Earth’s atmosphere is nearly 80% nitrogen (N) but then the element is not sufficiently available to the organisms. It is known that some of the organisms have the capacity to assimilate molecular nitrogen and convert it into assumable form (Fig. 11-1). In general, four main types of organisms are recognized which can assimilate molecular nitrogen.
These are symbiotic microorganisms inhabiting the roots of some angiosperms or gymnosperms; some heterotrophic soil bacteria which live in the Free State; photosynthetic bacteria and some photosynthetic blue-green algae. The biological nitrogen fixation is highly essential and most part of the organic nitrogen comes from this process.
As early as 1838 Boussingault observed that plants could fix atmospheric nitrogen. He attributed such a characteristic to the legumes which increased the nitrogen content of the soil. In 1886, Hellriegel and Wilfarth demonstrated that bacteria present in the nodules of legumes were responsible for this process.
It was later on discovered that the baterialsymbionts belong to genus Rhizobium. However, now it is known that some of the non-leguminous plants like Myrica gale, Alnus sp., etc. possess nodules and fix nitrogen. Later on, it was found that these nodules contained endophyticactinomycetes instead of bacteria.
The loose association or true symbiosis of the roots of higher plants (mainly grasses) with nitrogen fixing Spirillumlipoferum has also attracted much attention these days. The nitrogen fixing bacteria, Bacillus and Enterobacter have also been found in association with the rhizosphere of grasses. The formation of nodules has attracted the attention of morphologists and physiologists.
The initial step in nodulation is the formation of a substance in the legume roots which attracts Rhizobium bacteria. Then a material rich in hormones is secreted which causes curling of root hair. Bacteria partially destroy the cell walls by secreting some enzymes and enter the cortex in thread like mucilage material termed as ‘infection-thread’.
Consequently, the cortical cells are stimulated, divide and become polyploid. The repeated divisions of polyploid cells result in the formation of nodules. The nodule bacteria stop dividing, increase in size and shape and are termed as ‘bacteroids’. The host cells divide and increase the diameter of a nodule. Figure 11-3 shows stages in the formation of nodules.
The Rhizobium species are specific for different legume species. Recently, it has been found that roots of leguminous plants are having ‘lectins’, the plant glycoproteins or protein can detect a specific landscaping of different sugars on bacterial cell surface and thus determine the specificity. It is essential to have sufficient inoculation of a specific Rhizobium species.
Similarly, appropriate Rhizobium cultures are sweared to the seeds of legumes (e.g. pea, beans) or even soil is inculated at the time of sowing.
The nitrogen fixed in the nodules is immediately converted to amino acids by transaminases and other assimilatory enzymes. Organic nitrogen is translocated generally as amides, particularly asparagine in legume species. The functional nodules contain a pink pigment called leghemoglobin which is functionally similar to mammalian hemoglobin, the red pigment of blood.
This pigment is involved in the nitrogen fixation by protecting the nitrogenase enzyme from possible O2 inactivation. This pigment is located in between bacteroids and the surrounding membrane. It takes O2 produced in the vicinity of nitrogen fixation and passed on to the electron transport chain away from the site.
Studies on the localization and genetic of leghemoglobin synthesis indicate that it takes place in the cytoplasm of the host cells and is regulated by genes of the legume plant. Bacteroid enzymes, however, seem to be responsible for the synthesis of the heme component. This pigment is absent in photosynthetic bacteria and algae.
In these cases, the nitrogenase enzyme which is also having the function of reducing protons to dehydrogen, protects itself by means of an oxyhydrogen reaction against inactivation by oxygen in heterotrophic bacteria, photosynthetic bacteria and blue-green algae.