A Novel ZjbZIP33-ZjPRX1 Module Positively Regulates Lignin Formation in the Jujube Fruit Stone

Fruit stones play an important role in fruit development and are closely related to fruit quality. Lignification of the fruit stone (endocarp) is a typical feature of stone fruits. Stoneless jujube is a valuable jujube germplasm resource, and the degree of lignification of its fruit stone is low. However, the underlying mechanisms of lignification in the jujube endocarp remain unclear. In this study, we characterised a novel Class III peroxidase, PRX1, via comparative transcriptome analysis of the jujube endocarp and investigated its potential molecular mechanism in endocarp lignin synthesis. Subcellular localisation analysis revealed that ZjPRX1 was localised to the cytoplasm. According to the results of an in situ hybridisation analysis, the tissue-specific expression of ZjPRX1 was greatest in the endocarp. Moreover, ZjPRX1 overexpression in jujube fruits and calluses caused significant changes in the amount of lignin that accumulated. Scanning electron microscopy revealed that the cell walls of the stems of ZjPRX1-OE Arabidopsis lines were significantly thicker than those of WT Arabidopsis. Yeast one-hybrid, dual-luciferase reporter and electrophoretic mobility shift assays indicated that ZjbZIP33 activated ZjPRX1 expression by binding specifically to the G-box element in the ZjPRX1 promoter. IAA and GA₃ induced ZjbZIP33-ZjPRX1 expression and lignin formation. Overall, the results revealed a novel module, that is, ZjbZIP33-ZjPRX1, that positively regulates lignin formation in the jujube fruit stone. This study contributes to a better understanding of the molecular basis of lignin synthesis in the endocarp of jujube fruits and identifies candidate genes for optimising stone fruit breeding.