Symbiotic Nitrogen Fixation

Summary

Symbiotic nitrogen fixation is a mutually beneficial association between a prokaryotic N₂-fixing organism and a eukaryotic, and usually photosynthetic host. The best known example is that between legume and Rhizobium. Symbioses involving crop and pasture legumes account for ~50% of all the N used in agriculture with rates as much as 600 kg ha^-1.

Nodulation in legumes can follow several different pathways, but in many legumes involves root-hair infection. This has both a visible and a molecular component, and involves signalling between host and Rhizobium. In this process rhizobia bind to immature root hairs, inducing deformation and curling; dissolve the cell wall at the point of attachment, and enclosed within a plant-derived infection thread move down the root hair in the direction of the root cortex. Signal molecules include flavonoids excreted from germinating seed and roots, and which trigger expression of nodulation genes in Rhizobium, and lipo-chitooligosaccharides or “nod factors” produced by the rhizobia. The latter are extremely powerful growth hormones which alone can induce many of the changes associated with nodulation.

Given the complexity of the nodulation process, it is not surprising that specificities can occur between host and Rhizobium. Thus not all legumes form nodules, and those that do those that do can only be infected by specific groups of rhizobia. Cross inoculation infectiveness groups and sub groups occur, and even once nodules are formed, they may or may not fix nitrogen. More than 100 different inoculant rhizobia are needed to ensure effective nitrogen fixation in the currently important legumes species. To maximize N₂ fixation one must select for both the best Rhizobium strain and the best host cultivar. Breeding for enhanced nitrogen fixation is both possible and desirable.

Environmental factors will often constrain nitrogen fixation. Thus soil pH is more inhibitory to nodulation per se than to either Rhizobium or host legume. Again selection for acid tolerant strains and rhizobia is possible, and can markedly affect nodulation success and N₂ fixation. Several nutrients are also required in greater amount in the nodulated legume than in the same plant supplied N. They include Mo (for nitrogenase), Fe (nitrogenase, infection, leghemoglobin), sulfur (protein synthesis), phosphorus (energy), Ca (Infection) and Co (cyanocobalamine). Plants deficient in these elements will usually show N deficiency symptoms.

Knowledge of the actinorhizal-Frankia and Azolla-Anabaena symbioses lags behind that available in legumes, but a number of commonalities have been identified.