Introduction

Although the central role of amino acid biosynthesis in plant metabolism and development is evident, progress in the understanding of the molecular mechanisms by which such pathways are regulated is only recent. To elucidate the genetic regulation of a pathway, mutants have proved to be invaluable in the analysis of microbial systems. The same is true for plants, but again there are only a few pathways for which mutants with defective or altered essential functions are available. However, the improvements in plant mutant isolation at cell and whole plant levels, and in molecular biology have now provided mutants affected in amino acid metabolism as well as amino acid biosynthetic genes or cDNAs (for a review, see Last, 1992).

The branched biosynthetic pathway of aspartate-derived amino acids (lysine, methionine, threonine) and the related pathway leading to isoleucine, valine and leucine have received much interest for various reasons. First, it gives rise to essential amino acids which, if poorly represented as lysine in cereals and methionine in legumes, limit the nutritional quality of crop plants as diet for human beings and monogastric animals. Secondly, three classes of potent herbicides (sulphonylureas, imidazolinones and triazolopyrimidin) kill plants through the inhibition of acetolactate synthase, an enzyme common to the isoleucine and valine pathways. Key regulatory enzymes of the aspartate-derived amino acid biosynthesis would also be suitable targets for efficient herbicides while little affecting the environment.