Role of the Shikimic Acid Pathway in the Formation of Tryptophan in Higher Plants : Evidence for an Alternative Pathway in the Bean

Abstract

IT has been assumed generally, without adequate supporting evidence, that tryptophan and other indole compounds are generated via the shikimic acid pathway in higher plants. Evidence for this bio-synthetic pathway has been derived mainly from microbial studies and from theories based on the unity of biochemistry^1–3. Proof that shikimic acid is a precursor to the aromatic amino-acids, phenyl-alanine and tyrosine, has been clearly obtained by McCalla and Neish⁴. It has also been demonstrated that higher plants possess a mechanism to convert quinic acid to shikimic acid and phenylalanine and tyrosine^5–7. The latter aromatic acids are assumed to be derived from prephenic acid, which is transformed to phenylpyruvic and p-hydroxyphenyl-pyruvic acids, and afterwards to phenylalanine and tyrosine, respectively¹. Since anthranilic acid, a precursor to tryptophan, may be derived from a separate branch of the aromatic pathway in microorganisms⁸, one might expect a similar situation to exist in higher plants as well. Recently, Wightman et al.⁹ have indeed reported that shikimate-U-¹⁴C gives rise to tryptophan and gramine in young barley seedlings in the light. Similar investigations on the conversion of quinate-U-¹⁴C and shikimate-U-¹⁴C to tryptophan in bean seedlings in the light and dark have been under investigation in this laboratory.