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 i n the arbuscular mycorrhizal (AM) fungus Glomus mosseae (BEG 12) during a d ifferential RNA display experiment. Further downstream sequence was obtaine d by a nested-PCR approach. Eight introns were found in 2.6 kb sequence. Th e fragment encodes a putative protein with high homology (53% identity) to the C terminus of S. cere isiae TOR2 and its homologues in Schizosaccharomy ces 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 l ife cycle stages of the AM fungus. TOR2 is known to be involved in the cont rol of the cell cycle in S. cere isiae and the organization of the actin cy toskeleton 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 f ound to decrease hyphal growth of G. mosseae sporocarps but not to affect s pore germination. This result confirms that DNA replication is not needed f or germination but during the presymbiotic growth. The immunostaining of ge rminated sporocarps of G. mosseae with antibodies against tubulin showed th e presence of mitotic spindles in some secondary spores, confirming previou s findings of DNA replication during presymbiosis. The possibility that GmT OR2 controls the cell cycle arrest in AM fungi in the absence of the plant as a response to nutrient starvation is discussed.