Discovery, Herbicidal Activity and Biosynthesis of a Novel Natural Tetramic Acid from Alternaria Species

Abstract The tetramic acid moiety is a pivotal structural unit in numerous natural products. As an analogue of the simplest tetramic acid compound tenuazonic acid (TeA), sec‐pentyl‐TeA (S‐TeA) exhibits double herbicidal activity of TeA. Here, this work identifies S‐TeA as a novel natural product synthesized by Alternaria alternata and three other filamentous fungi. Chiral analysis confirm the absolute configuration of natural S‐TeA as (5 S , 6 S ). Configuration‐bioactivity studies reveal that natural (5 S , 6 S )‐S‐TeA is the eutomer and possesses the highest herbicidal activity among all tested diastereomers. Biosynthetic analyses demonstrate that threonine is the precursor to S‐TeA, beginning with the production of 2‐amino‐3‐methylhexanoic acid (AMHA) via eight enzymes from the branched‐chain amino acid (BCAA) biosynthetic pathway, including threonine deaminase, 2‐isopropylmalate synthase (IPMS), 3‐isopropylmalate dehydratase (IPMDH), isopropylmalate dehydrogenase (ISMD), acetolactate synthase, ketol‐acid reductoisomerase, dihydroxy acid dehydratase, and BCAA aminotransferase. Subsequently, AMHA undergoes acetylation and cyclization by non‐ribosomal peptide synthetases to form S‐TeA. Distinct differences in the biosynthetic pathways of S‐TeA and TeA are identified. In vitro studies confirm the critical roles of three unique enzymes IPMS, IPMDH, and ISMD in S‐TeA biosynthesis, which are absent in TeA biosynthesis. These findings provide a solid basis for developing S‐TeA as a natural product herbicide.