Gibberellin precursor biosynthesis inhibition by EPTC and reversal by R-25788

S-ethyl dipropylthiocarbamate (EPTC) inhibited gibberellic acid (GA) precursor biosynthesis in a cell-free enzyme preparation from unruptured, etiolated sorghum (Sorghum bicolor L. cv. G522 DR) coleoptiles. EPTC, 1 μM, inhibited incorporation of [¹⁴C]mevalonic acid into kaurene 60%, while 10 μM EPTC inhibited ¹⁴C incorporation into kaurene 90%. The precursor of kaurene cyclization (GGPP) increased in ¹⁴C content at both EPTC concentrations. R-25788 reversed the EPTC inhibition of kaurene synthesis. Kaurene oxidation was modified by both EPTC and R-25788. Hypothesized modes of action for EPTC and R-25788 are (a) inhibition of GA synthesis, (b) increased peroxidase activity resulting in increased lignification, (c) increased detoxification by sulfoxidation and carbamoylation, and (d) inhibition of fatty acid synthesis and/or desaturation. These hypotheses are discussed with three of them being incorporated into one working unit which correlates with EPTC and R-25788 symptom phenology. The fourth hypothesis could also fit into this general pattern.