaining of germinated sporocarps of G. mosseae with antibodies against tubulin showed the presence of mitotic spindles in some secondary spores, confirming previous findings of DNA replication during presymbiosis. The possibility that GmTOR2 controls the cell cycle arrest in AM fungi in the absence of the plant as a response to nutrient starvation is discussed.