Trace the events starting from the coming in contact of Rhizobium to a leguminous root till nodule formation. Add a note on importance of leg haemoglobin.

Assertion $\left(\mathrm{A}\right):$ Free-living bacteria such as Rhizobium and Frankia can help in the biological fixation of dinitrogen in the roots of angiospermous plants.

Reason $\left(\mathrm{R}\right):$ Prokaryotes exclusively contain nitrogenase enzyme which can catalyze the oxidation reaction of dinitrogen to glutamate.

The correct answer is:

The nitrogen cycle is given below. Identify A and B in it.

Rhizobium forms a symbiotic association with roots in legumes and fixes atmospheric nitrogen. Which of the following statement is correct about this process?

Microbes are useful to human beings in diverse ways. If so, name the following:

Symbiotic nitrogen-fixing bacterium.