Clover likes bacteria, and certain bacteria like it. Nodule bacteria (rhizobia) like to live together with plants from the legume family. In addition to clover, these include important forage plants such as soya beans and peas. Both partners benefit from the presence of the other. This coexistence of different species for mutual benefit is called symbiosis.

 

Plants need nitrogen to grow, which is present in the air as a gas (N2), but which they cannot utilise in this form. The nodule bacteria convert N2 into ammonia (NH3) or ammonium (NH4+) and thus make it available to the plants. In return, the plant produces substances that the bacteria need to live, such as certain sugars. The symbiosis with the rhizobia allows the plants to thrive on nitrogen-poor soils.

Agriculture utilises this interaction: the cultivation of legumes on agricultural land supplies the soil with nitrogen, which is available to other plants grown there in the following season. This symbiosis is based on a fixed pattern: both partners recognise each other through components on their surfaces. The rhizobia attach themselves to the root hairs of the plant and cause them to curl. The bacteria then penetrate the plant and form a tube. A new organ, the bacteroid, forms at the tip of the tube (see illustration). This is where the enzyme that converts the nitrogen works. This nitrogenase is very sensitive to oxygen. Therefore, another enzyme, leghaemoglobin, ensures a low oxygen concentration. Like our related blood pigment haemoglobin, leghaemoglobin is red in colour. If the level of oxygen becomes too low, leghaemoglobin can release it again. It therefore fulfils the same function as haemoglobin in human blood: the binding and release of oxygen. However, leghaemoglobin binds oxygen ten times more strongly than haemoglobin in human blood, which means that much less oxygen remains in the root nodule.