The Influence of Phosphate Deficiency on Legume Symbiotic N2 Fixation

Legumes are well recognized for their nutritional and health benefits as well as for their impact in the sustainability of agricultural systems. Phosphate (Pi) deficiency is a major nutritional factor limiting legume production, particularly in acidic and calcareous soils. Pi deficiency limits N2 fixation, since it has been described to have a strong impact on the growth and survival of both rhizobia and host plant. Legumes have evolved complex mechanisms to cope with Pi limitation. The maintenance of symbiotic N2 fixation seems to be a key aspect to assure legume productivity in low Pi environments. During the last decades, physiological components and molecular players underlying Pi-deficiency adaptation responses have been elucidated. Molecular, biochemical, physiological, and morphological responses are triggered to stimulate soil Pi uptake or to optimize its intracellular use efficiency and allocation over all plant organs. Research conducted on model species such as Medicago truncatula or important pulses such as common bean, soybean, and white lupin have provided valuable clues about the different mechanisms ensuring Pi homeostasis in nodules under Pi-limited conditions. In this chapter, we provide a general overview on the recent achievements on the impact of Pi deficiency on symbiotic N2 fixation in a broad range of legume species. A critical discussion of the main results and open questions is provided, highlighting the different approaches used to address the current needs of agriculture under the climate change context.