Transcriptome-based analysis of lignin accumulation in the regulation of fruit stone development and endocarp hardening in Chinese jujube

Stone fruits, also known as drupes, have evolved an extremely hard wood-like shell called a stone to protect the seeds. Recently, the market value of stoneless cultivars has risen dramatically, which highlights the need to cultivate stoneless fruit. Therefore, determining the underlying mechanism of fruit stone development is urgently needed. By employing the stone-containing jujube cultivar ‘Youhe’ and two stoneless Chinese jujube cultivars, ‘Wuhefeng’ and ‘Daguowuhe’, we comprehensively studied the mechanism of fruit stone development in jujube. Anatomical analysis and lignin staining revealed that the stone cultivar ‘Youhe’ jujube exhibited much greater lignin accumulation in the endocarp than the two stoneless cultivars. Lignin accumulation may be the key factor in fruit stone formation. By analyzing the transcriptome data and identifying differentially expressed genes (DEGs), 49 overlapping DEGs were identified in the comparisons of ‘Youhe’ jujube vs. ‘Wuhefeng’ jujube and ‘Youhe’ jujube vs. ‘Daguowuhe’ jujube. ZjF6H1-3 and ZjPOD, which are involved in lignin synthesis, were identified among these DEGs. The overexpression and silencing of ZjF6H1-3 and ZjPOD in wild jujube seedlings further confirmed their roles in lignin synthesis. In addition, two bHLH transcription factors were included in the 49 overlapping DEGs, and bHLH transcription factor binding motifs were found in the promoters of ZjF6H1-3 and ZjPOD, indicating that bHLH transcription factors are also involved in lignin synthesis and stone formation in Chinese jujube. This study provides new insights into the molecular networks underlying fruit stone formation and can serve as an important reference for the molecular design and breeding of stoneless fruit cultivars of jujube and fruit trees.