Xylem-based long-distance transport and phloem remobilization of copper in Salix integra Thunb.
Graphical abstract
Introduction
Copper (Cu), as one of the essential micronutrients, and is involved in protein synthesis, photosynthesis and critical membrane activities in plants (Weinstein et al., 2011). However, Cu can also be toxic at doses beyond the optimal concentrations of approximately >10 μM, resulting in growth inhibition, oxidative damage and nutrient loss (Cui et al., 2019; Ryan et al., 2013). Cu contamination in soils has primarily been caused by mining, smelting, fertilizer/fungicide use and waste water irrigation (Li et al., 2014; Zhao et al., 2018). Effective remediation techniques that successful manage risk for contaminated lands are in great demand worldwide. Due to the low biomass of herbaceous hyperaccumulators, fast-growing woody plants such as Salix and Populus have attracted much attention for metal phytoextraction and have shown significant potential for soil remediation (Luo et al., 2016). Previous studies have demonstrated that Salix, a non-hyperaccumulator woody plant species, can accumulate considerable amounts of heavy metals (Konlechner et al., 2013; Robinson et al., 2000; Vollenweider et al., 2006). For example, Salix caprea could accumulate 116 μg⋅ g−1 Cd and 4680 μg⋅ g−1 Zn dry weight in leaves when grown at a metal contaminated site (Unterbrunner et al., 2007). Salix integra Thunb (S. integra) could accumulate 691 μg⋅ g−1 Cu in roots, exhibiting a great potential for phytoremediation of Cu-contaminated soil via phytostabilization in wetland environment (Cao et al., 2017). Therefore, elucidation of the mechanisms of Cu transport and accumulation in willow species is of significant interest, and the findings from such studies could be used to both understand and optimize phytoremediation efficiency.
Metals can be rapidly taken up by root cells, with subsequent storage in root tissues, long-distance transport via xylem and phloem, sequestration in cell walls and vacuoles, or detoxification by cytosolic metal chelation or antioxidant defense systems (Luo et al., 2016). Although several processes (e.g. metal uptake by roots, detoxification in shoot and xylem loading) have been shown to influence the efficiency of root-shoot translocation in crops or herbaceous hyperaccumulators (Ando et al., 2013; Deng et al., 2016; Zhao et al., 2018), little is known about the long-distance transport of heavy metals in non-hyperaccumulator species, particularly fast-growing woody plants. In general, metal accumulation in plants is not only associated with root uptake but also relies on internal redistribution and remobilization of the stored elements within various plant organs and tissues (Wu et al., 2010). Element redistribution in plants results primarily from transport in xylem, translocation from xylem to phloem, and remobilization through phloem transport (Taiz and Zeiger, 2010). Xylem is a direct route for metal transport from roots to shoots via water transpiration (Shen and Ma, 2001). However, element transport through the phloem (from old/mature leaves to young leaves) is more selective and active, and is a significant pathway for element allocation within plants (Erenoglu et al., 2002). Furthermore, nutritional elements are generally mobile in the phloem and can be reallocated from old/mature leaves (sources) to young tissues (sinks) (Hu et al., 2019; Page et al., 2006); however, due to the limitations of extraction and analytical methods, it is difficult to obtain this information in woody plants. Both upward and downward movement of elements have been reported in phloem transport. For example, Fismes et al. (2005) observed that in three vegetable species (bean, lettuce and radish), 63Ni was transferred throughout the entire plant following the foliar application; the metal accumulated primarily in young leaves and roots. Lu et al. (2013) reported that 68Zn could be remobilized from mature leaves to the youngest leaf tissue in Sedum alfredii Hance. As such, we speculate that Cu accumulation in new emerging tissues could be increased by phloem translocation in S. integra.
During the long-distance transport via xylem and phloem, the heavy metals might be complexed with organic acids and amino acids (Ando et al., 2013). In Alyssum serpyllifolium, Ni was complexed predominantly with citric acid in the xylem sap, suggesting that metal transport via the xylem involves organic acids (Alves et al., 2011). Evidence for the long-distance transport of heavy metals associated with organic acids has also been reported for other plant species (Deng et al., 2016; Lu et al., 2013; Fu et al., 2019). These findings implied that the chemical form of the heavy metals during transport via xylem and phloem, as well as the remobilization via the phloem, will vary with metal type, metal concentration and plant species.
