Arbuscular mycorrhizal (AM) fungi are obligate symbionts that depend on living host plants to complete their life cycle1,2. This feature, which leads to their unculturability in the absence of plants, strongly hinders basic research and agricultural application of AM fungi. However, at least one AM fungus can grow and develop fertile spores independently of a host plant in co-culture with the bacterium Paenibacillus validus3. The bacteria-derived substances are thought to act as stimulants or nutrients for fungal sporulation, but these molecules have not been identified. Here, we show that (S)-12-methyltetradecanoic acid4,5, a methyl branched-chain fatty acid isolated from bacterial cultures, stimulates the branching of hyphae germinated from mother spores and the formation of secondary spores in axenic culture of the AM fungus Rhizophagus irregularis. Extensive testing of fatty acids revealed that palmitoleic acid induces more secondary spores than the bacterial fatty acid in R. irregularis. These induced spores have the ability to infect host plant roots and to generate daughter spores. Our work shows that, in addition to a major source of organic carbon6,7,8,9, fatty acids act as stimulants to induce infection-competent secondary spores in the asymbiotic stage and could provide the key to developing the axenic production of AM inoculum.
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We thank I. Nishida, H. Imai, T. Maeda, K. Ebine and M. Yamato for their critical suggestions, and Y. Ogawa, R. Oguchi, R. Okita, M. Ohashi, Y. Sugiura, S. Tanaka, A. Tokairin and Y. Yoshinori for their experimental support. We appreciate K. Yamaguchi, S. Shigenobu, Functional Genomics Facility and Data Integration and Analysis Facility and the NIBB Core Research Facilities for technical support. This work was supported by ACCEL (JPMJAC1403) from the Japan Science and Technology Agency; Grant-in-Aid for Scientific Research on Innovative Areas (22128006); Grant-in-Aid for Scientific Research (A) (15H01751) from the Japan Society for the Promotion of Science; and NIBB Collaborative Research Program (18-341).
The authors declare no competing interests.
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Supplementary Figs. 1–8 and Supplementary Tables 1–5.
Differentially expressed genes in response to palmitoleic acid (C16:1Δ9Z) and/or between hyphae and spores.
Genes co-regulated both by palmitoleic acid and intraradical mycelium.
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Kameoka, H., Tsutsui, I., Saito, K. et al. Stimulation of asymbiotic sporulation in arbuscular mycorrhizal fungi by fatty acids. Nat Microbiol 4, 1654–1660 (2019). https://doi.org/10.1038/s41564-019-0485-7
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