• Rhizobium bacteria, basically form root nodules (stem nodules in Sesabania rostrata) in leguminous plants and fix atmospheric nitrogen in a symbiotic association.
• These nodules act as mini-factory for fixation of atmospheric N.
• The Rhizobium bacteria gives nitrogen to the plant and the plant gives protection to the
• bacteria from oxygen damage by harboring it inside the root nodule.
• The plant roots also supply essential minerals and newly synthesized substances to the bacteria.
• Rhizobium inoculation is a well-known agronomic practice to ensure adequate N supply for legumes in place of N fertilizer.
• Rhizobium gets entry into the legume root hairs and induces nodulation in plant roots.
• There are two types of nodules effective and ineffective ones. Effective nodules are formed by effective strains of Rhizobium.
• They are well developed, pink color due to the presence of pigment possess leghaemoglobin.
• The bacteriod tissue is well developed and well organized with plenty of bacteriods.
• On the contrary ineffective strains of Rhizobium form ineffective nodules which are generally small and contain poorly developed bacteriod tissue showing accumulation of glycogen.
• The red pigment in the nodules acts as a biological value in regulating the supply of oxygen into the bacteriod tissue.

Rhizobium is

• Host specific, can fix 50-100 Kg N/ha, 10-35% increase in yield
• Also reduces seed rot, foot & root rot of leguminous and non-leguminous (okra, tomato) plants.
• Need optimum P and Mo
• Rhizobia also produce Phytohormones such as auxins, cytokinins, gibberllins and abscicic acid.
• Also known to stimulate seed germination in a wide range of plant species.