| Dendroremediation, which is the use of trees in cleaning up heavy metals in soils, is popular in the application of phytoremediation. Willow species(Salix spp.) have been suggested for use in the remediation of contaminated soils due to their high biomass production, fast growth, and high accumulation of heavy metals. There are more than 500 willow species around the world, of which more than 200 species are native to China, and the tolerance and accumulation of metals may vary among different willow genotypes, and the assessment of this variability is vital for selecting willow species/varieties for phytoremediation applications. S. integra is a fast-growing shrub willow, and in China, it is generally cultivated by short-rotation plantation, with the new shoots harvested for use in weaving. Some of the cultivated varieties, such as Yizhibi and Weishanhu, displayed excellent in biomass production and environmental adaptation.In the present research, we studied on the variations of uptake and accumulation of lead and cadmium in S.integra genotypes. The characteristics of Pb distribution in leaves, stem and root of S. integra were investigated by synchrotron radiation X-ray fluorescence analysis. To investigate the molecular mechanisms of Cd tolerance and detoxification, we employed the RNA-seq platforms to explore the transcriptome of S.integra and to investigate its transcriptional changes in response to Cd treatment. The differentially expressed genes in the leaves of S.integra was also examined by quantitative-Real-Time-Polymerase Chain Reaction (q-RT-PCR). The results were as follows:1. The tolerance and accumulation of Pb varied among the three willow varieties depending on the Pb concentration. All three genotypes had a high tolerance index (TI) and EC50 value (the effective concentration of Pb in the nutrient solution that caused a 50% inhibition on biomass production), but a low translocation factor (TF), indicating that Pb sequestration is mainly restricted in the roots of S. integra. Among the three genotypes, Dahogntou was more sensitive to the increased Pb concentration than the other two varieties, with the lowest EC50 and TI for root and above-ground tissues. In this respect, Weishanhu and Yizhibi were more suitable for phytostabilization of Pb-contaminated soils.2. The highest content of cadmium was observed in root tissues for both genotypes of S. integra, and the cadmium content in phloem tissues was lower than that in roots. The cadmium content in leaves showed the lowest level among roots, phloem and xylem. The cadmium content in the aboveground (including phloem, xylem and leaf) tissues of two genotypes of S.integra increased significantly compared to the control with the increase of cadmium treatment in the range of 10-70 μmol·L-1, and it came to the highest cadmium accumulation at the concentration of 70 μmol·L-1 cadmium, while when exposed to 90 μmol·L-1 cadmium solution, the cadmium content declined in the aboveground parts. The same accumulation patterns were found in roots for two varieties of S. integra in the range of 10-70 μmol·L-1 cadmium treatment. However, when the roots were exposed to 90 μmol·L-1 cadmium treatment, the cadmium content in’Weishanhu’declined sharply, but still rose for that in’Yizhibi’. This suggests the roots of’Yizhibi’have greater potentential in uptake of cadmium than the’Weishanhu’when exposed to higher concentration of cadmium.3. Using illumina paired-end sequencing, approximately 60.05 million high-quality reads were obstained. De novo assembly yielded 80,105 unigenes with an average length of 703 bp. A total of 50,221 (63%) unigenes were further functionally annotated by comparing their sequences to different proteins and functional domain databases. Totally 13,973 unigenes were annoted in PlantCyc database, including 607 pathways, of which 287 unigenes were related to metal transportation and cellular detoxification, almost play roles in Ascorbate-glutathione cycle, degradation of superoxide ion pathway and GSH mediated detoxification process II, etc.4. Compared to the protein databases from fully sequenced species,37,283 (46.5%) unigenes have homologous proteins in the Nr protein database, among which 63.96%(23,846) unigenes showed significant homology with sequences of Populus trichocarpa, which is only 29.8% of all unigenes. Only 4.15% and 2.95% of the mapped sequences have a high similarity with sequences of Ricinus communis and Vitis vinifera.5. Based on the digital gene expression profiling sequencing, we discovered 896 differentially expressed genes in leaves of S.integra with cadmium and without cadmium, of which 293 were up-regulated significantly, and 603 were down-regulated under cadmium stress. Among the 293 up-regulated genes, most were annoted the hypothetical protein of Populus trichocarpa, which mean the response processes with cadmium in S. integra were controlled by many genes with unknown function. And we also found several genes related to stress responses, like metallothionein(MT), metal tolerant proteins(MTP1), Zrt/Irt-like protein(ZIP), heavy metal ATPase (HAM) and phytochelatin syntheses(PCS). This work will provide references to discover the functions of these genes under cadmium stress.6. To evaluate the potential adaptation and remediation of Salix for heavy metal contamination. Six tree species, including S. integra, were used in a field experiment on vegetation restoration in the abandoned lead-zinc tailings for 2 years. Plant species expressed different tolerance to the tailings. Most species appeared the heavy metal-induced etiolation, defoliation, or growing stopped. There were significant differences in the growth rate of height and base diameter, root growth, and biomass among the species. The mine tailings reduced the root distribution depth and stimulated the max length of lateral roots, the fine root ratio, and increased to defferent extents the mass allocation proportion to roots in all species. Accoding to the evaluation, S. integra would be more suitable for the remediation of Pb/Zn tailings. |
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