ABSTRACT: Adventitious root (AR) formation is essential for grafted seedling propagation, yet its suppression in specific scion-rootstock combinations remains a significant challenge. To determine the mechanisms by which watermelon scions inhibit AR formation in pumpkin rootstocks, this study utilized a double-root-cutting system across three comparative treatments: non-grafted pumpkin (P), self-grafted pumpkin (P/P), and watermelon-grafted pumpkin (W/P). Histological analysis revealed that the watermelon scion completely inhibits cellular dedifferentiation and primordium initiation. Hormonal profiling confirmed that W/P rootstocks contained significantly lower indole-3-acetic acid (IAA) and higher trans-zeatin riboside (tZR) levels than P and P/P treatments, abolishing the requisite auxin-to-cytokinin ratio for AR induction. Metabolomic analysis revealed significant alterations in 32 key metabolites across the tryptophan, zeatin, and flavonoid/phenylpropanoid pathways, notably including indole-3-carboxylic acid, adenine, isopentenyladenosine, quercetin, and kaempferol. Transcriptomic profiling highlighted the profound enrichment of plant hormone signal transduction pathways, characterized by the systemic down-regulation of core auxin signaling components (AUX1, AUX, ARFs, SAUR) and the simultaneous up-regulation of cytokinin signaling elements (AHPs, ARRs). Furthermore, WGCNA identified specific transcription factors (CAMTA3, ILR3, AIL6) and functional hub genes (GH3.6, CYP735A, LOG, XTH, Pectinesterase) involved in the coordinated suppression of AR development. Collectively, this study elucidates the scion-induced transcriptional and metabolic disruptions that antagonize AR organogenesis, providing crucial genetic targets for improving rooting efficiency in elite grafted cucurbits.
