Copyright 2006 Accademia dei Georgofili
A typical example of a Nitrogen-Fixing Symbiosis (NFS) is represented by the interaction between Legume plants and bacteria of the family Rhizobiacee. The result of this symbiotic interaction is a new plant organ developed as result of the bacteria infection on the root system of the legume plant, the nitrogen fixing root nodule. Inside the nodules, the bacteria found the optimal compartment for the fixation of the atmospheric nitrogen. In terms of metabolic exchange, the fixed N is exported in the form of ammonium to the plant cells of the nodules and in different amino acids forms to the rest of the plants, whereas the photosyntate products are transferred from the shoot to the nodule to supply the microbes partner with the amount of energy needed for the very expensive N fixation reaction. It was calculated that about 20% of the plant photosynthate is allocated to the nodule to allow the fixation of the atmospheric nitrogen. In legumes, development of nitrogen fixing nodules is induced by perception of lipochitin-oligosaccharide signals (Nod factors) secreted by the Rhizobium symbiont. Besides, nitrogen starvation is a prerequisite for formation, development and functioning of root nodules and high level of combined N in the form of nitrate and/or ammonium, through unknown signaling pathways can be perceived by legume plants and completely abolish nodule formation.
