P deficiency: A major limiting factor for rhizobial symbiosis

Abstract

Together with nitrogen (N), phosphorus (P) has been described as the main plant macronutrient limiting growth. Although P is abundant in many soils, its availability for plants is low. For this reason, P is provided to plants largely through the application of P-enriched fertilizers. However, since rock phosphate reserves (the main source of P) are predicted to be depleted in the near future, it is crucial to understand the processes linked with a better P use efficiency. P is a target structural constituent of energetic compounds (ATP, ADP), nucleic acids, phosphate sugars, etc., that are essential for cell metabolism and plant development. Current knowledge highlights that low P availability negatively affects above- and below-ground organ growth, as a consequence, in part, of poor photosynthetic performance. While essential for all plants, the P requirement of N2-fixing plants has been described as larger than that of non N2-fixing plants, mainly as a consequence of the large P demand for biological N2 fixation (BNF) processes. Moreover, three main factors have been suggested to affect BNF under low P conditions: carbon supply, N-feedback and O2 diffusion have been identified as the main factors conditioning N2 fixation under low P availability conditions. In this chapter, we summarize current knowledge regarding P content in plant performance, with special emphasis on N2-fixing plants and the symbiotic relationship.