Integrated study on subcellular localization and chemical speciation of Pb reveals root strategies for Pb sequestration and detoxification in Salix integra

Salix integra has a high tolerance to lead (Pb) stress, and has been applied in the phytostabilization of Pb-Zn mine tailings in East China. Mechanisms for Pb sequestration and detoxification were poorly understood in Salix plants. The present study aimed to elucidate where Pb was localized and how Pb was combined in roots of S. integra for a better understanding of Pb tolerance strategies. Histochemical methods combined with synchrotron-based X-ray fluorescence microscopy, transmission electron microscopy and fourier transform infrared spectroscopy were used to explore the distribution and chemical forms of Pb at cellular and subcellular levels in S. integra. The accumulation of Pb displayed a fast linear uptake in roots and stems in the short-term period, whereas a slower Freundlich-like uptake in leaves. Micro-XRF and histochemical staining revealed that the absorbed Pb in roots was mostly restricted in the cortex, and partly translocated in the stele. At the subcellular level, most Pb in roots was localized in the cell walls and trophoplast fraction, and the free -OH and pectin C—O/C—O—S groups may be involved in the cell wall sequestration. It was also found that the majority of Pb in roots existed as phosphate and oxalate. S. integra has a high uptake potential of accumulating Pb in their roots. The absorbed Pb was primarily localized in the cell walls and combined with -OH or pectin groups, and precipitation as phosphate or oxalate may be responsible for the detoxification in root cells.