Explain the methods of amino acid synthesis in plants.

(N/A) $ \Rightarrow $ At physiological $ pH $, ammonia is protonated to form $ NH_{4}^{+} $ ions.
$ \Rightarrow $ While most plants can assimilate both nitrate and ammonium ions, the latter is quite toxic to plants and hence cannot accumulate in them.
$ \Rightarrow $ $ NH_{4}^{+} $ is used to synthesize amino acids in plants through two main processes: $ (i) $ Reductive Amination and $ (ii) $ Transamination.
$ (i) $ Reductive Amination: In this process, ammonia reacts with $ \alpha $-ketoglutaric acid to form glutamic acid as shown below:
$ \alpha $-ketoglutaric acid $ + NH_{4}^{+} + NADPH \xrightarrow{\text{Glutamate Dehydrogenase}} \text{glutamate} + H_{2}O + NADP $
$ (ii) $ Transamination: This involves the transfer of an amino group from one amino acid to the keto group of a keto acid. Glutamic acid is the main amino acid from which the amino group $ (NH_{2}) $ is transferred to form other amino acids. The enzyme transaminase catalyzes these reactions.
$ \text{Amino-donor} + \text{Amino-acceptor} \xrightarrow{\text{Transaminase}} \text{Keto-acid} + \text{New amino acid} $
$ \Rightarrow $ Additionally, two important amides, asparagine and glutamine, are formed from aspartic acid and glutamic acid by adding another amino group. Since amides contain more nitrogen, they are transported via xylem. Some plants (e.g., soybean) also export fixed nitrogen as ureides.