The goal of our studies is to determine the features of amino acid biosynthesis unique to plants using Arabidopsis thaliana. We propose a molecular genetic approach to the dissection of the tryptophan pathway by isolating tryptophan- requiring mutants. This pathway is of particular importance in plants because of its role in the biosynthesis of auxin, flavanoids, lignin precursor and other phenolic compounds. The fluorescence of one of the mutants we have already isolated will be used as a genetic marker to obtain more Trp mutants and to understand the relationship of tryptophan biosynthesis to the production of the hormone IAA. As a complementary approach, we will clone the genes encoding the tryptophan biosynthetic enzymes. The complete sequence of one of these genes, the tryptophan synthetase, which has already been cloned will be determined. The cloned genes used as probes will reveal mechanisms of regulation, hormonal control, and intracellular localization. Our analysis of the glutamate synthase (GOGAT) should provide key information about the relationship between amino acid metabolism and photosynthesis. Our analysis of the HMG CoA reductase should also provide important insights concerning the aspects of steroid biosynthesis that are novel to plants. With the cloned reductase gene as a probe we will able to determine the effects of hormones (gibberilins and abcissic acid) on the regulation of this enzyme. The isolation of a tryptophan auxotroph of Arabidopsis thaliana was of the first demonstrations of amino acid auxotrophy in plants. Such a breakthrough opens up an area of inquiry into plant gene regulation and metabolism which has not been previously possible.
