Phenylalanine Ammonia-Lyase: A Core Regulator of Plant Carbon Metabolic Flux Redistribution—From Molecular Mechanisms and Growth Modulation to Stress Adaptability

Phenylalanine ammonia-lyase (PAL) is the core branch-point enzyme connecting plant primary aromatic amino acid metabolism to the phenylpropanoid pathway, which determines carbon flux redistribution between growth and defense and is essential for plant adaptation to various environments. Extensive research has clarified PAL’s conserved homotetrameric structure, MIO cofactor-dependent catalytic mechanism, and its roles in plant growth, development, and stress responses. However, there is a lack of comprehensive review studies focusing on PAL-mediated carbon metabolic flux redistribution, specifically covering its structural and evolutionary foundations, the links between this flux regulation and plant growth/development, its multi-layered regulatory network, and its roles in stress adaptation, limiting a comprehensive understanding of its evolutionary and functional diversity. This review systematically covers four core aspects: first, the molecular foundation, encompassing PAL’s structural features and catalytic specificity governed by the MIO cofactor; second, evolutionary diversity spanning from algae to angiosperms, with emphasis on unique regulatory mechanisms and evolutionary significance across lineages; third, the multi-layered regulatory network, integrating transcriptional control, post-translational modifications, epigenetic regulation, and functional crosstalk with phytohormones; and fourth, functional dynamics, which elaborate PAL’s roles in organ development, including root lignification, stem mechanical strength, leaf photoprotection, flower and fruit quality formation, and lifecycle-wide dynamic expression, as well as its mediated stress adaptations and regulatory networks under combined stresses. These insights provide a theoretical basis for targeted manipulation of PAL to optimize crop carbon allocation, thus improving growth performance, enhance stress resilience, and promote sustainable agriculture.