Abstract
Arbuscular mycorrhizal (AM) symbiosis is a mutually beneficial interaction between fungi and land plants and promotes global phosphate cycling in terrestrial ecosystems. AM fungi are recognised as obligate symbionts that require root colonisation to complete a life cycle involving the production of propagules, asexual spores. Recently, it has been shown that Rhizophagus irregularis can produce infection-competent secondary spores asymbiotically by adding a fatty acid, palmitoleic acid. Furthermore, asymbiotic growth can be supported using myristate as a carbon and energy source for their asymbiotic growth to increase fungal biomass. However, the spore production and the ability of these spores to colonise host roots were still limited compared to the co-culture of the fungus with plant roots. Here we show that a combination of two plant hormones, strigolactone and jasmonate, induces the production of a large number of infection-competent spores in asymbiotic cultures of Rhizophagus clarus HR1 in the presence of myristate and organic nitrogen. Inoculation of asymbiotically-generated spores promoted the growth of host plants, as observed for spores produced by symbiotic culture system. Our findings provide a foundation for the elucidation of hormonal control of the fungal life cycle and the development of inoculum production schemes.
Highlights
Arbuscular mycorrhizal (AM) symbiosis is a mutually beneficial interaction between fungi and land plants and promotes global phosphate cycling in terrestrial ecosystems
The effective concentration of myristate for asymbiotic culture of R. irregularis was determined in the previous research[15], we adopted a fatty acid concentration of 500 μM for supplying to the medium
In asymbiotic culture supplemented with potassium myristate, AM fungus expanded its habitat by generating straight, thick hyphae with small bunches of short branches, but sporulation rates, the number of secondary spore-forming parent spores per germinated parent spores, were very low 6 weeks after incubation (WAI) in the three trials (Fig. 1b)
Summary
Arbuscular mycorrhizal (AM) symbiosis is a mutually beneficial interaction between fungi and land plants and promotes global phosphate cycling in terrestrial ecosystems. We show that a combination of two plant hormones, strigolactone and jasmonate, induces the production of a large number of infection-competent spores in asymbiotic cultures of Rhizophagus clarus HR1 in the presence of myristate and organic nitrogen. Inoculation of asymbiotically-generated spores promoted the growth of host plants, as observed for spores produced by symbiotic culture system. Co-culture of the AM fungus Rhizophagus irregularis and mycorrhiza-helper bacterium Paenibacillus validus demonstrated that AM fungi can complete their life cycle in the absence of host plants[12,13]. Myristate initiated the asymbiotic growth of AM fungi and can serve as a carbon and energy source[15] These findings would lead to the development of new research tools for AM studies and novel production systems of AM fungal inoculants. It has been shown that AM fungi directly take up recalcitrant and labile forms of organic nitrogen[19]
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