Which bacterium is the most important in forming nodules on the roots of legume plants and engaging in symbiotic nitrogen fixation?

Rhizobium

The correct answer is Option (2) - Rhizobium.

Rhizobium is the most important symbiotic nitrogen-fixing bacteria that forms nodules on the roots of legume plants. Legumes, such as peas, beans, lentils, and clover, have a unique ability to form a mutualistic relationship with certain bacteria, specifically Rhizobium species.

The symbiotic relationship between legumes and Rhizobium begins when the bacteria infect the root hairs of the legume plant. The plant responds by forming specialized structures called nodules, which provide a suitable environment for Rhizobium to thrive. Inside the nodules, Rhizobium bacteria reside in special cells known as bacteroids. The bacteroids fix atmospheric nitrogen into ammonium, which can then be utilized by the plant for growth and development.

This symbiotic relationship between legumes and Rhizobium is beneficial for both parties. The legume plants receive a direct supply of fixed nitrogen, reducing their dependence on external nitrogen sources like fertilizers. In return, the legumes provide carbohydrates and other nutrients to the Rhizobium bacteria, supporting their growth and metabolism.

The other options mentioned in the question are incorrect:

(1) Aspergillus and (3) Penicillium are both fungi, not bacteria, and they are not known for their nitrogen-fixing abilities.

(4) Streptococcus is a genus of bacteria, but it is not involved in symbiotic nitrogen fixation with legume plants.