Extraradical mycelium network of arbuscular mycorrhizal fungi allows fast colonization of seedlings under in vitro conditions.

Actively growing extraradical hyphae extending from mycorrhizal plants are an important source of inoculum in soils which has seldom been considered in vitro to inoculate young plantlets. Seedlings of Medicago truncatula were grown in vitro in the extraradical mycelium network extending from mycorrhizal plants. After 3, 6, 9, 12, and 15 days of contact with the mycelium, half of the seedlings were harvested and analyzed for root colonization. The other half was carefully transplanted in vitro on a suitable growth medium and mycelium growth and spore production were evaluated for 4 weeks. Seedlings were readily colonized after 3 days of contact with the mycelium. Starting from 6 days of contact, the newly colonized seedlings were able to reproduce the fungal life cycle, with the production of thousands of spores within 4 weeks. The fast mycorrhization process developed here opens the door to a broad range of in vitro studies for which either homogenous highly colonized seedlings or mass-produced in vitro inoculum is necessary.

Absence of carbon transfer between Medicago truncatula plants linked by a mycorrhizal network, demonstrated in an experimental microcosm.

Carbon transfer between plants via a common extraradical network of arbuscular mycorrhizal (AM) fungal hyphae has been investigated abundantly, but the results remain equivocal. We studied the transfer of carbon through this fungal network, from a Medicago truncatula donor plant to a receiver (1) M. truncatula plant growing under decreased light conditions and (2) M. truncatula seedling. Autotrophic plants were grown in bicompartmented Petri plates, with their root systems physically separated, but linked by the extraradical network of Glomus intraradices. A control Myc-/Nod- M. truncatula plant was inserted in the same compartment as the receiver plant. Following labeling of the donor plant with 13CO2, 13C was recovered in the donor plant shoots and roots, in the extraradical mycelium and in the receiver plant roots. Fatty acid analysis of the receiver's roots further demonstrated 13C enrichment in the fungal-specific lipids, while almost no label was detected in the plant-specific compounds. We conclude that carbon was transferred from the donor to the receiver plant via the AM fungal network, but that the transferred carbon remained within the intraradical AM fungal structures of the receiver's root and was not transferred to the receiver's plant tissues.

Transport of radiocaesium by arbuscular mycorrhizal fungi to Medicago truncatula under in vitro conditions.

The capacity of arbuscular mycorrhizal (AM) fungi to take up and translocate radiocaesium (Cs) to their host has been shown using the root-organ culture (ROC) system. However, the absence of photosynthetic tissues, lack of a normal root hormonal balance and incomplete source-sink relationships may bias the bidirectional transfer of elements at the symbiotic interface and complicate transport studies. Accordingly, we developed a novel culture system [i.e. the Arbuscular Mycorrhizal-Plant (AM-P) in vitro culture system], where AM fungi and an autotrophic host plant develop under strict in vitro conditions. With this system, we unambiguously demonstrated the capacity of AM fungi to transport Cs. The extraradical fungal hyphae took up 21.0% of the initial supply of 134Cs. Translocation to the plant represented 83.6% of the 134Cs taken up. Distribution of 134Cs in the host plant was 89.8% in the mycorrhizal roots and 10.2% in the shoot. These results confirm that AM fungi can take up, translocate and accumulate Cs. They further demonstrate unambiguously and for the first time that Cs can be transferred from AM fungi to host tissues. These results suggest a potential involvement of AM fungi in Cs biogeochemical cycle and in plant Cs accumulation.

Development of an autotrophic culture system for the in vitro mycorrhization of potato plantlets.

An autotrophic culture system was developed for the in vitro mycorrhization of potato plantlets. Roots of micropropagated plantlets were associated to an arbuscular mycorrhizal fungus under in vitro conditions, while shoots developed under open air conditions. Several thousand spores, an extensive extraradical mycelium and an abundant root colonization were obtained. Spores were able to colonize new plantlets under the same conditions. These results support the capacity of the autotrophic culture system to continuously culture arbuscular mycorrhizal fungi and may serve as a powerful tool to investigate various aspects of the symbiosis for which a source-sink relationship and photosynthetic active tissues are necessary.