Which microorganisms live symbiotically on leguminous plants?

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

$Frankia$ is free-living in soil,but as symbionts,can fix atmospheric nitrogen.

We find that $Rhizobium$ forms nodules on the roots of leguminous plants. Also,$Frankia$,another microbe,forms nitrogen-fixing nodules on the roots of the non-leguminous plant $Alnus$. What kind of relationship is observed between mycorrhiza and pine trees?

Leghaemoglobin helps in

Explain the stages of root nodule formation.