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.

不定根（Adventitious roots, ARs）对嫁接西瓜幼苗至关重要，在养分吸收、抗逆性与嫁接成效中发挥关键作用。然而，接穗调控内源激素以影响不定根形成的具体机制仍未被充分阐明。本研究以‘HXX’作为接穗、‘铁砧3号’作为砧木，构建了西瓜幼苗（WP）实验体系。去除接穗子叶（WP-1）可显著促进不定根发育；与之相反，去除接穗真叶（WP-2）的影响极小，而同时移除接穗子叶与真叶（WP-3）则呈现中等程度的响应效果。内源激素动态分析显示，WP-1组的吲哚-3-乙酸（indole-3-acetic acid, IAA）水平持续升高，且脱落酸（abscisic acid, ABA）与茉莉酸（jasmonic acid, JA）含量较低；而WP-2与WP-3组在不定根发育后期的激素谱呈现分化特征。转录组测序结果表明，差异表达基因（differentially expressed genes, DEGs）富集于多条激素信号通路。在差异基因数量达到峰值的第4天，三个处理组共有的显著表达差异基因富集于茉莉酸、生长素、乙烯及细胞分裂素的激活信号通路与响应过程。bHLH、ERF、MYB、NAC等转录因子在不定根发育阶段显著表达，发挥关键调控作用。京都基因与基因组百科全书（Kyoto Encyclopedia of Genes and Genomes, KEGG）富集分析显示，5条激素信号转导通路共涉及82个差异表达基因。加权基因共表达网络分析（weighted gene co-expression network analysis, WGCNA）鉴定出与不定根激素水平正相关的基因模块，并筛选出核心基因，包括乙烯转录因子（CRF4、ABR1、ERF054、ERF098）、生长素响应因子（SAUR21、SAUR32）及其他调控因子（CSA、HSP、bHLH93、ZAT5、ZAT13、NAC、MYB与C3H）。本研究结果初步揭示了接穗通过激素调控不定根发育的作用机制，为优化西瓜嫁接栽培实践提供了理论基础。