Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi

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Abstract

Arbuscular mycorrhizal (AM) fungi form mutualistic, symbiotic associations with the roots of more than 80% of land plants1. 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 appressorium2,3,4, the structure used to penetrate the plant root. Host roots are known to release signalling molecules that trigger hyphal branching5,6,7, 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 Orobanche8. 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.

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Figure 1: Hyphal branching of G. margarita induced by lipophilic fractions from root exudates of L. japonicus using the paper disk diffusion method.
Figure 2: Purification of BF by HPLC on a semi-preparative C 18 reverse-phase column.
Figure 3: Spectroscopic analysis of BF isolated from L. japonicus.
Figure 4: Hyphal branching activity of BF and natural strigolactones on G. margarita.

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Acknowledgements

We thank M. Kawaguchi for discussions and critical reading of the manuscript, Y. Sugimoto for providing (± )-sorgolactone, GR24 and (+ )-strigol, and K. Yoneyama for discussions and critical reading of the manuscript. This work was supported by Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, and a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan.

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Correspondence to Kohki Akiyama.

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Supplementary Table S1

Table of physicochemical data for synthetic (±)-5-deoxy-strigol. (DOC 20 kb)

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Akiyama, K., Matsuzaki, K. & Hayashi, H. Plant sesquiterpenes induce hyphal branching in arbuscular mycorrhizal fungi. Nature 435, 824–827 (2005) doi:10.1038/nature03608

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