Melatonin facilitated Pb accumulation in bermudagrass by modulating radial transport pathways

Radial transport of lead (Pb) within roots determines the extent of Pb loading into the xylem, facilitating its subsequent root-to-shoot translocation. However, the regulatory role and underlying mechanism of exogenous melatonin in modulating Pb radial transport in bermudagrass roots remain unclear. Here, morphological and molecular basis by which melatonin affects the Pb radial transport in bermudagrass were explored. Results revealed that exogenous melatonin (0-200 μM) altered Pb tolerance, accumulation and translocation in bermudagrass. Notably, a low-concentration melatonin treatment (4 μM) enhanced Pb uptake and translocation, resulting in a 1.65-fold higher Pb accumulation in the shoots compared to the only Pb stress treatment. Concerning the radial transport pathways, the symplastic transport dominated Pb radial transport across all melatonin concentrations. At 4 μM, melatonin upregulated key genes CdACBP1 and CdACBP4, facilitating Pb transport via the symplastic pathway. Concurrently, downregulation of lignin/suberin biosynthesis genes CdLAC3 and CdCYP86-B1 delayed Casparian strip and suberin lamellae formation. Melatonin also alleviated Pb-induced root growth inhibition, increasing root tip number by 80 %, thereby enhancing apoplastic Pb entry. The contribution of the apoplastic pathway significantly increased by 64 % compared to the only Pb stress treatment. Overall, this study is the first to confirm that melatonin promoted Pb radial transport and root-to-shoot translocation in bermudagrass by regulating the expression of Pb transporter genes, root growth, and root endodermal differentiation. These insights provided advance theoretical foundation for advancing the practical application of melatonin in phytoremediation of Pb-contaminated soils.