Which form of nitrogen enters into the plants?

The processes of nitrification and nitrate assimilation are:

What does the given reaction show?
Choose the correct option.

Complete the equation.
$\alpha -$ Ketoglutaric acid $+ NH_4^+ +$ ...$A$.... $\xrightarrow[\text{Dehydrogenase}]{\text{glutamate}} $ ...$B$... $+ H_2O + NADP$
Identify $A$ and $B$.

The conversion of $NO_{3}^{-} \rightarrow NO_{2}^{-} \rightarrow NH_{3}$ is called and is catalysed by:

Complete the equation for reductive amination.