Bacterial capacities to mineralize phytate increase in the rhizosphere of nodulated common bean (Phaseolus vulgaris) under P deficiency

Abstract

Phytate (a form of Inositol phosphate), which is one of the most abundant sources of organic phosphorus (Po) in soils, needs to be mineralized by microbial phosphatases to become available for plants. Phytases are the most active phosphatases for the dephosphorylation of phytate but the ecology of the phytate mineralising bacteria (PMB) remains poorly understood. The aim of this work was to determine if PMB represent an important component of the rhizosphere of legumes and if their density and their activities depend upon the soil-P availability. In this work the density, activity and phylogenetic affiliation of the PMB were characterized in the nodulated rhizosphere of a legume (Phaseolus vulgaris) grown in two soils with contrasting Low-P or P-sufficient content. Screened on solid medium, the density of PMB was higher in the rhizosphere of common bean than in the bulk soil only when plants were cultured on the low-P soil. In liquid culture, half of the rhizosphere isolates were able to use phytate as the sole P source and to release free inorganic P in the medium at rates much higher when bacterial strains were isolated from low-P than from P-sufficient soil. Enzymatic activity confirmed the ability of these rhizosphere isolates to mineralize phytate. Whatever the soil P level, the PMB belonged to the same genera Pseudomonas, Pantoae, Enterobacter and Salmonella, but not Bacillus. Our work demonstrates that low soil P availability increases both PMB density in the rhizosphere compared to the bulk soil and the activities of PMB populations through different growth patterns.