Melatonin Mediates Lead Accumulation Reduction via the RsWRKY26-RsHIPP26-RsASMT2 Module in Radish Taproot

Soil contamination with lead (Pb) severely affects the quality and safety of vegetable crops. Melatonin (MT) decreases Pb accumulation in plants. However, the molecular mechanism underlying MT-mediated Pb accumulation remains elusive in radish. Herein, a total of 11,265 long non-coding RNAs (lncRNAs)and 4730 lncRNA-protein-coding (PC) gene pairs were identified among the control, 200 mg/L Pb(NO₃)₂ and Pb plus MT treatment. Most differentially expressed genes were significantly enriched in oxidoreductase activity and oxidation-reduction processes. RsASMT2 expression was upregulated under Pb_MT treatment. RsASMT2 overexpression promoted reactive oxygen species scavenging and reduced Pb accumulation in Arabidopsis and radish roots under Pb stress. Ectopic expression of the TCONS_00123952-targeted RsHIPP26 gene reduced Pb accumulation in yeast cells. Moreover, RsHIPP26 overexpression decreased Pb accumulation in radish and Arabidopsis roots. Both the Y1H and DLA assays showed that RsWRKY26 was directly bound to the RsHIPP26 and RsASMT2 promoters and activated their transcription. RsWRKY26 overexpression resulted in lower Pb accumulation in radish roots. Collectively, MT-mediated RsWRKY26-RsHIPP26-RsASMT2 module coordinately reduces Pb accumulation in radish, establishing an effective strategy of product-safe radishes with low Pb accumulation and facilitating genetically improving Pb-uptake trait in radish breeding programs.