The picture shows a cross-section of a root nodule formed by the nitrogen-fixing bacteria Rhizobium in symbiosis with a plant from the legume family. Within the nodule, bacteria convert atmospheric nitrogen into a plant-available form. Several key minerals drive this vital process. Legume plants secrete flavonoids through their roots to attract rhizobia bacteria. In turn, the bacteria produce signalling molecules called nod factors that initiate the formation of nodules on the plant’s roots. The nodules house the bacteria and serve as an in-house nitrogen factory. The intracellular movement of calcium mediates the complex dialogue between the plant and bacteria.
Bacteria use calcium to sense and transduce the flavonoid signal from the plants, and the plant similarly relies on calcium to detect the bacteria’s prompt to initiate nodule formation. Within the nodules is where the bacteria convert atmospheric nitrogen into biologically available nitrogen. Molybdenum, vanadium and iron are trace element co-factors required to activate the nitrogenise enzyme pathway.
The red pigment inside root nodules is leghemoglobin – the plant equivalent of haemoglobin and an indicator of active N fixation.
This process requires an adequate supply of cobalt to synthesise leghemoglobin. Nodules without a reddish colour inside signify a lack of cobalt. This process illustrates the synergy between soil microbes and minerals. Neither one fulfils its function without the other. Minerals catalyse biological processes, and microbes potentiate mineral availability.
