Metabolic engineering strategies for optimized lignan production in plants

Plant secondary metabolites, including lignans, play essential roles in plant defense and adaptation, and their pharmacological properties are increasingly valued for human health. Lignans are dimers derived from the phenylpropanoid pathway whose biosynthesis is tightly controlled by dirigent proteins, laccases, and other redox-related enzymes. Recent advances in plant metabolic engineering have progressed from simple single-gene overexpression to integrated strategies that combine transcriptional regulation, metabolic flux optimization, and CRISPR-based genome editing. This review proposes the "Push-Pull-Release" framework to organize these approaches. This framework utilizes three complementary mechanisms: "Push" strategies to increase precursor supply through master transcription factors such as AtMYB85 and enzymatic overexpression; "Pull" methods to redirect flux by attenuating competing metabolic sinks, including CHS- and F5H-associated branches; and "Release" mechanisms that alleviate intrinsic pathway repression by targeting MYB repressors and post-translational regulators such as KFB proteins. Advanced control is further achieved through synthetic biology principles, including modular pathway reconstruction, multiplex genome editing, and spatiotemporal regulation through tissue-specific, inducible, and subcellular engineering strategies. Case studies on sesamin, podophyllotoxin, and a proposed SDG production framework in flax illustrate how these multi-layered strategies may be integrated in plant systems. However, maximizing lignan yield must be balanced against trade-offs in plant structural integrity and disease resistance. Accordingly, future lignan metabolic engineering should integrate multi-layered controls with spatiotemporal regulation and systematic phenotypic evaluation to achieve sustainable production while preserving plant fitness.