Plastidal Pathways for Nitrogen Assimilation and the Inhibition of Amino Acid Biosynthesis by Herbicides

There are many cellular processes occurring in plastids. One of them concerns nitrogen assimilation (nitrate and ammonium). Photosynthesis taking place in the same organelle plays a key role in direct or indirect assimilation of nitrogen. This is involved in the building of the fundamental life molecules: nucleotides, proteins, chlorophyll and numerous other metabolites and cellular components. This review enlightens the mechanisms involved in the transport, reduction and assimilation of nitrate implying specific transporters in the plastid envelopes, specific enzymes as glutamine synthetase/ glutamate synthase pathway in the plastid stroma from plants and algae studied in the lab as models. EPSP synthase, AHA synthase and GS participating to amino acid biosynthesis have been used as targets for widely distributed herbicides. This paper will review the molecular basis of their inhibiting action and the various approaches to obtain herbicide- tolerant crops.