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

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

Nodular diagnosis of contrasting recombinant inbred lines of Phaseolus vulgaris in multi-local field tests under Mediterranean climate

Common bean (Phaseolus vulgaris L.) has the capacity to fix atmospheric N-2 into the biosphere through its aptitude to establish a symbiosis with soil rhizobia. In order to search for environmental constraints that might limit this symbiosis a nodular diagnosis was performed in eighteen field sites chosen with farmers of the Setif agro-ecosystem. Common bean was used as a model grain-legume with six recombinant inbred lines (RILs) and one local genotype Djadida. At flowering stage, the biomass of plants and nodules was determined by excavating 20 cm in depth and around the root-system of ten plants per genotype and per site. The results indicate a large spatial variation in nodulation and growth between genotypes, and the distribution of soils in four soil clusters, based on physico-chemical properties. The inhibition of nodulation of all genotypes in soil of clusters A and B was associated with high residual soil mineral nitrogen (2.23 +/- 0.49 g kg(-1) soil). The low nodulation of all genotypes in the phosphorus (P) deficient soil of cluster C (6.73 +/- 3.63 mg kg(-1) soil) was partly compensated by increasing their efficiency in use of the rhizobial symbiosis (13%), estimated by the slope of the regression model of shoot biomass as a function of nodule biomass. Interestingly, significant correlations were found between nodulation of all genotypes and Olsen-P content in soils of clusters C (R-2 = 0.97, P < 0.001) and D (R-2 = 0.94, P < 0.05). It is concluded that the RILs selected for their efficient use of P for symbiotic nitrogen fixation show the highest nodulation and growth and that the nodular diagnosis can be used to assess the growth response of N-2-dependent grain-legume to soils with low availabilities of N and P