Short-chain dehydrogenase/reductase; part of the gene cluster that mediates the biosynthesis of abscisic acid (ABA), a phytohormone that acts antagonistically toward salicylic acid (SA), jasmonic acid (JA) and ethylene (ETH) signaling, to impede plant defense responses (PubMed:15240257, PubMed:16820452). The first step of the pathway catalyzes the reaction from farnesyl diphosphate to alpha-ionylideneethane performed by the alpha-ionylideneethane synthase aba3 via a three-step reaction mechanism involving 2 neutral intermediates, beta-farnesene and allofarnesene (PubMed:30226766). The cytochrome P450 monooxygenase aba1 might then be involved in the conversion of alpha-ionylideneethane to alpha-ionylideneacetic acid (Probable). Alpha-ionylideneacetic acid is further converted to abscisic acid in 2 steps involving the cytochrome P450 monooxygenase aba2 and the short-chain dehydrogenase/reductase aba4, via the intermediates 1'-deoxy-ABA or 1',4'-trans-diol-ABA, depending on the order of action of these 2 enzymes (Probable). Aba2 is responsible for the hydroxylation of carbon atom C-1' and aba4 might be involved in the oxidation of the C-4' carbon atom (PubMed:16820452). Hormone biosynthesis. Constitutively expressed at a low level. Reduces the production of abscisic acid (ABA) and accumulates a compound that corresponds probably to 1',4'-trans-diol-ABA. Belongs to the short-chain dehydrogenases/reductases (SDR) family.