Amino Acid Metabolism in Plants. V. Changes in Basic Indole Compounds and the Development of Tryptophan Decarboxylase Activity in Barley (Hordeum vulgare) during Germination and Seedling Growth

Abstract

In barley seedlings, tryptophan is the precursor of the simple indole alkaloid gramine, and also of tryptamine, which is important as a potential precursor of the plant growth hormone 3-indoleacetic acid. The present investigation was designed to study the distribution of free tryptophan and its derivatives within the seedlings, and to follow the changes in these compounds with time. Development of the enzyme tryptophan decarboxylase, which catalyzes the conversion of tryptophan to tryptamine, was also studied. An increase in free tryptophan was detected within 2 h of soaking the seed; this compound reached high values in very young tissues, and then declined. Gramine and its precursors, 3-aminomethylindole and N-methyl-3-aminomethylindole, were confined to the shoots; all three compounds appeared together at the inception of shoot growth. Quantitatively, gramine was the most important compound present and reached a concentration of 623 μg/g fresh weight (25 times that of free tryptophan) on the 9th day, and then declined. Isolated embryos were capable of synthesizing gramine at about one quarter the normal rate, indicating that these embryos have a considerable inherent capacity for tryptophan synthesis and are not wholly dependent on tryptophan released by the endosperm. Tryptophan decarboxylase and tryptamine were found only in the shoot, and both enzyme and product appeared after the 1st week of growth, when the rate of gramine synthesis was beginning to decline. 5-Hydroxytryptamine began to accumulate in both shoot and root after about 2 weeks of growth, and N-methyl-5-hydroxytryptamine was also present in the roots. The close parallel between the gramine pathway of the barley shoot and the analagous hordenine pathway of the root, in which tyrosine is the precursor amino acid, is discussed.