The monolignol pathway contributes to the biosynthesis of volatile phenylpropenes in flowers

Summary Volatile phenylpropenes play important roles in the mediation of interactions between plants and their biotic environments. Their biosynthesis involves the elimination of the oxygen functionality at the side‐chain of monolignols and competes with lignin formation for monolignol utilization. We hypothesized that biochemical steps before the monolignol branch point are shared between phenylpropene and lignin biosynthesis; however, genetic evidence for this shared pathway has been missing until now. Our hypothesis was tested by RNA i suppression of the petunia ( P etunia hybrida ) cinnamoyl‐CoA reductase 1 (Ph CCR 1), which catalyzes the first committed step in monolignol biosynthesis. Detailed metabolic profiling and isotopic labeling experiments were performed in petunia transgenic lines. Downregulation of Ph CCR 1 resulted in reduced amounts of total lignin and decreased flux towards phenylpropenes, whereas internal and emitted pools of phenylpropenes remained unaffected. Surprisingly, Ph CCR 1 silencing increased fluxes through the general phenylpropanoid pathway by upregulating the expression of cinnamate‐4‐hydroxylase ( C4H ), which catalyzes the second reaction in the phenylpropanoid pathway. In conclusion, our results show that Ph CCR 1 is involved in both the biosynthesis of phenylpropenes and lignin production. However, Ph CCR 1 does not perform a rate‐limiting step in the biosynthesis of phenylpropenes, suggesting that scent biosynthesis is prioritized over lignin formation in petals.