Short-chain dehydrogenase/reductase; part of the gene cluster that mediates the biosynthesis of the mycotoxin pyrichalasin H, a tyrosine-derived cytochalasan that inhibits the growth of rice seedlings, but also inhibits lymphocyte capping and actin polymerization and alters cell morphology (PubMed:31099577) (Probable). Pyrichalasin H is indicated as the responsible agent for the genus-specific pathogenicity of M.grisea toward crabgrass (PubMed:31099577). The first step in the pathway is catalyzed by the O-methyltransferase pyiA which methylates free tyrosine to generate the precursor O-methyltyrosine (PubMed:31099577). The hybrid PKS-NRPS pyiS, assisted by the enoyl reductase pyiC, are responsible for fusion of the O-methyltyrosine precursor and the polyketide backbone (PubMed:31099577). The polyketide synthase module (PKS) of pyiS is responsible for the synthesis of the polyketide backbone and the downstream nonribosomal peptide synthetase (NRPS) amidates the carboxyl end of the polyketide with the O-methyltyrosine precursor (PubMed:31099577). As the NRPS A-domain demonstrates substrate tolerance, pyiS can also use phenylalanine, tyrosine and even para-chlorophenylalanine as amino acid precursor, which leads to the production of novel cytochalasans, including halogenated cytochalasans (PubMed:31099577). Because pyiS lacks a designated enoylreductase (ER) domain, the required activity is provided the enoyl reductase pyiC (PubMed:31099577). Reduction by the hydrolyase pyiE leads to 1,5-dihydropyrrolone, which is substrate for dehydration and intra-molecular Diels-Alder cyclization by the Diels-Alderase pyiF to yield the required isoindolone-fused macrocycle (PubMed:32039410). The tailoring cytochrome P450 monooxygenases piyD and piyG catalyze the hydroxylation at C-18 and C-7, respectivily, whereas the short-chain dehydrogenase/reductase pyiH reduces the carbonyl at C-21 in preparation for the transfer of an acetyl group by the acetyltransferase pyiB (PubMed:31099577). These 3 reactions whose order is not clear yet, lead to the production of O-methylpyrichalasin J, a deacetylated pyrichalasin H (PubMed:31099577). Finally, pyiB to converts O-methylpyrichalasin J into the final product pyrichalasin H via acetylation of C-21 (PubMed:31099577). Mycotoxin biosynthesis. Leads to the loss of pyrichalasin H production, but accumulates novel cytochalasans. Belongs to the short-chain dehydrogenases/reductases (SDR) family.