Transcriptomic and Metabolomic Analyses Reveal Differing Phytohormone Regulation in Rhododendron Cultivars in Response to Azalea Lace Bug (Stephanitis pyrioides)

Rhododendron spp., valuable ornamental plants, frequently suffer from infestations of the azalea lace bug (Stephanitis pyrioides Scott, ALB). However, the hormonal regulatory mechanisms underlying Rhododendron defense against ALB are not well understood. In this study, integrated transcriptomic and metabolomic analyses were performed to investigate the phytohormone responses under ALB stress in two Rhododendron cultivars with distinct insect susceptibility: the resistant ‘Taile’ (TL), and the susceptible ‘Yanzhimi’ (YZM). Transcriptomic sequencing identified 10,052 and 3113 differentially expressed genes (DEGs) in ‘TL’ and ‘YZM’, respectively, after ALB infestation. KEGG pathway enrichment analysis revealed that the DEGs in ‘TL’ were significantly enriched in hormone signal transduction pathways, including gibberellin (GA), jasmonic acid (JA), salicylic acid (SA), and ethylene (ETH), with 21 out of 25 hormone-related DEGs being upregulated. In contrast, ‘YZM’ exhibited 18 upregulated and 13 downregulated DEGs and suppressed auxin and cytokinin signaling. Non-targeted metabolomic analysis detected increased indole-3-acetic acid (IAA), abscisic acid (ABA), and jasmonoyl–isoleucine (JA-Ile) levels in both cultivars. ‘TL’ also showed elevated levels of SA precursor (benzoic acid) and ethylene precursor (1-aminocyclopropane-1-carboxylate, ACC). These findings indicate that ALB infestation induces endogenous hormone signaling-related genes in Rhododendron leaves and regulates hormones such as SA and JA to counteract insect stress. This study provides theoretical insights into the molecular mechanisms of Rhododendron defense against insect herbivory and lays the foundation for breeding resistant cultivars.