NiTROGEN FIXATION BY rHIZOBIUM
Rhizobium is a genus of gram negative bacteria that lives in the soil and forms an endosymbiotic relationship with the roots of certain legumes by fixing nitrogen from the air into the roots. The bacteria forms colonies in the root nodules where they convert nitrogen from the atmosphere into ammonia which can be used by the plants for metabolism. The plants in turn provide the bacteria with organic compounds which are by products of photosynthesis ad needed by the bacteria for their metabolic activities. In 1888, Beijerinck a Dutchman, isolated and cultivated the first microorganisms from the root of ...view middle of the document...
3. Biological fixation - nitrogen-fixing bacteria which fix 60% of nitrogen gas
Nitrogen fixing bacteria like Rhizobium belong to the group biological fixation. The process of fixation involves the reduction of nitrogen gas to ammonia and is energy intensive. It requires 16 molecules of ATP and a complex variety of enzymes to break the nitrogen bonds so that it can combine with hydrogen. Its reduction can be written as:
energy (16 ATP)
N2 + 3H2-------- -------2NH3
Fixed nitrogen is made available to plants by the death and lysis of free living nitrogen-fixing bacteria or from the symbiotic association of some nitrogen-fixing bacteria with plants. Examples of nitrogen-fixing bacteria are shown in the table below.
Free living | | Symbiotic association with plants | |
Aerobic | Anaerobic | Legumes e.g. peas, beans | Non-legumes |
Azotobacter | clostridium | Rhizobium | Frankia |
Rhizobium is the most well-known species of a group of bacteria that acts as the primary symbiotic fixer of nitrogen. The bacteria lives in the soil and can infect the roots of leguminous plants, leading to the formation of lumps or root nodules where the nitrogen fixation takes place. A symbiotic interaction begins to take place once this is established. The bacterium’s enzyme system supplies a constant source of fixed nitrogen to the host plant while the plant provides nutrients and energy for the activities of the bacterium. A vast majority of legumes can form this relationship with rhizobium. In the soil, some rhizobium are free living and motile, feeding on the remains of dead organisms. Free living rhizobia cannot fix nitrogen into the roots of plants. They even have a different morphology from those that live in the root nodules. They are straight rods in structure while those in root nodules the nitrogen-fixing form exists as irregular cells referred to as ‘bacteroids’ and are either often club shaped or Y-shaped.
When the bacteria infect the plant roots, a set of genes in the bacteria control different aspects of the nodulation process. Each Rhizobium strain can infect certain species of legumes but not others. For example the pea is the host plant to Rhizobium leguminosarum. Specificity of certain genes are the main factors in determining which Rhizobium strain will infects which kind of legume. Just because a root noodle is formed doesn’t necessarily mean that nitrogen fixation will take place. A rhizobium can be effective or ineffective. Effective strains induce nitrogen-fixing nodules. Those unable to fix nitrogen are termed ineffective. Effectiveness is governed by a different set of genes in the bacteria from the specificity genes. The genes that govern the various stages of nodulation are called the Nod genes.
The process of nodulation begins with an interaction between the roots of the host plant...