In Vivo Measurement of Indole-3-acetic Acid Decarboxylation in Aging Coleus Petiole Sections

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Abstract
The concentration of IAA in plant tissues is regulated, in part, by its rate of decarboxylation. However, the commonly used in vitro assays for IAA oxidase may not accuratley reflect total in vivo decarboxylation rates. A method for measuring in vivo decarboxylation was utilized in which 14CO2 is collected following uptake of [1-14C]IAA by excised tissue sections. After a 30-min equilibration period, the evolution of 14CO2 followed an approximately linear course with respect to both time and tissue weight. Decarboxylation rates were measured by this method in petiole sections of the Princeton clone of C. blumei Benth. Both the 14CO2 evolved per milligram tissue and the percent of [1-14C]IAA uptake decarboxylated were highest in sections from the youngest petioles tested, and declined in the older tissue. TLC of acetonitrile extracts from the [1-14C]IAA-treated petioles showed a decreasing amount of free IAA and an increase at the retardation factor of indoleacetylasparate in the older sections. The decreased decarboxylation rates in the older petioles may be attributable to a generally lower metabolic rate and increased protection of the IAA conjugation.