Lignin: the primary component responsible for endocarp sclerosis in the development of Cornus officinalis fruit

Sclerified endocarp formation represents a defining developmental feature in Cornus officinalis drupes. We systematically investigated cytological dynamics and lignin metabolism to define the mechanistic basis of endocarp sclerification during Cornus officinalis fruit development. Results demonstrated that endocarp sclerification coincided with a significant increase in lignin content. Stone cell density exhibited developmental variation (239-529 piece·mg-1; mean: 400 piece·mg-1), while the microscopic characteristic index of stone cells progressively increased and stabilized at fruit maturity. Critically, the activities of pivotal lignin biosynthesis enzymes (PAL, CCR, CAD, POD) peaked during early fruit development and subsequently declined with progressive lignin accumulation-a trend corroborated by transcriptomic profiling. RNA-Seq analyses revealed stage-specific expression of lignin biosynthesis genes: PAL, C4H, CSE, POD, CAD, and CCR were highly expressed in young fruit but downregulated during later development. Conversely, COMT and F5H expression peaked at ripening, aligning with maximal lignin deposition and indicating their direct regulation of terminal lignin synthesis steps. Collectively, these findings demonstrate that Cornus officinalis endocarp hardening arises from synergistic stone cell differentiation and lignification, with phenylpropanoid metabolism initiated at juvenility and ripening-stage lignin deposition governed primarily by COMT and F5H.