Continuous light alters indole-3-acetic acid metabolism in Lemna gibba

Turnover is an important parameter of indole-3-acetic acid (IAA) metabolism, and IAA turnover may also be linked to the modulation of auxin- regulated responses. Previous studies have not, however, addressed whether environmental factors, including light, affect the turnover of IAA. We have used a model system consisting of normal Lemna gibba and a line of L. gibba that overproduces IAA precursors (MTR1) to measure IAA 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 [¹³C₆]IAA tracer followed by analysis by GC-MS. Free IAA levels and IAA uptake rates were also 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 less 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 the normal line. These studies suggest-based on the difference in response between the MTR1 and the normal line-that IAA turnover is regulated both by the rate of biosynthesis as well as the rate of catabolism.