The Dual Role of Lignin in Fruit Trees: Unraveling Regulatory Networks from Stress Resilience to Quality Control.

Lignin deposition in fruit trees represents a fundamental physiological trade-off: essential for structural integrity and stress adaptation, yet excessive or mistimed activation compromises fruit texture, palatability, and market value. This review synthesizes advances in lignin biosynthesis and its multilayered regulation in commercial fruit species. We describe how abiotic (drought, salinity, temperature extremes) and biotic (pathogens, pests) stresses trigger lignification through transcriptional, post-transcriptional, hormonal, and epigenetic mechanisms, centered on the conserved NAC-MYB cascade. This core module integrates WRKY/ERF transcription factors (TFs), microRNA networks, and hormone signaling. Transcriptional programs are further refined by microRNAs, alternative splicing, DNA methylation, histone acetylation, and phytohormone crosstalk (abscisic acid, ABA; jasmonic acid, JA; salicylic acid, SA; and brassinosteroids, BRs). We emphasize molecular crosstalk integrating abiotic and biotic stress signaling via shared TFs, reactive oxygen species (ROS), and epigenetic memory. We critically examine lignification's dual nature during development and postharvest storage, contributing to desirable traits (stone formation and skin toughness) but also driving defects (stone cell gritty texture and chilling-induced wooliness). Finally, we propose a strategic framework leveraging molecular breeding, targeted gene editing, and precision horticulture to fine-tune lignification, enabling climate-resilient cultivars without compromising fruit quality.