CONTINUOUS LIGHT ALTERS INDOLE-3-ACETIC-ACID METABOLISM IN LEMNA-GIBBA

Turnover is an important parameter of indole-3-acetic acid (IAA) metab olism, and IAA turnover may also be linked to the modulation of auxin- regulated responses. Previous studies have not, however, addressed whe ther environmental factors, including light, affect the turnover of IA A. We have used a model system consisting of normal Lemna gibba and a line of L. gibba that overproduces IAA precursors (MTR1) to measure IA A turnover in intact plants exposed to constant light, constant light followed by 24 h darkness, and a diurnal (12 h light) cycle. Turnover rates were determined by pulse labeling with [C-13(6)]IAA tracer follo wed by analysis by GC-MS. Free IAA levels and IAA uptake rates were al so determined. These studies showed that IAA uptake in both lines was greater in the light in both light conditions (continuous and diurnal) than in the dark. In the dark, IAA turnover time in both lines was le ss than 1 h and IAA levels were 4-6 ng/g fr. wt. Under constant light, IAA levels were higher in MTR1 and turnover time was much longer in t he normal line. These studies suggest-based on the difference in respo nse between the MTR1 and the normal line-that IAA turnover is regulate d both by the rate of biosynthesis as well as the rate of catabolism. (C) 1998 Elsevier Science Ltd. All rights reserved.