Metabolic engineering of a tyrosine-specific phenylpropanoid pathway in plants

While all plants use L-phenylalanine for phenylpropanoid biosynthesis, grasses can also initiate the pathway from L-tyrosine. Curiously, no plant has evolved an exclusive tyrosine-derived route. We generate plants with phenylpropanoid biosynthesis initiated from phenylalanine, tyrosine, or both by expressing a Brachypodium phenylalanine/tyrosine ammonia-lyase (PTAL) in Arabidopsis WT and c4h mutants. Engineering a bifunctional phenylpropanoid pathway in WT plants did not negatively impact growth, while introducing a tyrosine-specific pathway in the c4h mutant could overcome the seedling-lethal phenotype. Interestingly, restored c4h mutants relying solely on the tyrosine route displayed developmental defects linked to the strong overaccumulation of the auxin transport inhibitor cis-cinnamic acid. Our findings suggest that the requirement of this widely overlooked plant metabolite could be the crucial factor for the evolutionary retention of the canonical phenylpropanoid biosynthesis route via L-phenylalanine in plants.