Symbiotic nitrogen-fixing bacterium is . . . . . . .

What are the conditions necessary for fixation of atmospheric nitrogen by $Rhizobium$? What is their role in $N_2$-fixation?

Which of the following statements is correct regarding nitrogen fixation in legumes?

The anaerobic condition for nitrogen fixation is maintained by ..........

What are the steps involved in the formation of a root nodule?

Arrange the stages of root nodule formation in the correct sequence:
$I-$ Infection thread is produced.
$II-$ Bacteria are carried to the cortex of the root.
$III-$ Bacteria attach to the root hair.
$IV-$ Bacteria are released from the thread into the cells.
$V-$ Differentiation into nitrogen-fixing cells occurs.