Phosphorus use efficiency for N2 fixation in the rhizobial symbiosis with legumes

Symbiotic nitrogen fixation (SNF) by legumes may provide an ecologically acceptable complement or substitute for mineral nitrogen fertilizers that farmers try to minimize for economic limitation and environment sake. However, phosphorus (P) deficiency is a major limiting factor for legume–rhizobia symbioses, particularly in acidic and calcareous soils. Nevertheless, the expression of legume SNF potential under P deficiency may be improved. Indeed genotypes contrasting in P use efficiency (PUE) for SNF could be found for such legumes spp. as Phaseolus vulgaris, Vigna unguiculata, or Medicago truncatula in hydroaeroponic culture under glasshouse conditions after inoculation with specific reference rhizobia. In order to assess the contribution of phosphatase activity to PUE for SNF, an in situ RT-PCR methodology was used to localize and quantify the mRNA of candidate genes in nodules of common bean grown under deficient versus sufficient P supply. The transcript localization of phytase, phosphoenolpyruvate phosphatase, and trehalose 6 phosphate phosphatase was found to be tissue specific and differ among any phosphatase APase genes, P treatments, and legume genotypes. In order to assess whether the genotypic difference in PUE for SNF may contribute to adaptation of the rhizobial symbiosis to low-P soils, the nodulation and growth of contrasting common-bean recombinant lines from the cross of BAT477 and DOR364 were assessed in fields of farmers in reference agroecosystems of the Mediterranean basin following a participatory approach. The most efficient genotype, namely RIL115, was superior in most fields where nodulation was above the threshold for significant contribution to legume nutrition. From those fields with low-P soils where adaptation was observed for low-efficiency genotypes, bacteria were isolated from nodules and rhizosphere in order to search for rhizobia or rhizobacteria that could increase the P efficiency for the legume symbiosis. Since mycorrhizae may contribute to the PUE, the tripartite symbiosis was tested in hydroaeroponic culture, showing that some species may limit nodulation by contrast with Glomus intradices that increased it under P deficiency. It is concluded that (i) phosphatases contribute to the use of organic P for N2 fixation and to the increase in nodule permeability to O2 under P deficiency; (ii) nodulation in agroecosystems varies considerably in space and time; (iii) high PUE for SNF may increase the N2-dependent growth of common bean in low-P soils and the overall P bioavailability; and (iv) some local rhizobia may contribute to the PUE for SNF.