Redefining phytoalexins as engineered defenses for plant disease resistance

Phytoalexins are inducible secondary metabolites that play a pivotal role in the plant’s innate immunity. They function as antimicrobial agents and signal molecules in response to pathogen attack. Structurally diverse groups—such as flavonoids, terpenoids, and alkaloids enable plants to mount broad-spectrum defenses. Although natural phytoalexins are central and evolutionarily conserved components of plant defense, their rapid turnover, spatial restriction, and susceptibility to pathogen detoxification can sometimes limit duration or spectrum of protection, particularly under high disease pressure. Therefore, in addition to enhancing their biosynthesis and stability, targeted structural modifications enabled by molecular engineering may further optimize their activity and strengthen durable resistance in crops. Molecular engineering approaches, including transcription factor engineering, metabolic engineering, synthetic biology, CRISPR/Cas9 genome-editing, and epigenetic regulation, offer power tools to enhance phytoalexin biosynthesis and functionality. Emerging strategies aim to develop specialized phytoalexins with improved stability, potency, and a broader range of action. When integrated with modern breeding and biotechnological platforms, these molecular innovations can enhance crops resilience. Despite challenges such as metabolic trade-offs, and potential growth–defense imbalances, engineered phytoalexins represent a promising avenue for next-generation plant defense. This review summarizes recent developments, challenges, and prospects of phytoalexins as designer defenses in the molecular engineering era. • Phytoalexins are inducible secondary metabolites with antimicrobial and signaling roles • Phytoalexins’ structural diversity underpins broad-spectrum defense • Molecular engineering enhances phytoalexin biosynthesis and activity • Designer phytoalexins offer improved stability, potency, and pathogen specificity • Engineered phytoalexins can reduce pesticide use and promote sustainable agriculture