FAD-linked oxidoreductase; part of the gene cluster that mediates the biosynthesis of betaenones, phytotoxic polyketides involved in leaf spot disease in sugar beets (PubMed:25530455). The first step of the pathway is the synthesis of dehydroprobetaenone I by the polyketide synthase bet1 and the enoyl reductase bet3 via condensation of one acetyl-CoA starter unit with 7 malonyl-CoA units and 5 methylations (PubMed:25530455). The C-terminal reductase (R) domain of bet1 catalyzes the reductive release of the polyketide chain (PubMed:25530455). Because bet1 lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase bet3 (PubMed:25530455). The short-chain dehydrogenase/reductase bet4 then catalyzes reduction of dehydroprobetaenone I to probetaenone I (PubMed:25530455). The cytochrome P450 monooxygenase bet2 catalyzes successive epoxidation, oxidation (resulting from epoxide opening) and hydroxylation to install a tertiary alcohol in the decaline ring to yield betaenone C from dehydroprobetaenone I and betaenone B from probetaenone I (PubMed:25530455). The FAD-linked oxidoreductase (orf1) is probably responsible for the conversion of betaenone C to betaenone A via an intramolecular aldol reaction between C-1 and C-17 to form the bridged tricyclic system in betaenone A (By similarity). betaenone C = betaenone A Mycotoxin biosynthesis. Belongs to the oxygen-dependent FAD-linked oxidoreductase family.