Deciphering the Role of Phytohormones Regulated by the Scion in the Adventitious Root Development of Grafted Watermelon Seedlings

Adventitious roots (ARs) are crucial for grafted seedlings, playing vital roles in nutrient absorption, stress resistance, and grafting efficacy. However, how scions regulate endogenous hormones to influence AR formation remains poorly understood. In this study, we constructed watermelon seedlings (WP) using “HXX” as the scion and “Tie Zhen No. 3” as the rootstock. Removing scion cotyledons (WP-1) significantly promoted AR development, while removing true leaves (WP-2) had minimal effect. Simultaneous removal of both (WP-3) increased endogenous levels of IAA (Indole-3-Acetic Acid), abscisic acid (ABA), and jasmonic acid (JA). Transcriptome sequencing revealed that differentially expressed genes (DEGs) are enriched in various hormone signaling pathways. On the fourth day, when the number of differential genes was the highest, the DEGs significantly expressed in all three treatment groups were enriched in the activation signaling pathways and responses of JA, auxin, ethylene, and cytokinins. Transcription factors such as bHLH, ERF, MYB, and NAC were significantly expressed during the development of ARs, playing a key regulatory role. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified 82 DEGs across five hormone signal transduction pathways. Weighted gene co-expression network analysis (WGCNA) identified modules positively correlated with AR hormones, highlighting hub genes such as ethylene transcription factors (CRF4, ABR1, ERF054, ERF098), auxin response factors (SAUR21, SAUR32), and other regulators (CSA, HSP, bHLH93, ZAT5, ZAT13, NAC, MYB, C3H). These findings provide molecular insights into how scions regulate AR development through hormones, offering a foundation for improving watermelon grafting practices.