Which of the following protects nitrogenase inside the root nodule of a leguminous plant?

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 $Alnus$. Is it necessary for a microbe to be in close association with a plant to provide mineral nutrition? Explain with the help of one example.

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

Nitrogenase enzyme is found in $Nostoc$ in the cell of:

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?

Assertion : Nitrogen-fixing enzyme in legume root nodules functions at low oxygen concentration.
Reason : Low oxygen concentration is provided by leghaemoglobin.