Changes during early ectomycorrhiza formation by Pisolithus tinctorius on Acacia mangium, and their impact on nodule formation by Bradyrhizobium sp.

Abstract

Changes in the root morphology and rhizosphere pH during ectomycorrhiza formation have been reported for various plants. These changes are mediated by root-growth-regulating compounds and organic acids secreted into the rhizosphere by the ectomycorrhizal fungus. Plate culture experiments were conducted to study the effects of early stages of ectomycorrhiza formation by Pisolithus tinctorius on the root morphology, rhizosphere pH, and nodulation of Acacia mangium seedlings. Also, experiments were conducted to find out the effects of putative compounds produced by P. tinctorius on nodule formation by Bradyrhizobium in A. mangium. In the presence of P. tinctorius, the root morphology of A. mangium seedlings was significantly altered (length of the tap root decreased, number of lateral roots cm -1 increased, and length of the lateral roots decreased), the pH of the rhizosphere region was reduced, and nodule formation by Bradyrhizobium was inhibited. The role of indole-3-acetic acid (IAA) produced by P. tinctorius in root morphogenesis was confirmed by using different concentrations of IAA and auxin transport inhibitor, 2,3,5-triiodobenzoic acid (TIBA). However, there is no evidence for the involvement of fungal IAA in nodule inhibition. Concentrations of IAA that induced changes in root morphology also induced nodule formation by Bradyrhizobium. Aminoethoxyvinylglycine (AVG), inhibitor of ethylene biosynthesis, did not reverse the effect of P. tinctorius on nodule formation indicating that inhibition of nodules was not due to ethylene-mediated mechanism. Low rhizosphere pH, probably due to production of organic acids, can be one of the factors responsible for nodule inhibition during ectomycorrhiza formation. ©2005 Balaban.