Myristate can be used as a carbon and energy source for the asymbiotic growth of arbuscular mycorrhizal fungi

Abstract

19Arbuscular mycorrhiza (AM) is one of the most widespread mutualistic symbioses, which is 20 formed between the majority of land plants and soil-borne fungi belonging to Glomeromycotina. 21 AM fungi are obligate symbionts that cannot complete their natural life cycle without a host. 22 Recent evidence suggests that lipids synthesized by a host are transferred to AM fungi that 23 possess no fatty acid synthase genes in their genome and that mutations in lipid biosynthesis-24 related genes of the host lead the symbiotic interaction to fail (1-3). We hypothesized that lipids 25 derived from plants are crucial for AM fungal growth and reproduction. In this study, we 26 evaluated whether AM fungi can grow on medium supplied with fatty acids under asymbiotic 27 conditions without the host. Myristate led to an extensive hyphal growth of Rhizophagus 28 irregularis and an increase in biomass production. Other examined fatty acids showed no effect 29 on biomass production. Myristate also induced secondary spore formation. The myristate-30 induced spores can germinate, colonize carrot hairy roots, and form the next generation of 31 mature daughter spores. A fluorescently labeled fatty acid probe was taken up by branched 32 hyphae of AM fungi. Tracer experiments using 13 C-labeled myristic acid showed that myristate 33 and its metabolites were utilized for the synthesis of triacylglycerol and cell wall components 34 of AM fungi. Furthermore, myristate activated ATP generation in the fungal hyphae. Here we 35 demonstrate that myristate is utilized as a carbon and energy source for biomass production and 36 3 sporulation under asymbiotic conditions. 37 38 Introduction 39 Arbuscular mycorrhizal (AM) fungi belonging to the subphylum Glomeromycotina (4) form 40 symbiotic associations with over 70% of land plant species (5). AM fungi provide hosts with 41 minerals taken up via hyphal networks in soil and in return receive carbon sources, such as 42 sugars and lipids derived from plant photosynthates. This is regarded as an obligate symbiotic 43 relationship, where the life cycle of AM fungi is completed through colonization with their host. 44Only a few reports have been published on AM fungal culture without hosts. One report showed 45 that the co-cultivation of the AM fungus Glomus intraradices with strains of Paenibacillus 46 validus separated from each other with dialysis membranes induced secondary spore formation 47 of the AM fungus (6). Another study reported that some fatty acids, including palmitoleic acid 48 and (S)-12-methyltetradecanoic acid, induced fertile secondary spores in asymbiotic cultures of 49 AM fungi (7). These results suggest that AM fungi may be cultured independently from host 50 plants under artificial conditions. In nature, the life cycle of AM fungi proceeds as follows. 51Resting spores of AM fungi germinate, and then germ tubes emerge from the spores and 52 elongate into the soil. After colonization into plant roots, AM fungi form a highly branched 53 hyphal structure, called an arbuscule, in ...

Responses of Arbuscular Mycorrhizal Symbiosis to Abiotic Stress: A Lipid-Centric Perspective

“…Since some interesting work has recently highlighted pure cultures of AM fungi, especially those concerning the uptake of environmental lipids by fungal hyphae (Sugiura et al, 2019) and the promotive effects of ABA on sporulation (Liu et al, 2019), now is the right time to devote increased effort to strengthen these efforts. For instance, it is essential to clarify whether the enzymes hydrolyzing PLs in senescent arbuscules originate from plants or AM fungi.…”

“…Therefore, the transfer of lipids from plant cells to arbuscules is indispensable for AM symbiosis (Jiang et al, 2017). Recently, axenic culture of R. irregularis indicated that EH can take up lipids directly from the media and that myristate is the most effective fatty acid to promote hyphal growth, while (S)-12-methyltetradecanoic acid promotes both hyphal growth and sporulation (Kameoka et al, 2019;Sugiura et al, 2019).…”

Palmitvaccenic acid (Δ11-cis-hexadecenoic acid) is synthesized by an OLE1-like desaturase in the arbuscular mycorrhiza fungusRhizophagus irregularis

“…Recently, methyl-myristic acid (methyl-14:0) was shown to induce spore formation of R. irregularis [9]. Furthermore, myristic acid (14:0) supplementation to the medium was sufficient to support hyphal growth in axenic culture, 14:0 was taken up by the fungus and metabolized [10]. Taken together, these results suggest that a 14:0 or 16:0-containing lipid most likely is transferred from the host to the fungus where it serves as carbon source in addition to carbohydrates.…”

Myristate and the ecology of AM fungi: significance, opportunities, applications and challenges

“…These results mean that AM fungi require an external supply of lipids. Recently, Sugiura et al (2019), in a study published as a preprint on bioRxiv, provided evidence that AM fungi can complete their life cycle in the absence of a host when supplied with certain fatty acids, myristates (C:14, a common organic acid in plant root exudates; Li et al, 2017), in a variety of formulations (also see an earlier study on fatty acid effects on AM fungal growth by the same group; Kameoka et al, 2019). The significance of this finding for understanding the biology of these fungi and for their independent culture are immediately apparent.…”

“…Before addressing these questions, it needs to be acknowledged that the study by Sugiura et al (2019) was a pioneering study. As such, it needs to still be independently verified as it was carried out with only one isolate, Rhizophagus irregularis, under controlled laboratory conditions.…”