• Hard endocarp or stone significantly impacts fruit quality and downstream processing, making it a critical factor in fruit production and utilization. • Lignin deposition in the secondary cell wall determines the hardness of the stone, composed of coniferyl, sinapyl, and p-coumaroyl alcohols. • The structural genes encoding lignin biosynthetic enzymes include LAC12–1, PAL2, C4H, C3H, CSE, CCoAOMT, PRX42, PRX73 and PRX1 . • Delignification of seed stones is regulated by miR397a, Pistillata, miR31-3p, miR8-5p, MYB-32, MYB24, bZIP48, and bZIP33, FUL , and REPLUMLESS . • Phytohormones regulate endocarp and stone cell lignification by controlling transcription factors and lignin biosynthesis genes. A hard endocarp (i.e., stone) inside fruit is a characteristic of drupe fruits such as jujube, peach, mango, etc. Hard stone significantly affects the quality and downstream processing of fruits. The complex aromatic polymer lignin deposition in the secondary cell wall determines stone hardness. Lignin comprises phenylpropanoid units formed by hydroxycinnamoyl alcohol, which includes coniferyl, sinapyl, and p -coumaroyl alcohols. Lignin biosynthesis pathway involves a series of complex enzymatic reactions initiated from phenylalanine ammonia lyase and ends up polymerizing lignin monomers by laccase and peroxidase enzymes. Phytohormones, particularly auxin, gibberellins, and Ca²⁺ signaling, further modulate endocarp lignification by regulating transcriptional networks and lignin biosynthetic genes, thereby fine-tuning secondary cell wall thickening and stone hardness in drupe fruits. Lignin biosynthesis is controlled by both structural genes and transcriptional regulators. The structural genes encoding lignin biosynthetic enzymes include LAC12–1, PAL2, C4H, C3H, CSE, CCoAOMT, F5H, CAD , and PRX1 . In addition, several transcription factors regulating secondary cell wall and lignin deposition, such as MYB24, bZIP48 , and bZIP33 play key regulatory roles. Conversely, delignification or suppression of stone formation is associated with transcription factors ( Pistillata, MYB32, FUL , and REPLUMLESS ) and post-transcriptional regulators, including miR397a, miR31-3p , and miR8-5p . Accurate alteration in the expression of these genes will result in the attainment of stoneless fruits for cheap and hazel-free downstream processing.
Tahir et al. (Sun,) studied this question.