Evaluation of Indole-3-acetic Acid Decarboxylating Activity in Hypocotyl Sections of Etiolated Lupinus albus Seedlings

Abstract

Summary It has been suggested that the endogenous Indole-3-acetic acid (IAA) level in plant tissues is regulated, in part, by its decarboxylation. However, in vivo decarboxylation assays do not permit one to attribute a definitive role to this metabolic pathway in the control of the normal plant growth. Therefore a method was developed in which 14 CO 2 is collected during incubation in [1- 14 C]-IAA of excised hypocotyl sections of etiolated lupin ( Lupinus albus ) seedlings to measure the in vivo IAA decarboxylation. IAA decarboxylation by purified peroxidase (HRP) was utilized as a model system to calibrate the C0 2 measurements as well as a reference system to evaluate the relative incidence of decarboxylation in the IAA metabolism by lupin sections. The 14 C0 2 evolved per mg tissue was linear for 1.5 h. In this period 6-15 % of total 14 CO 2 was produced. After a temporary fall in the rate (from 1.5 to 3 h), decarboxylation increased again and evolution of 14 C02 was detected until 20-24 h. It is suggested that an inactivation of the decarboxylating activity occurs during the enzymatic IAA breakdown, and new activity is synthesized to explain the restoration of the IAA decarboxylation. The percentage of initial IAA decarboxylated by lupin hypocotyl sections was higher in rapidly-growing hypocotyls of 7 day old seedlings than in non-growing hypocotyls of 18 days. Localization of the section along the hypocotyl influenced decarboxylation mainly at an intermediate age, decarboxylation being higher in apical (growing) than in middle (non growing) sections. Thus a correlation between decarboxylation and cell elongation is suggested.