Naturally impaired side-chain shortening of aromatic 3-ketoacyl-CoAs reveals the biosynthetic pathway of plant acetophenones

Acetophenones, which show scattered distribution across phylogenetically distant Plants and fungi, play diverse roles in plant-plant, plant-insect, plant-microbiome and even animal-insect interactions. However, the enzymatic basis of acetophenone biosynthesis in Plants remains unknown. Here we elucidate the complete biosynthetic pathway of picein (4-hydroxyacetophenone glucoside) from 4-coumaroyl-CoA using pear (Pyrus) as a study system. We demonstrate that in certain pear cultivars, the acetophenone moiety originates from an impaired side-chain shortening reaction of an aromatic 3-ketoacyl-CoA intermediate, a key step in the β-oxidative biosynthesis of benzoic acid. This impairment results from a loss-of-function mutation in a peroxisomal 3-ketoacyl-CoA thiolase. The accumulated aromatic 3-ketoacyl-CoA is subsequently hydrolysed by a thioesterase and undergoes spontaneous decarboxylation to yield the acetophenone moiety. This rare metabolic phenomenon highlights that not only neofunctionalization but also loss-of-function mutations can drive diversification in plant secondary metabolism. Forward genetic approaches are powerful to shed light on such 'hidden' or recessive pathways in Plants.