Akiyama Kohki, Matsuzaki Ken-ichi, Hayashi Hideo
Division of Applied Biological Chemistry, Graduate School of Agriculture and Biological Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.
Nature. 2005 Jun 9;435(7043):824-7. doi: 10.1038/nature03608.
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. Their spores can germinate and grow in the absence of a host, but their hyphal growth is very limited. 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. Here we have isolated a branching factor from the root exudates of Lotus japonicus and used spectroscopic analysis and chemical synthesis to identify it as a strigolactone, 5-deoxy-strigol. Strigolactones are a group of sesquiterpene lactones, previously isolated as seed-germination stimulants for the parasitic weeds Striga and Orobanche. The natural strigolactones 5-deoxy-strigol, sorgolactone and strigol, and a synthetic analogue, GR24, induced extensive hyphal branching in germinating spores of the AM fungus Gigaspora margarita at very low concentrations.
丛枝菌根（AM）真菌与80%以上的陆地植物根系形成互利共生关系。在没有宿主根系的情况下，这些真菌无法完成其生命周期。它们的孢子在没有宿主的情况下可以萌发和生长，但其菌丝生长非常有限。关于AM真菌与其宿主植物之间信号传导和识别的分子机制，人们了解甚少。在宿主识别的最初阶段之一，AM真菌的菌丝在附着胞形成之前，即在宿主根系附近表现出广泛的分支，附着胞是用于穿透植物根系的结构。已知宿主根系会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。在这里，我们从百脉根的根系分泌物中分离出一种分支因子，并通过光谱分析和化学合成将其鉴定为独脚金内酯5-脱氧独脚金醇。独脚金内酯是一类倍半萜内酯，之前作为寄生杂草独脚金和列当属种子萌发的刺激物被分离出来。天然独脚金内酯5-脱氧独脚金醇、独脚金内酯和独脚金素，以及一种合成类似物GR24，在极低浓度下就能诱导AM真菌珠状巨孢囊霉萌发的孢子产生广泛的菌丝分支。
