Short chain dehydrogenase; part of the gene cluster that mediates the biosynthesis of terretonin, a fungal meroterpenoid that acts as a mycotoxin. The first step of the pathway is the synthesis of 3,5-dimethylorsellinic acid (DMOA) by the polyketide synthase trt4. DMOA is then prenylated into farnesyl-DMOA by the polyprenyl transferase trt2. Methylation by the methyltransferase trt5 then leads to farnesyl-DMOA methyl ester which is further subject to epoxidation by the FAD-dependent monooxygenase trt8 to yield epoxyfarnesyl-DMOA methyl ester. Cyclization of epoxyfarnesyl-DMOA methyl ester by the terpene cyclase trt1 leads to a tetracycle intermediate which is in turn converted to preterretonin. Dehydrogenase trt9 comes next to transform preterretonin to preterrenoid. The FAD-dependent monooxygenase trt3 is then required for the C-hydroxylation at C16 of preterrenoid to yield terrenoid. The cytochrome P450 trt6 catalyzes three successive oxidations to transform terrenoid into an unstable intermediate, which then undergoes the D-ring expansion and unusual rearrangement of the methoxy group to afford the core skeleton of terretonin. Trt14 catalyzes the D-ring expansion of terretonin involving intramolecular methoxy rearrangement as well as the hydrolysis of the expanded D-ring and the methyl ester moiety. Finally, the nonheme iron-dependent dioxygenase trt7 accomplishes the last two oxidation reactions steps to complete the biosynthesis of terretonin. Terretonin C is produced via spontaneous decarboxylation of the terretonin precursor. Another shunt product of the terretonin biosynthesis is dihydrofarnesyl-DMOA, derived from epoxyfarnesyl-DMOA through hydrolysis of the epoxide