P metabolism and transport in AM fungi

The arbuscular mycorrhizal symbiosis is mutualistic, based on reciprocal transfer of P from the fungus to the plant and carbon from the plant to the fungus. Thus P is a most important 'currency' in the symbiosis. After absorbing P from the soil solution, the fungi first incorporate it into the cytosolic pool, and the excess P is transferred to the vacuoles. The vacuolar P pool probably plays a central role in P supply to the plant. The main forms of inorganic P in fungal vacuoles are orthophosphate and polyphosphate, but organic P molecules may also be present. Long distance translocation of P from the site of uptake in the external mycelium to the site of transfer to the plant is probably achieved via transfer of vacuolar components. This transport would be mediated either by protoplasmic streaming or the motile tubular vacuole-like system. The site of release of P into the interfacial apoplast and thence to the plant is most probably the fungal arbuscules. The biochemical and biophysical processes involved in P metabolism and transfer between cellular compartments in the symbiosis are currently not well understood. Some recent investigations of substrate specificities of phosphatase-type enzymes in AM fungi and other eukaryotic microorganisms, however, have shed new light on earlier results and permit the construction of a hypothetical scheme of P-flow, including possible regulatory factors. Steps in this scheme are experimentally testable and should stimulate future researc

Characterization of arbuscular mycorrhizal fungal communities with respect to zonal vegetation in a coastal dune ecosystem

Coastal dune vegetation distributes zonally along the environmental gradients of, e.g., soil disturbance. In the preset study, arbuscular mycorrhizal fungal communities in a coastal dune ecosystem were characterized with respect to tolerance to soil disturbance. Two grass species, Elymus mollis and Miscanthus sinensis, are distributed zonally in the seaward and landward slopes, respectively, in the primary dunes in Ishikari, Japan. The seaward slope is severely disturbed by wind, while the landward slope is stabilized by the thick root system of M. sinensis. The roots and rhizosphere soils of the two grasses were collected from the slopes. The soils were sieved to destruct the fungal hyphal networks, and soil trap culture was conducted to assess tolerance of the communities to disturbance, with parallel analysis of the field communities using a molecular ecological tool. In the landward communities, large shifts in the composition and increases in diversity were observed in the trap culture compared with the field, but in the seaward communities, the impact of trap culture was minimal. The landward field community was significantly nested within the landward trap culture community, implying that most members in the field community did not disappear in the trap culture. No nestedness was observed in the seaward communities. These observations suggest that disturbance-tolerant fungi have been preferentially selected in the seaward slope due to severe disturbance in the habitat. Whereas a limited number of fungi, which are not necessarily disturbance-sensitive, dominate in the stable landward slope, but high-potential diversity has been maintained in the habitat

Plant symbiotic microorganisms in acid sulfate soil: significance in the growth of pioneer plants

Acid sulfate soil is generated by chemical and microbial oxidization of sulfide-rich minerals/sediments. Although revegetation of the soil is difficult due to low-pH and poor nutrient availability, pioneer plants may adapt to such an extreme environment via associating with mycorrhizal fungi and/or N-fixing bacteria for acquisition of mineral nutrients. In this study, an abandoned quarry in which acid sulfate soil was found was chosen to investigate the influence of soil acidity on the levels of colonization by the microsymbionts, the identities of the microsymbionts that associated with pioneer plants and the dependency of pioneer plants on the microsymbionts. The levels of arbuscular mycorrhizal (AM) colonization in pioneer grass, forbs and legume shrubs grown in the field were assessed, and no significant decline in the levels with an increase in soil acidity was observed. Most of the legume shrubs formed root nodules. Several AM fungi and bradyrhizobia were cultured from the rhizosphere soils of pioneer plants grown in the quarry and identified based on the sequences of the small subunit ribosomal RNA genes. Pot experiments revealed that the microsymbionts isolated from the field significantly promoted the growths of pioneer grasses and legume shrubs in acid sulfate soil at pH 3.4. These results suggest that plant-microbial symbiotic associations play significant roles in the growth of pioneer plants in acid sulfate soil

Release of acid phosphatase from extraradical hyphae of arbuscular mycorrhizal fungusRhizophagus clarus

Arbuscular mycorrhizal (AM) fungi enhance plant uptake of available phosphorus (P) from soil through their extraradical hyphae. The mechanism underlying this P uptake enhanced by AM fungi is the increase in the surface area for absorption of available P. Little is known about utilization of unavailable P by AM fungi. We investigated whether extraradical hyphae of AM fungi release acid phosphatase (ACP). Sterilized Andosol was packed in pots that were separated into the mycorrhizal and hyphal compartments with a nylon net of 30-μm pore size. Seeds of Allium fistulosum L. were inoculated or uninoculated with the AM fungus Rhizophagus clarus (Nicolson & Schenck) Walker & Schüßler. Mullite ceramic tubes were buried in the soil of each compartment, and soil solution was collected. A. fistulosum L. and Linum usitatissimum L. inoculated with R. clarus were grown in sand culture and in vitro monoxenic culture, respectively. Uninoculated A. fistulosum L was grown in hydroponic culture to collect root exudate. The soil solution, hyphal extracts, root extract and root exudates were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. Shoot P concentration, shoot P content and shoot dry weight were higher in the inoculated treatment than in the uninoculated treatment. Activity staining of the gel revealed that ACP activity at 187 kDa was observed in the soil solution in the inoculation treatment, and in the hyphal extract collected from sand culture and in vitro monoxenic culture, but neither in the root exudate of non-mycorrhizal plant grown in the hydroponic culture nor in the root extracts irrespective of mycorrhizal status. Those results provide strong evidence that the corresponding activity in the soil solutions in soil culture is of R. clarus CK001 origin. These findings suggest that the fungus releases ACP from extraradical hyphae into the hyphosphere

Ninety-year-, but not single, application of phosphorus fertilizer has a major impact on arbuscular mycorrhizal fungal communities

Background and aims Arbuscular mycorrhizal (AM) fungi play a significant role in P nutrition of crops in agriculture, but P accumulation in the soil, e.g., application of P-fertilizer, generally reduces AM fungal colonization. The impact of long-term application of chemical fertilizer on AM fungal communities was investigated with respect to the time scale. Methods Soils were collected from four plots with different fertilizer management in the long-term experimental field established in 1914. Lotus japonicus was grown in the soils in a greenhouse, while Glycine max was grown in the plots in the field. DNA was extracted from their roots, and the diversity and community compositions were analyzed based on occurrence of the AM fungal phylotypes defined by sequence similarity in the LSU rDNA. Results The 90-year-application of N and K in the absence of P increased AM fungal diversity and resulted in formation of a distinctive fungal community compared with those in the other treatments. This effect was not cancelled by single application of P. Whereas the impact of balanced application of N, P, and K was ambiguous. Conclusion These observations suggest that the presence/absence of P-fertilizer has a major impact on AM fungal communities, but the action may appear only on a long time scale. Electronic supplementary material The online version of this article (doi:10.1007/s11104-012-1398-x) contains supplementary material, which is available to authorized users