Comparative Transcriptional Analysis and Functional Validation of Aluminum Stress-Responsive <i>RsALS3</i> Gene in Two <i>Rhododendron</i> Cultivars

<i>Rhododendrons</i> naturally inhabit acidic soils where aluminum (Al) toxicity severely restricts plant growth, yet the molecular basis underlying cultivar-dependent differences in Al tolerance remains poorly understood. In this study, we compared the transcriptional and physiological responses of an Al-resistant cultivar (Kangnaixin) and an Al-sensitive cultivar (Baijinpao) under Al stress. Transcriptome analysis was performed to identify Al-responsive differentially expressed genes (DEGs), followed by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses to elucidate functional categories and metabolic pathways involved in stress adaptation. In addition, the Al tolerance-related gene <i>RsALS3</i> was cloned and functionally characterized through heterologous overexpression in <i>Arabidopsis thaliana</i>. The two cultivars exhibited distinct transcriptional profiles in response to Al stress, with DEGs significantly enriched in abiotic stress responses, membrane-associated functions, and key metabolic pathways, including starch and sucrose metabolism, phenylpropanoid and flavonoid biosynthesis, and photosynthesis-related processes. These results suggest that Al stress disrupts membrane integrity and alters carbon metabolism in <i>Rhododendron</i>. Functional validation demonstrated that <i>RsALS3</i> overexpression moderately alleviated Al-induced toxicity in <i>A. thaliana</i>, as evidenced by reduced leaf damage and improved photosynthetic efficiency. Although the observed phenotypic differences were modest, and some chlorophyll fluorescence kinetics data did not reach strong statistical significance. The overall physiological trends support a potential role of <i>RsALS3</i> in Al stress adaptation. Collectively, these findings provide insight into cultivar-specific Al stress responses in <i>Rhododendron</i> and identify <i>RsALS3</i> as a promising candidate gene for further investigation aimed at improving adaptation to acidic soils.