Introduction

The observation reported in 1980 by Amrhein and his colleagues (Amrhein, Schab & Steinrücken, 1980; Steinrücken & Amrhein, 1980) that the herbicide glyphosate was a highly specific inhibitor of the shikimate pathway enzyme 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase served to focus the attention of many agricultural scientists on the possibility of finding other herbicides which acted by inhibiting the biosynthesis of macromolecular precursors and especially amino acids. Although knowledge of most pathways of amino acid biosynthesis in higher plants was still fairly primitive the idea that amino acid biosynthetic enzymes were good targets for herbicides was soon reinforced by the recognition that the sulfonyl urea and imidazolinone based herbicides (La Rossa & Schloss, 1984; La Rossa & Falco, 1984; Shaner, Anderson & Stidham, 1984; see Chapter 7) acted by inhibiting acetolactate synthase, a key enzyme in branched chain amino acid biosynthesis, and phosphinothricin (Leason et al., 1982; see Chapter 8) acted by inhibiting glutamine synthetase. All these compounds had resulted from random screening procedures but it is now widely believed in the agrochemical industry that it should be possible to accelerate the development of new herbicides by the judicious application of knowledge of the biochemistry and molecular biology of amino acid biosynthesis.

The principal information required for rational herbicide design is the structure and mechanism of the target enzymes. This immediately highlights a difficulty. There was, and continues to be, a general lack of mechanistic and structural information for plant amino acid biosynthetic enzymes.