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Phosphorus use efficiency in symbiotic N2 fixation for coupling biogeochemical cycles in agrosystems with legumes
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Fewer chemical inputs, as fertilizers or pesticides, becomes of paramount importance for the safety and impact on the environment of agricultural production of food. This implies new requirements with regard to the selection of legume-crop varieties for grain cropping systems. Low phophorus availability in about 40% of the world’s arable land limits crop yield, most particularly for leguminous crops when their growth depends upon symbiotic N2 fixation (SNF). Therefore, our work aims to increase the phosphorus use efficiency (PUE) for SNF, and its contribution to a more effective coupling between the P and N bio-geochemical cycles in agriculture andforestry. Myo-inositol hexakisphosphate (phytate) constitutes the main source of organic P in soils, but is unavailable to plants. Phytases are the only phosphatases able to hydrolyse phytate efficiently into inorganic Pi, thus increasing soil-P bioavailability for plants. In this work we demonstrate the existence of phytases, bothhistidine acid and beta-propeller, among rhizobia nodulating legume spp, and show their expression within nodule infected-cells. Plant phytase-gene expression was also found in nodules, and shown by in situ RT-PCR, to increase significantly under P-deficiency. The latter varies among recombinant inbred lines of Phaseolus vulgaristhat are contrasting in their PUE for SNF. A subsequent virtuous cycle of P and N fertility will be addressed in relation with the objective of the interdisciplinary research strategy of FABATROPIMED. The overall objective is to increase the benefit of grain-legumes for cereal systems and the environment by promoting the interactionbetween soil micro-organisms for plants to acquire and use N and P most efficiently.The field activities include a participatory approach with farmers in reference agroecosystems offering a wide range of agroclimatical and socioecological situations
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