Towards Growth of Arbuscular Mycorrhizal Fungi Independent of a Plant Host

Abstract

When surface-sterilized spores of the arbuscular mycorrhizal fungus (AMF) Glomus intraradices Sy167 were germinated on agar plates in the slightly modified minimum mineral medium described by G. Bécard and J. A. Fortin (New Phytol. 108:211-218, 1988), slime-forming bacteria, identified as Paenibacillus validus, frequently grew up. These bacteria were able to support growth of the fungus on the agar plates. In the presence of P. validus, hyphae branched profusely and formed coiled structures. These were much more densely packed than the so-called arbuscule-like structures which are formed by AMF grown in coculture with carrot roots transformed with T-DNA from Agrobacterium rhizogenes. The presence of P. validus alone also enabled G. intraradices to form new spores, mainly at the densely packed hyphal coils. The new spores were not as abundant as and were smaller than those formed by AMF in the monoxenic culture with carrot root tissues, but they also contained lipid droplets and a large number of nuclei. In these experiments P. validus could not be replaced by bacteria such as Escherichia coli K-12 or Azospirillum brasilense Sp7. Although no conditions under which the daughter spores regerminate and colonize plants have been found yet, and no factor(s) from P. validus which stimulates fungal growth has been identified, the present findings might be a significant step forward toward growth of AMF independent of any plant host.The roots of more than 80% of land plants are colonized by arbuscular mycorrhizal fungi (AMF). Under diverse stress conditions, the hyphae of the fungi exploit water and minerals from soils better than the roots and effectively transfer them to the plant macrosymbiont. Thus, arbuscular mycorrhiza is likely the most important symbiosis in nature. Its study is hampered by the fact that AMF are obligate biotrophs (19,20). However, as originally described by B. Mosse and C. M. Hepper (25,26) and subsequently elaborated in much more detail by others, some AMF, such as Gigaspora spp. and Glomus intraradices, can fulfil their life cycles up to spore formation when grown in a dual system with root tissues from either carrots (5), tomatoes (3), or clover (13). The root factors which enable the AMF to grow in this monoxenic culture have not yet been fully elucidated. When spores are placed on agar containing medium only, they germinate and form so-called runner hyphae. After some weeks, however, this growth stops, the hyphae septate, and the cytoplasm is retracted. In the monoxenic culture with carrot roots, completion of the life cycle of AMF requires the synthesis of lipids which are deposited into the newly formed spores (4) and CO 2 fixations for anabolism (6). These reactions are apparently turned on by unidentified factors produced and excreted into the medium by the plant tissues before any contact of the hyphae with a root tissue cell.Numerous investigations have indicated that rhizosphere bacteria have strong impacts on the growth of AMF (1, 2, 18). The so-called plant growth-promoting rhizoba...

Bacterial-Fungal Interactions: Hyphens between Agricultural, Clinical, Environmental, and Food Microbiologists

“…Extracellular bacteria can affect fungal development and spore production, to the benefit or the detriment of the fungus. Bacteria stimulate spore germination in several fungi, including the plant-pathogenic oomycete Phytophthora alni (68), the saprophytic cheese-associated fungus Penicillium roqueforti (152), several bark beetle fungal symbionts (1), and the arbuscular mycorrhizal fungus Glomus intraradices Sy167 (161,162). Interestingly, antibiotic treatment to "cure" Rhizopus of the Burkholderia endobacteria living within its hyphae results in a fungus that no longer produces reproductive sporangia or spores (299).…”

Mycorrhizal responses to biochar in soil – concepts and mechanisms

“…flavonoids (AMF) and furans (ECM), that facilitate the growth of fungal hyphae and the subsequent colonization of plant roots by ECM (Founoune et al 2002;Duponnois and Plenchette 2003;Aspray et al 2006;Riedlinger et al 2006) and AM fungi (Duponnois and Plenchette 2003;Hildebrandt et al 2002Hildebrandt et al , 2006. Hildebrandt et al (2002Hildebrandt et al ( , 2006 have demonstrated that certain compounds (including raffinose and other unidentified metabolites) produced by strains of Paenibacillus can directly enhance the growth of AMF extraradical mycelium. Additionally, Kothamasi et al (2006) demonstrated that other species of bacteria, such as Pseudomonas aeruginosa, can solubilize important plant nutrients, especially phosphate, making them part of a group of bacteria called phosphate solubilizing bacteria (PSB).…”

Spore Associated Bacteria (SAB) of Arbuscular Mycorrhizal Fungi (AMF) and Plant Growth Promoting Rhizobacteria (PGPR) Increase Nutrient Uptake and Plant Growth Under Stress Conditions

“…Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs that forms symbiotic or mutualistic association with roots of about 80% of plant species (Rai, 2001;Bianciotto et al, 2000;Hildebrandt et al, 2002;Cekic et al, 2012). The interaction between AMF and host plants under different conditions has received greater attention because of their mutual association (Miransari, 2009) especially when they are subjected to stresses.…”