Monoxenic in vitro production and colonization potential of AM fungus Glomus intraradices.

The paper reports the establishment of mycorrhizal infection of a non-mycorrhizal Ri-T-DNA transformed carrot root when co-cultured with a surface sterilized sweet potato root segment colonized by arbuscular mycorrhizal (AM) fungus G. intraradices on minimal M medium. Extensive fungal hyphal emergence from each cut end of the mycorrhizal sweet potato root piece was observed in one week old cultures. These hyphae caused infection on contacting the transformed-carrot- root segment and produced many hyphae and spores both inside and outside the zone of the root after 6 week of growth. Axenically produced fungal propagules proliferated on the surface of fresh minimal M medium when sub-cultured without any root segment. On repeated sub-culturing, these propagules did not lose their ability to grow and produced many juvenile small spore-like vesicles during the non-symbiotic phase. Although these spores were morphologically and anatomically similar to their soil borne counter parts, they were much smaller. When placed in the vicinity of a fresh hairy root on the minimal medium or a Sudan grass seedling in sand culture, the axenically produced AM fungal propagules caused root infection, but the infection characteristics were significantly different to the original culture in terms of shape (spherical vs oval) and size (20 microm vs 45 microm) of the intraradical vesicles, and absence of 'H' branches. Sudan grass seedlings inoculated with the axenically cultured fungus showed significantly (P < 0.05) higher dry weights plant'. When compared to the plants inoculated with sand cultures, the growth parameters and the percentage infection were not significantly different. However, when both sources of inocula were used together, a synergistic effect on plant growth as well as root infection was observed.