Ectomycorrhizal Association Enhances Al Tolerance by Inducing Citrate Secretion in Pinus densiflora

Abstract

To investigate the effects of ectomycorrhizal association on the aluminum (Al) tolerance of Pinus densiflora Sieb. & Zucc., seedlings with or without ectomycorrhizal association with the fungus Pisolithus tinctorius (Pers.) Coker & Couch were exposed to 1 mM Al. Association with P. tinctorius alleviated Al-induced inhibition of root elongation and biomass growth in the mycorrhizal seedlings. Secretion of malate and citrate, both low-molecular-weight organic acids that could detoxify Al by the formation of stable complexes, was investigated in P. tinctorius mycelia and in pine roots with and without P. tinctorius association. Citrate secretion from the P. tinctorius mycelia in vitro was stimulated by Al. Citrate secretion from the roots of the ectomycorrhizal seedlings was also stimulated by Al, but was not detected in the non-mycorrhizal seedlings. These results suggest that citrate secreted from the roots of the ectomycorrhizal seedlings was produced in the hyphae of P. tinctorius. Citrate secretion may play a role in enhancing the Al tolerance of host seedlings.