SELECTION AND CHARACTERIZATION OF ALPHA-METHYLTRYPTOPHAN-RESISTANT LINES OF LEMNA-GIBBA SHOWING A RAPID RATE OF INDOLE-3-ACETIC-ACID TURNOVER

Turnover rate is an important aspect of the regulation of plant proces ses by plant growth substances. To study turnover of indole-3-acetic a cid (IAA), two alpha-methyltryptophan-resistant lines (MTR1 and MTR2) of Lemna gibba were generated by nitrosomethyl urea treatment of an in bred line derived from L. gibba G-3. In this report we describe: (a) t he development of a selection system using this near isogenic line of L. gibba; (b) techniques for chemical mutation of the lines and select ion for alpha-methyltryptophan resistance; and (c) the partial charact erization of the selected lines. MTR lines contained 3-fold higher lev els of anthranilate synthase activity. The enzyme in the MTR lines req uired higher levels of tryptophan for feedback inhibition. MIR lines a lso contained 8-fold higher levels of tryptophan, 3-fold higher levels of free IAA, and similar levels of total IAA compared to the inbred l ine. Turnover rates in the inbred and selected lines were calculated, using the first-order rate equation, based on the decrease over time i n isotopic enrichment of C-13(6)-IAA introduced into L. gibba during a 1-h pulse period. Isotope enrichment in IAA was determined by using g as chromatography-mass spectrometry. Both MTR lines had an approximate ly 10-fold higher rate of IAA turnover than the parent inbred line.