Which of the following is responsible for nitrogen fixation in an aquatic fern?

The nitrogen-fixing symbiotic organism present in $Azolla$ is:

Symbiotic nitrogen fixation in non-leguminous plants is carried out by

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

Which of the following are the part or example of symbiotic mutualistic association?
$I.$ Yeast
$II.$ Rhizobium
$III.$ Mycorrhiza
$IV.$ Oscillatoria

Nodulated roots make the plant rich in