Enhanced phenylalanine biosynthesis amplifies light-stress-driven phenylpropanoid production in Arabidopsis

Phenylpropanoids are L-phenylalanine (Phe)-derived specialized metabolites with crucial roles in plant stress adaptation. Among various abiotic stresses, high-light (HL) is a major threat that leads to oxidative damage and therefore upregulates the biosynthesis of antioxidant phenylpropanoids, including anthocyanins, in plants. Phenylpropanoid production is initiated by the conversion of Phe by phenylalanine ammonia-lyase (PAL) enzyme, which acts as the key gatekeeper controlling carbon influx from the Phe pool into phenylpropanoid biosynthesis. Despite the importance of Phe as a precursor for phenylpropanoid production, we have limited knowledge of how Phe precursor availability influences downstream pathways, particularly in the context of stress adaptation. To tackle this question, Arabidopsis thaliana wild-type and production of anthocyanin pigment 1 dominant (pap1D) mutant, a line genetically enhanced for anthocyanin production, were subjected to HL treatment, followed by proteome and metabolome analyses. Our multi-omics data indicated that Phe biosynthesis was insufficiently responsive to meet the increased precursor demand from the downstream phenylpropanoid pathway, likely making Phe availability a rate-limiting factor for HL-induced phenylpropanoid production. We then generated pap1D double mutants with Phe-overaccumulating mutants. Untargeted metabolomics revealed that enhanced Phe availability had little impact on phenylpropanoid accumulation under standard growth conditions but additively promoted the accumulation of specific phenylpropanoid intermediates and anthocyanin species under HL stress. This metabolic difference between light treatment and genotypes was correlated with the activation of PAL enzymatic activity. This study demonstrates that enhanced Phe biosynthesis amplifies HL-induced phenylpropanoid biosynthesis.