Excessive amounts of Cu from contaminated soils can be bioaccumulated in plants and other organisms, causing toxicity and threatening the environmental safety. Thus, a series of experiments investigating xylem and phloem transport of Cu were conducted set up to further our understanding of metal accumulation and transport in fast-growing willow (S. integra). The aims of the current study were: (1) to explore the long-distance transport pattern of Cu through S. integra xylem and phloem; (2) to examine and compare the potential chelating ligands for Cu in xylem sap and phloem exudate; (3) to establish a comprehensive understanding of the Cu transport mechanism in S. integra. An understanding of the mechanisms of long-distance transport of Cu in willow species would inform and enable optimization of the rational design of technologies for the remediation of metal-contaminated soils.
Section snippets
Plant pre-cultivation
One-year-old S. integra branches were selected from a local nursery at the Research Institute of Subtropical Forestry, Chinese Academy of Forestry, Hangzhou, Zhejiang province, China, in April 2019. Branches of S. integra with diameter of approximately 0.8 cm were cut into 12 cm in lengths, were then inserted into Styrofoam, and were pre-cultured in a 20 L plastic box containing 1/4 strength Hoagland solution without Cu as described in Cao et al. (2018) for 45 d. S. integra seedlings were
Cu and other nutrient elements in the xylem sap and phloem exudates
The Cu concentration in the xylem sap of S. integra was 3.25 and 6.32 μM in the control and 10 μM Cu treatment, respectively (Table 1). However, the Cu concentration in the phloem exudate was between 0.11–0.23 μM. Significant differences within the Cu treatments (P < 0.01) were observed in both the xylem sap and phloem exudate. The Cu concentration in the xylem sap and phloem exudates in the 10 μM Cu treatment was 0.94 and 1.09-fold higher than the respective control. Exposure to 10 μM Cu
Cu transport via xylem in S. integra
In the present study, the Cu concentration in root was markedly higher than that in above-ground tissues after the 5-d exposure period (Fig. 3b), which is in line with previous results demonstrating that Cu preferentially localizes in the roots (Cao et al., 2019; Liao et al., 2000; Zhao et al., 2018). The significant decrease of TF and BAF was observed as solution Cu level elevated, and the considerable amount of Cu retained in the roots of S. integra suggests rate limited translocation to
Conclusions
The current study clearly demonstrated the xylem transport and phloem remobilization of Cu in shrub willow S. integra by using multiple advanced methods.
High level of Cu in xylem sap and high Cu intensity in μ-XRF in xylem tissues directly confirm the significance of xylem transport of Cu from roots to shoots. Differential spatial distribution of Cu in root apex and lateral zone indicates that Cu was more preferred to transport to root stele. The re-rooting and stable isotope 65Cu trace
Credit author statement
Dr. Yini Cao: Conceptualization, investigation, data analysis, writing the paper; Dr. Chuanxin Ma: data analysis and writing the paper; Mr. Hongjun Chen and Dr. Jianfeng Zhang: Sample preparation, determination and data analysis; Dr. Guangcai Chen: Conceptualization, investigation, manuscript revision, Funding acquisition and project administration; Drs. Jason C. White and Baoshan Xing: Manuscript comments and revision.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work. We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgements
This work was supported by the Fundament Research Funds of CAF (Grant No. CAFYBB2019SZ001) and National Natural Science Foundation of China (Grant No.31470619). The μ-XRF analysis of this research was carried out at the 4W1B beamline of Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences. The authors sincerely acknowledge Drs. Dongliang Chen, Juncai Dong and all staff members of 4W1B, for their support in measurements and data reduction. The
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2022, Science of the Total EnvironmentCitation Excerpt :The dried samples were ground using a stainless-steel mall mill and passed through a 0.25 mm sieve in preparation for elemental measurement. The dry organs (0.25 g) were digested in concentrated nitric acid (HNO3) and hydrogen peroxide (H2O2) (4:1, v/v) using a hot block system (ED36, Lab Tech, Germany) (Cao et al., 2020a, b). The digestion was diluted to 50 ml with 2% (v/v) diluted HNO3 solution.