Rhizobium

A classical example of nitrogen fixation is the symbiosis leguminous plants and bacteria of the genus Rhizobium.The seat of symbiosis is within the nodules that appear on the plant roots. In the symbiotic association between Rhizobium and leguminous plant, two organisms interact in such a way as to influence and coordinate the expression of both prokaryotic and eukaryotic gene.

The communications that occur between the plant and the rhizobia during nodule formation and maintenance constitutes a novel opportunity to study signal transduction in a plant system. The expression of nodulation genes in the bacteria is activated by signals from plant roots and as a result the bacteria synthesise signals that induce a nodule meristem and enable the bacteria to enter this meristem via a plant-made infection thread. The chemical signals synthesised by the bacteria are based on a modified amino acid (homoserine lactone) carrying a variable acyl chain substituent, and are called acyl homoserine lactones (AHLs).

By detecting and reacting to these chemicals, individual cells can sense how many cells surround them and whether there are enough bacteria, i.e.a quorum , to initiate the change towards acting in a multi cellular fashion. This is known as 'quorum sensing'.

Beijerinck was the first to isolate and cultivate the bacterium responsible for nodulation and he named it Bacillus radicicola which is now placed in the genus Rhizobium.

Rhizobium is found as a free-living organism in the soil but does not fix atmospheric nitrogen in that state. Only after its association with a leguminous plant and after the formation of root nodules does it fix atmospheric nitrogen in the nodules.

Source : microbiologyprocedure.com