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.

Jujube witches’ broom (JWB) caused by phytoplasma has a severely negative effect on multiple metabolisms in jujube. The GST gene family in plants participates in the regulation of a variety of biotic and abiotic stresses. This study aims to identify and reveal the changes in the jujube GST gene family in response to phytoplasma infection. Here, 70 ZjGSTs were identified in the jujube genome and divided into 8 classes. Among them, the Tau-class, including 44 genes, was the largest. Phylogenetic analysis indicated that Tau-class genes were highly conserved among species, such as Arabidopsis, cotton, chickpea, and rice. Through chromosome location analysis, 37.1% of genes were clustered, and 8 of 9 gene clusters were composed of Tau class members. Through RT-PCR, qRT-PCR and enzyme activity detection, the results showed that the expression of half (20/40) of the tested ZjGSTs was inhibited by phytoplasma infection in field and tissue culture conditions, and GST activity was also significantly reduced. In the resistant and susceptible varieties under phytoplasma infection, ZjGSTU49-ZjGSTU54 in the cluster Ⅳ showed opposite expression patterns, which may be due to functional divergence during evolution. Some upregulated genes (ZjGSTU45, ZjGSTU49, ZjGSTU59, and ZjGSTU70) might be involved in the process of jujube against JWB. The yeast two-hybrid results showed that all 6 Tau-class proteins tested could form homodimers or heterodimers. Overall, the comprehensive analysis of the jujube GST gene family revealed that ZjGSTs responded actively to phytoplasma infection. Furthermore, some screened genes (ZjGSTU24, ZjGSTU49-52, ZjGSTU70, and ZjDHAR10) will contribute to further functional studies of jujube-phytoplasma interactions.

The juvenile-to-adult phase change with first flowering as the indicator plays a crucial role in the lifecycle of fruit trees. However, the molecular mechanisms underlying phase change in fruit trees remain largely unknown. Shikimic acid (ShA) pathway is a main metabolic pathway closely related to the synthesis of hormones and many important secondary metabolites participating in plant phase change. So, whether ShA regulates phase change in plants is worth clarifying. Here, the distinct morphological characteristics and the underlying mechanisms of phase change in jujube (Ziziphus jujuba Mill.), an important fruit tree native to China with nutritious fruit and outstanding tolerance abiotic stresses, were clarified. A combined transcriptome and metabolome analysis found that ShA is positively involved in jujube (‘Yuhong’ × ‘Xing 16’) phase change. The genes in the upstream of ShA synthesis pathway (ZjDAHPS, ZjDHQS and ZjSDH), the contents of ShA and the downstream secondary metabolites like phenols were significantly upregulated in the phase change period. Further, the treatment of spraying exogenous ShA verified that ShA at a very low concentration (60 mg · L^-1) can substantially speed up the phase change and flowering of jujube and other tested plants including Arabidopsis, tomato and wheat. The exogenous ShA (60 mg · L^-1) treatment in jujube seedlings could increase the accumulation of endogenous ShA, enhance leaf photosynthesis and the synthesis of phenols especially flavonoids and phenolic acids, and promote the expression of genes (ZjCOs, ZjNFYs and ZjPHYs) involved in flowering pathway. Basing on above results, we put forward a propose for the underlying mechanism of ShA regulating phase change, and a hypothesis that ShA could be considered a phytohormone-like substance because it is endogenous, ubiquitous, movable and highly efficient at very low concentrations. This study highlights the critical role of ShA in plant phase change and its phytohormone-like properties.