The common bean was first domesticated more than 6000 years ago in South and Central America.  Today it is cultivated on all continents.  It constitutes an important source of protein for the human diet in numerous southern countries.  However, yields in these regions are particularly low (0.5-0.7 t/ha) when compared with those achieved in developed countries (2.5-3.5 t/ha).
Thanks to symbiosis with bacteria (rhizobia), the leguminous bean can grow on poor soils without the addition of costly and potentially pollutant nitrate fertilizers.  In fact, these soil bacteria induce the formation of nodules on the roots – and sometimes the stems – of legumes, in which they transform atmospheric nitrogen (N2) into ammonium (NH4+) that is used by the plant to synthesize proteins.  
However, this nitrogen fixing requires intense respiration that depends on the permeability of nodules to oxygen.  But this permeability is disturbed by a lack of water, excess salt or even phosphorus deficits, all of  which constitute major environmental limitations to rhizobial symbiosis in the soils of Mediterranean and tropical regions.

Breeding high-performance lines to ensure symbiotic nitrogen fixation under osmotic stress

In the context of a collaborative project by the CIAT and INRA on the biodiversity of beans in Latin America, symbioses were identified with a high potential for symbiotic nitrogen fixation and efficient phosphorus use.  The most successful lines (such as BAT 477, also bred for its drought tolerance) were crossed with varieties of agronomic importance such as DOR 304, which is widely cultivated for its tolerance of the bean golden mosaic virus.
It is from the progeny of this cross that contrasting lines were bred in the context of the Agropolis platform, and then used during the European AQUARHIZ project in order to understand the mechanisms and determine the genes involved in the bean's ability to achieve improved yields in poor soils (phosphorus and nitrogen deficits) under osmotic stress (lack of water, excess of salt) in Mediterranean regions.

Identifying the genes implicated in plants with a high potential for symbiotic nitrogen fixation

RNA in the nodular cortex, where variations in permeability occur, was extracted from different lines and transferred to GeneXpro (University of Frankfurt) where the screening of candidate genes was performed using the SuperSage method, developed initially for the human genome.  More than 50 genes were shown to be over- or under-expressed in nodules in response to a salt shock.  In particular, genes were found that coded for phosphatases (the enzymes responsible for phosphate solubilization).
In order to understand the function of these genes in symbiosis, molecular biology methodologies have been developed to localize, or even quantify, the expression of genes in different nodular tissues, including the inner cortex. 

Comparison of the molecular data thus obtained at a cellular level, and those arising from biophysical measurements at the scale of the whole plant, made it possible to establish that phosphatases are implicated in the adaptation of nitrogen fixation to moderate salinity in the bean.

The lines bred during the project proved to be of sufficient interest for them to be retained as research tools for future international programmes (concerning the bacteriology and screening of leguminous genes) in order to improve the bean and the use of legumes in poor soils, particularly in the context of the integrated European Union project GRAINLEGUME, the PHASEOMICS consortium (on bean genomics), the cooperative research group FABAMED (on nitrogen fixation in the Mediterranean basis) and the AIEA-FAO N&P project.