Chemical signals controlling the arbuscular mycorrhiza symbiosis

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Arbuscular mycorrhizal(AM) fungi form mutualistic, symbiotic associations with the roots of more than 80% of land plants. The fungi are incapable of completing their life cycle in the absence of a host root. Little is known about the molecular mechanisms that govern signalling and recognition between AM fungi and their host plants. In one of the first stages of host recognition, the hyphae of AM fungi show extensive branching in the vicinity of host roots before formation of the appressorium, the structure used to penetrate the plant root. Host roots are known to release signalling molecules that trigger hyphal branching, but these branching factors have not been isolated.We have already developed the paper disc method as a bioassay system for branching factors. We isolated a branching factor from the root exudates of Lotus japonicus and identified it as a strigolactone, 5-deoxy-strigol by spectroscopic analysis and chemical synthesis. Branching factor from root exudates of carrot seemed to be a member of strigolactones, and was tentatively identified as 4a-hydroxy-strigol or 8a-hydroxy-strigol. Branching factors were also purified from root exudates of Sorghum, but the chemical structure was not determined because of the very tiny quantity of the active substance. Natural 5-deoxy-strigol induced extensive hyphal branching in germinating spores of the AM fungus Gigaspora maragarita in the range of 1 ng.- 30 pg per disc.Strigolactones are known to be seed-germination stimulants for the parasitic weed Striga and Orobanche. It has been suggested that strigolactones play a role in the natural ecological host-parasite relation because strigol was originally isolated from the false host cotton. Our finding that strigolactones induce the branching of germinating spore strongly suggested that strigolactones play more significant role in nature.