Efficiency of acid phosphatases secreted from the ectomycorrhizal fungus Hebeloma cylindrosporum  to hydrolyse organic phosphorus in podzols

Abstract

Ectomycorrhizal fungi may improve the phosphate nutrition of their host plants by secreting, into the soil solution, acid phosphatases (AcPases) able to release orthophosphate (Pi) from soil organic phosphorus (Po). Using cation-exchange chromatography, we separated four fractions with AcPase activity secreted by the ectomycorrhizal fungus Hebeloma cylindrosporum grown in a pure culture under P-starved conditions. Each AcPase active fraction displayed strong ability in vitro to hydrolyse a wide range of phosphate monoesters, but none of them efficiently hydrolysed phytate. Their efficiency to release Pi from soil NaHCO(3)-extractable Po was studied in a sandy podzol used intact or autoclaved. Soils were collected in a 15-year-old Pinus pinaster stand, receiving regular fertilization or not. Autoclaving increased the NaHCO(3)-extractable Po concentrations by 55% in unfertilized and by 32-43% in fertilized soils. The efficiency of each AcPase fraction was affected significantly by the soil fertilization regime and the soil treatment (intact vs. autoclaved). The proportion of labile Po enzyme ranged from 0% to 11% and 14% to 48% after 1 h of incubation in bicarbonate solutions extracted from intact and autoclaved soils, respectively. This work suggests that AcPases secreted from H. cylindrosporum could be efficient in recycling Po pools from soil microorganisms that may be delivered by soil autoclaving.

Diversity in phosphorus mobilisation and uptake in ectomycorrhizal fungi

“…The ability of Hebeloma species to increase the proportion of extracellular pNPPase compared to the surface-bound activity in the case of Pi depletion was also reported by Tibbett et al (1998). The high ability to release acid phosphatases into the external medium was recently confirmed in another isolate of H. cylindrosporum by Louche et al (2010). In this study, four fractions containing ACPase activity were separated from the culture medium, suggesting that this fungal species could be able to produce several isoforms of acid phosphatase.…”

“…Only four isolates had free extracellular phytase activity (one strain of S. collinitus, S. granulatus, and both strains of H. cylindrosporum; Quiquampoix and Mousain 2005). However, the rate of Pi release was low compared to that measured for para-nitrophenol release, with values ranging from 1% to 8% in the four fractions separated from the culture medium of H. cylindrosporum (Louche et al 2010). …”

“…This is particularly important in the case of inositol phosphate, a polyanion that can be easily combined with mineral surfaces (iron and aluminium oxides, calcium) and thus made unavailable to phytases (Giaveno et al 2010). However, the ability to hydrolyse a large spectrum of phosphomonoesters, as shown by Louche et al (2010), may be an advantage in soil conditions. The phosphatases could also be involved in recapturing of excreted plant or fungal compounds (BarrettLennard et al 1993) or breaking down and recycling the phospholipids from old hyphae (Nygren and Rosling 2009), avoiding therefore the loss of P into the environment.…”

Ecological importance of soil bacterivores for ecosystem functions

“…By prospecting a large volume of soil away from plant roots, and by releasing carboxylates (Finlay 2008) and enzymes (Courty et al 2006;Louche et al 2010 enhance plant N and P acquisition (Plassard and Dell 2010;Tibbett and Sanders 2002;Wallenda and Read 1999). The presence of mycorrhizal fungi can alter the positive effects of bacterivores on plant performance through two main pathways.…”

Plant and microbe genomics and beyond: potential for developing a novel molecular plant nutrition approach

“…Ectomycorrhizal fungi improve nutrient status of plants, as they are able to release organic anions or phosphatases to mobilize unavailable P (Louche et al 2010;Plassard and Dell 2010). Furthermore, bacteria play a major role in the mineralization of P through trophic relationships as they can produce phytases to degrade phytate, the main form of soil organic P. Bacteria are also more effective than other microorganisms or plants at immobilizing free Pi.…”