Transcriptional Activation of <i>ZjPME36</i> by ZjPosF21 and Repression by ZjWRKY48 Together Regulate Fruit Size in Chinese Jujube

Pectin methylesterases (PMEs) are essential enzymes that catalyze the demethylesterification of pectins, thereby facilitating their degradation. They have been implicated in fruit ripening, but their role in fruit size regulation remains unclear. This study found that ZjPME36 was more highly expressed in small-fruit cultivars than in large-fruit cultivars of Chinese jujube ( Ziziphus jujuba Mill.). Overexpression of ZjPME36 in tomato significantly reduced the fruit size by restricting mesocarp cell expansion. Concurrently, the content of PME increased significantly, while both the degree of pectin methylesterification and total pectin content decreased. Furthermore, the transcription factors ZjPosF21 and ZjWRKY48 were identified as an activator and a repressor of ZjPME36, respectively. They coordinately contribute to the final fruit size of jujube by altering pectin content. This research sheds new light on the molecular pathway for PME-mediated fruit size regulation in jujube.