Carbon metabolic adjustment in soybean nodules in response to phosphate limitation: A metabolite perspective

The necessity to gain deep insights regarding the overall metabolic efficiency of soybean functional acclimation to tackle phosphate (Pi) starvation has led us to discuss the current knowledge on the carbon (C) metabolic changes in soybean nodules under such Pi stress conditions in this review. Soybean possesses numerous coping strategies to conserve Pi utilization, while decreasing the C cost to promote nodule function during Pi stress. This could be achieved through the modulation of carbohydrate importation, mobilization of storage reserves, modification of respiratory pathways and exportation of nitrogenous (N) products. Under such circumstances, soybean regulates the C partitioning among the various nutrient-acquiring structures and the C flux through different metabolic pathways (primary and secondary). However, substantial genetic variation leading to nodule acclimation to Pi stress is notable and dependent on the crop genotypes and/or rhizobial strains that are in symbiosis. Several Pi deficiency-induced responses (e.g., mycorrhizal association, exudative burst and secondary metabolism) can act to increase the Pi levels in nodules, which are, unavoidably, often associated with a heavy burden to the overall C budget of the host. Alternative non-adenylate respiratory pathways (e.g., glycolytic bypass) and ureide export (i.e., less amide transport) are metabolically less expensive, and thus more favorable during Pi stress.