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. Using pear as a study system, the biosynthetic pathway of acetophenones has been elucidated: naturally impaired side-chain shortening of aromatic 3-ketoacyl-CoAs leads them to undergo hydrolysis, followed by decarboxylation, to yield acetophenones.