The power of the phytoglobin-NO cycle in the regulation of nodulation and symbiotic nitrogen fixation

Abstract

Recent years have seen a large increase in our understanding of the multifaceted roles of nitric oxide (NO) in plant biology, particularly in the regulation of symbiotic interactions with soil microorganisms. NO serves key functions in the initiation and maintenance of legume–rhizobium symbiosis. Extensive molecular crosstalk between the host legume and the soil rhizobium results in the initiation of symbiosis and the establishment of a highly specialized organ called the nodule, which provides the conditions required for bacterial nitrogen fixation. Within the nodules, the rhizobium bacteria are housed in bacteroids, in which the oxygen is maintained at very low levels allowing the bacterial nitrogenase to function. This enzyme is oxygen sensitive (half maximal inhibitory concentration (IC50) oxygen for nitrogenase is 2.9 µM; Goldberg et al., 1987). Hence, a key feature of the nodule is the maintenance of a strong oxygen diffusion barrier, in which leghaemoglobin is used to transfer oxygen to the plant mitochondria to sustain respiration.