Chemical signals controlling the arbuscular mycorrhiza symbiosis

丛枝菌根（AM）真菌与超过80%的陆地植物的根形成共生关系。在没有寄主根的情况下，真菌无法完成它们的生命周期。AM真菌和寄主植物之间的信号传导和识别的分子机制知之甚少。在宿主识别的最初阶段之一，AM真菌的菌丝在宿主根附近显示出广泛的分支，然后形成附着胞，这是用于穿透植物根的结构。已知寄主根释放触发菌丝分支的信号分子，但这些分支因子尚未被分离出来。我们已经开发了纸盘法作为分支因子的生物测定系统。从日本莲根分泌物中分离出一个分支因子，经光谱分析和化学合成鉴定为独脚金内酯，5-脱氧独脚金内酯。胡萝卜根分泌物的分枝因子可能是独脚金内酯的一个成员，初步鉴定为4 -羟基或8 -羟基独脚金内酯。从高粱根系分泌物中也纯化出了分支因子，但由于活性物质含量极低，无法确定其化学结构。天然5-脱氧首曲醇在1 ng范围内诱导AM真菌Gigaspora maragarita萌发孢子广泛的菌丝分支。-每盘30 pg。独角麦内酯是已知的种子萌发刺激剂为寄生杂草Striga和orobche。有人认为独角兽内酯在自然生态的寄主-寄生虫关系中起作用，因为独角兽内酯最初是从假寄主棉花中分离出来的。我们的发现表明，单角子内酯能诱导萌发孢子的分枝，这表明单角子内酯在自然界中起着更重要的作用。

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.