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

Quinoa (Chenopodium
quinoa 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 CqBBX8 in tobacco promoted Sb uptake
by upregulating metal transporter genes and aggravated oxidative damage
by repressing antioxidant enzymes and enhancing NtRbohE expression. Results confirmed that CqBBX8 binds to the promoters
of two putative Sb transporter genes, CqNRAMP and CqZIP, and activates their expression. It also binds to
the promoters of CqPOD and CqRbohE, 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.