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The regulation of the structural composition and complexity of the mycelium of arbuscular mycorrhizal (AM) fungi is not well understood due to their obligate biotrophic nature. The aim of this study was to investigate the structure of extraradical mycelium at high and low availability of carbon (C) to the roots and phosphorus (P) to the fungus. We used monoxenic cultures of the AM fungus Rhizophagus irregularis (formerly Glomus intraradices) with transformed carrot roots as the host in a cultivation system including a root-free compartment into which the extraradical mycelium could grow. We found that high C availability increased hyphal length and spore production and anastomosis formation within individual mycelia. High P availability increased the formation of branched absorbing structures and reduced spore production and the overall length of runner hyphae. The complexity of the mycelium, as indicated by its fractal dimensions, increased with both high C and P availability. The results indicate that low P availability induces a growth pattern that reflects foraging for both P and C. Low C availability to AM roots could still support the explorative development of the mycelium when P availability was low. These findings help us to better understand the development of AM fungi in ecosystems with high P input and/or when plants are subjected to shading, grazing or any management practice that reduces the photosynthetic ability of the plant.

Arbuscular mycorrhizal fungi (AMF) have a key role in plant phosphate (Pi) uptake by their efficient capture of soil phosphorus (P) that is transferred to the plant via Pi transporters in the root cortical cells. The activity of this mycorrhizal Pi uptake pathway is often associated with downregulation of Pi transporter genes in the direct Pi uptake pathway. As the total Pi taken up by the plant is determined by the combined activity of mycorrhizal and direct pathways, it is important to understand the interplay between these, in particular the actual activity of the pathways. To study this interplay we modulated the delivery of Pi via the mycorrhizal pathway in Pisum sativum by two means: (1) Partial downregulation by virus-induced gene silencing of PsPT4, a putative Pi transporter gene in the mycorrhizal pathway. This resulted in decreased fungal development in roots and soil and led to reduced plant Pi uptake. (2) Changing the percentage of AMF-colonized root length by using non-, half-mycorrhizal or full-mycorrhizal split-root systems. The combination of split roots, use of ³²P and ³³P isotopes and partial silencing of PsPT4 enabled us to show that the expression of PsPT1, a putative Pi transporter gene in the direct pathway, was negatively correlated with increasing mycorrhizal uptake capacity of the plant, both locally and systemically. However, transcript changes in PsPT1 were not translated into corresponding, systemic changes in actual direct Pi uptake. Our results suggest that AMF have a limited long-distance impact on the direct pathway., (© 2013 Scandinavian Plant Physiology Society.)

The exchange of carbohydrates and mineral nutrients in the arbuscular mycorrhizal (AM) symbiosis must be controlled by both partners in order to sustain an evolutionarily stable mutualism. Plants downregulate their carbon (C) flow to the fungus when nutrient levels are sufficient, while the mechanism controlling fungal nutrient transfer is unknown. Here, we show that the fungus accumulates nutrients when connected to a host that is of less benefit to the fungus, indicating a potential of the fungus to control the transfer of nutrients. We used a monoxenic in vitro model of root organ cultures associated with Glomus intraradices, in which we manipulated the C availability to the plant. We found that G. intraradices accumulated up to seven times more nutrients in its spores, and up to nine times more in its hyphae, when the C pool available to the associated roots was halved. The strongest effect was found for phosphorus (P), considered to be the most important nutrient in the AM symbiosis. Other elements such as potassium and chorine were also accumulated, but to a lesser extent, while no accumulation of iron or manganese was found. Our results suggest a functional linkage between C and P exchange., (© 2011 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.)

We investigated element accumulation in the arbuscular mycorrhizal fungus Glomus intraradices. Fungal spores and mycelia growing in monoxenic cultures were analyzed. The elemental composition was quantified using particle-induced X-ray emission (PIXE) in combination with scanning transmission ion microscopy. In the spores, Ca and Fe were associated mainly with the spore wall, while P and K showed patchy distributions and their concentrations were correlated. Excess of P in the hyphal growth medium increased the P and Si concentrations in spores and increased the K/Ca ratio in spores. Increased P availability decreased the concentration of Zn and Mn in spores. We concluded that the availability of P influences the uptake and accumulation of several elements in spores. It is demonstrated that PIXE analysis is a powerful tool for quantitative analysis of elemental accumulation in fungal mycelia.