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Metabolism and Enzymology

Stable Isotope Labeling, in Vivo, of D- and L-Tryptophan Pools in Lemna gibba and the Low Incorporation of Label into Indole-3-Acetic Acid ¹

U.S. Department of Agriculture, Agricultural Research Service, Plant Hormone Laboratory, Product Quality and Development Institute, Beltsville Agricultural Research Center, Beltsville, Maryland 20705, Department of Botany, University of Maryland, College Park, Maryland 20742

We present evidence that the role of tryptophan and other potential intermediates in the pathways that could lead to indole derivatives needs to be reexamined. Two lines of Lemna gibba were tested for uptake of [¹⁵N-indole]-labeled tryptophan isomers and incorporation of that label into free indole-3-acetic acid (IAA). Both lines required levels of L-[¹⁵N]tryptophan 2 to 3 orders of magnitude over endogenous levels in order to obtain measurable incorporation of label into IAA. Labeled L-tryptophan was extractable from plant tissue after feeding and showed no measurable isomerization into D-tryptophan. D-[¹⁵N]tryptophan supplied to Lemna at rates of approximately 400 times excess of endogenous D-tryptophan levels (to yield an isotopic enrichment equal to that which allowed detection of the incorporation of L-tryptophan into IAA), did not result in measurable incorporation of label into free IAA. These results demonstrate that L-tryptophan is a more direct precursor to IAA than the D isomer and suggest (a) that the availability of tryptophan in vivo is not a limiting factor in the biosynthesis of IAA, thus implying that other regulatory mechanisms are in operation and (b) that L-tryptophan also may not be a primary precursor to IAA in plants.

³ Present address: USDA/ARS Plant Hormone Laboratory, Beltsville, MD 20705.

¹ Supported by the National Science Foundation grant DCB-8917378 and by funds from the U.S. Department of Agriculture, Agricultural Research Service (USDA/ARS). A USDA/ARS postdoctoral fellowship (for support of B. G. B) is also gratefully acknowledged.

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