Which one among the following kinds of organisms resides in the roots of pulse plants to do nitrogen fixation ?

Nitrogen fixation in pulse plants (legumes) is carried out by specialized bacteria, most notably the genus Rhizobium. These bacteria reside in root nodules where they form a symbiotic relationship with the host plant [c1][c3]. While atmospheric nitrogen (N2) is inert and unusable by plants, these prokaryotic organisms utilize the enzyme nitrogenase to convert it into ammonia (NH3) or nitrates, which the plant can readily assimilate for growth [t1][t2]. In this mutualistic association, the pulse plant provides carbohydrates to the bacteria, while the bacteria provide fixed nitrogen, enhancing soil fertility [c4][t5]. Other organisms like fungi, protozoa, and viruses do not possess the biological machinery for this specific symbiotic nitrogen fixation in legumes [t1][t8]. Rhizobia are specifically identified as gram-negative soil bacteria that infect the roots of legumes to facilitate this process [t10].

Sources

1. [1] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 5: Geomorphic Processes > Biological Activity > p. 45
2. [2] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > The Nitrogen Cycle > p. 20
3. [3] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Pulses (Legumes) > p. 28
4. [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC98982/
5. [5] https://www.nature.com/scitable/knowledge/library/biological-nitrogen-fixation-23570419/
6. [6] https://en.wikipedia.org/wiki/Rhizobia