From Defense Executor to Engineering Target: Harnessing Lignin for Crop Resistance.

Plants have evolved sophisticated defense mechanisms involving pathogen and pest perception, signal transduction, and immunity execution. While immune receptors and signaling components underlying the first two steps have been extensively studied over the past three decades, the downstream physical and chemical executors that plants deploy to combat pathogen invasion have received comparatively less attention. Lignin is a fundamental structural polymer in the plant secondary cell wall. In recent years, an increasing number of studies have demonstrated that plants actively employ lignin fortification as a defense strategy against a broad spectrum of biotic stresses, underscoring lignin's crucial role in immunity execution. In this review, we comprehensively summarize the genetic contributions of lignin-related genes to plant immunity and elucidate the regulatory mechanisms governing the dynamic activation of monolignol biosynthesis, transport, and polymerization following pathogen/pest attack. In addition, we discuss the physiological and biochemical basis of lignin-mediated immunity and highlight outstanding questions in the field. Furthermore, we summarize recent advances in enhancing crop resistance through biotechnological modulation of lignin content and composition. Overall, this review underscores the role of lignin as a stalwart defense executor and positions the lignin pathway as a strategic target for crop engineering toward enhanced resistance against diverse biotic threats.