Question

How do plants synthesize proteins which are nitrogenous compounds?

Answer 1

Proteins are long chains of Amino acids, joined by Peptide bonds. Head on to Amino acid synthesis, where all the amino acid biosynthetic pathways are described. Note that these pathways are present both in animal and plant cells. While for animals it is easy to obtain amines and ammonia by “dismantling” amino acids they got from whatever they ate or from those they don’t need at the moment or have too much of, for plants, things get a bit more complicated. 

Answer 2

In plants, inorganic nitrogen (i.e. NO3– and NH4+) taken up by roots is incorporated into glutamine and glutamate (primary nitrogen assimilation), which is used to synthesize other amino acids and nitrogenous compounds by transamination. ... Amino acid transfer is also very active at the cellular level.