Histochemical visualization of phosphatase released by arbuscular mycorrhizal fungi in soil

A histochemical procedure was developed to visualize the excretion of phosphatase by arbuscular mycorrhizal (AM) fungus in soil. Red clover (Trifolium pratense L.) plants were grown in three-compartment pots under controlled conditions. Each pot was separated into three compartments by 30-mum nylon mesh, the central one for plant growth and two outer ones for growth of arbuscular mycorrhizal hyphae. Plants in the central compartments were either uninoculated or inoculated with AM fungus (Glomus mosseae). Plants were harvested after 9 weeks. The activities of acid and alkaline phosphatases and organic P depletion were measured at different distances from the central compartment. The acid phosphatase released by AM hyphae in soil was also assessed in situ using transmission electron microscopy. The results showed that AM hyphae increased the activities of acid and alkaline phosphatases and decreased the concentration of soil organic phosphorus in the hyphal compartments. Acid phosphatase in soil was observed around the AM hyphae by an in situ histochemical method. To our knowledge, this is the first direct evidence that AM fungus excretes phosphatases to soil.