Question

Identify the bacteria found in root nodules of leguminous plants
(a) Escherichia coli
(b) Rhizobium
(c) Azotobacter
(d) Granulibacter

Answer 1

Hint: These are gram-negative, motile, and non-sporulating rod-shaped bacteria that cannot fix nitrogen independently. They are found in the soil and require a plant host for nitrogen fixation.

Complete answer:
The root nodules of leguminous plants contain bacteria belonging to the genus Rhizobia. It is a paraphyletic group of bacteria that are gram-negative, motile, and non-sporulating rod-shaped. They cannot fix nitrogen independently and hence require a plant host that is a plant from the Fabaceae family. These are soil bacteria and infect the roots of legumes to form nodules where they fix atmospheric nitrogen and convert it into a plant usable form. The nitrogen is then used for the growth of the host plant. When the legume plant dies, the nodules formed in the root get broken, and Rhizobia are released back to the soil again so that they can infect a new plant.

Additional Information:
-Rhizobia are considered a unique group of bacteria because they are the only type of bacteria that live in a symbiotic relationship with the legumes. This symbiotic relationship involves a signal exchange between both partners and under the effect of this exchange, symbiotic structures develop in both partners. The root nodule can be seen in the figure given below –
-The bacteria Escherichia coli is found in the intestine of humans and animals. Also, it is found in the environment, milk, and food too. Mostly they are harmless but some cause diarrhea, urinary tract infections, respiratory illness, bloodstream infections, and other illnesses.
-The bacteria Azotobacter is found in soils that are neutral and alkaline in nature, in water, and also in association with some plants. It improves seed germination and increases nutrient availability in crops.
-The bacteria Granulobacter is found in fruits, fermented food, plants, soil, and water. Some members of this family are used in industries for the commercial production of vinegar.

So, the correct answer is ‘Rhizobium’.

Note:
-Azotobacter is also a nitrogen fixer but it is not symbiotic like the Rhizobium.
-Azotobacter is a free-living aerobic form of bacteria that is oval-shaped.
-E.coli is normally found in the intestine of humans and also in the gut of some other animals. Most of these bacteria are harmless and help in the digestion of food.