Melatonin Alleviates Antimony Toxicity in Quinoa by Suppressing CqBBX8-Mediated Antimony Uptake and Oxidative Stress

Quinoa (<i>Chenopodium quinoa</i> Willd.) is a promising nutrient-rich crop, but its cultivation is threatened by widespread antimony (Sb) contamination in soils. While melatonin (MT) is known to alleviate abiotic stresses, its role in Sb tolerance is poorly understood. We screened 14 quinoa varieties and identified two cultivars with contrasting Sb accumulation: a high-accumulator (CL2) and a low-accumulator (No. 1). MT application effectively mitigated Sb-induced phytotoxicity, reducing Sb accumulation in roots by 33.81% and 55.43% in CL2 and No. 1, respectively, with significant reductions also observed in the shoots. Transcriptomic analysis identified 699 common differentially expressed genes, among which the transcription factor CqBBX8 was prioritized for functional characterization due to its strong induction by Sb and suppression by MT. Heterologous overexpression of <i>CqBBX8</i> in tobacco promoted Sb uptake by upregulating metal transporter genes and aggravated oxidative damage by repressing antioxidant enzymes and enhancing <i>NtRbohE</i> expression. Results confirmed that CqBBX8 binds to the promoters of two putative Sb transporter genes, <i>CqNRAMP</i> and <i>CqZIP</i>, and activates their expression. It also binds to the promoters of <i>CqPOD</i> and <i>CqRbohE</i>, thereby differentially regulating these redox-related genes. Our findings establish CqBBX8 as a pivotal positive regulator of Sb uptake and sensitivity and reveal that MT enhances tolerance primarily by suppressing the CqBBX8 pathway. This work provides a novel mechanistic framework and a genetic target for breeding low-Sb crops to ensure food safety in contaminated regions.