Scion organ removal alters hormone levels and gene expression associated with adventitious root development in grafted watermelon seedlings.

Adventitious roots (ARs) are crucial for grafted watermelon seedlings, playing vital roles in nutrient absorption, stress resistance, and grafting efficacy. However, the way in which 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. Scion cotyledons removal (WP-1) significantly promoted AR development. In contrast, true leaf removal (WP-2) had minimal effect, while simultaneous removal of both (WP-3) elicited intermediate responses. Endogenous hormone dynamics showed that WP-1 maintained progressively increasing indole-3-acetic acid (IAA) with lower abscisic acid (ABA) and jasmonic acid (JA) levels, whereas both WP-2 and WP-3 exhibited divergent hormonal profiles in ARs during later development stages. 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. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified 82 DEGs across five hormone signal transduction pathways. The 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 and SAUR32), and other regulators (CSA, HSP, bHLH93, ZAT5, ZAT13, NAC, MYB, and C3H). These findings provide preliminary evidence of the scion's regulatory role in AR development through hormones, offering a foundation for improving watermelon grafting practices.