Is cortical root colonization required for carbon transfer to arbuscular mycorrhizal fungi?: Evidence from colonization phenotypes and spore production in the reduced mycorrhizal colonization (rmc) mutant of tomato

For the first time, the phenotypes formed in the reduced mycorrhizal colonization (rmc) Solanum lycopersicum L. (tomato) mutant with different arbuscular mycorrhizal (AM) fungi were used to explore the potential of different fungal structures to support development of external fungal mycelium and spores. The life cycle of AM fungi with rmc was followed for up to 24 weeks. Results showed that production of external mycelium was slight and transitory for those fungi that did not penetrate the roots of rmc (Pen-) (Glomus intraradices DAOM181602 and Glomus etunicatum). For fungi that penetrated the root epidermis and hypodermis (Coi-, Glomus coronatum and Scutellospora calospora) the mycelium produced varied in size, but was always smaller than with the wild-type 76R. Spores were formed by these fungi with 76R but not with rmc. The only fungus forming a Myc+ phenotype with rmc, G. intraradices WFVAM23, produced as much mycelium with rmc as with 76R. We observed lipid accumulation in hyphae and vesicles in both plant genotypes with this fungus. Mature spores were formed with 76R. However, with rmc, spores remained small and (presumably) immature for up to 24 weeks. We conclude that significant carbon transfer from plant to fungus can occur in Coi- interactions with rmc in which no cortical colonization occurs. We speculate that both carbon transfer and root signals are required for mature spores to be produced.