Respective and combined roles of rhizobia and ectomycorrhizal fungi in the plant growth, symbiotic efficiency, nutrients and metals contents of a leguminous tree species growing on ultramafic soils

Acacia spirorbis subsp. spirorbis Labill. is a widespread tree legume in New Caledonia that establishes mutual symbioses with both N 2 -fixing rhizobia and ectomycorrhizal (ECM) fungi. Moreover, A. spirorbis is able to naturally grow on ultramafic soils, characterized by a lack of nutrients but containing toxic concentrations of metals. In this study, we investigated the relationships between the symbiotic efficiency, the plant development, nutrients and metals contents in ultramafic soils. Seedlings of A. spirorbis were grown in the greenhouse on an ultramafic substrate, and inoculated with three strains of Bradyrhizobium sp. and/or with three isolates of the ECM species Pisolithus albus , used alone or in combination. After 12 months of growth, we measured plant biomass, nodulation, mycorrhization, shoot nutrient and metal contents. Our results showed that a single inoculation with P. albus and dual inoculation with Bradyrhizobium sp. significantly enhanced plant development under ultramafic conditions, with better plant growth, efficient absorption of major nutrients (N, P, K and Ca) and alleviation of Cr and Mn uptake. Symbiotic efficiency was shown to depend on specific combinations of rhizobia and ECM. Interactions between symbionts were detected, with a significant increase in root nodulation in the presence of ECM fungi. We conclude that the rhizobial and fungal symbioses associated with A. spirorbis play a key role in A. spirorbis adaptation to ultramafic conditions by boosting plant development and reducing metal uptake.