Tyramine hydroxycinnamoyl transferase in the roots of wheat and barley seedlings

Abstract

Tyramine hydroxycinnamoyl transferase (THT) activity was detected in extracts of the roots of wheat and barley seedlings. Maximal activity occurred two to three days after germination in the dark at 25°. Activity then declined rapidly, but could still be detected after 10 days of germination. Activity in the shoots was much lower than in the roots throughout this period. THT activity was detected in all the wheat and barley cultivars tested, but was very low in oats. The transferases extracted from wheat and barley seedlings exhibited the same requirements for optimal activity and had similar specificity. Maximal activity was obtained using tyramine and sinapoyl-CoA as substrates, but tyramine was also rapidly conjugated to feruloyl-CoA and to a lesser extent to p-coumaroyl-CoA. Phenethylamine and p-methoxyphenethylamine also acted as substrates, but were less efficient than tyramine. None of the amides synthesized in vitro could be detected in extracts of the roots of wheat seedlings, even in the varieties in which THT was the most active.