Tolerance, toxicity and transport of Cd and Zn in Populus trichocarpa

Abstract

Metal inputs to terrestrial ecosystems are of great concern due their toxicity to biota, especially for elements with no biological function such as cadmium. Fast-growing trees such as poplars may have potential in phytoremediation schemes. We assessed accumulation, metal partitioning, gene expression (Pt-HMA4) and overall tolerance to, and interaction between, cadmium (Cd) and zinc (Zn) in Populus trichocarpa 'Trichobel'. We predicted that Zn would have an antagonistic effect in Cd accumulation and anticipated some level of tolerance to these metals. Poplars were grown in sandy substrate under different metal applications, ranging from 1 to 243 mg kg-1 Cd; or 30 to 7,290 mg kg-1 Zn; and also two combined treatments: 27 mg kg-1 Cd with 90 or 270 mg kg-1 Zn. Growth parameters and metal contents in shoots and roots were determined. Transcriptional levels of the Pt-HMA4 gene were assessed in roots and leaves. P. trichocarpa showed a surprisingly high tolerance to Cd, with root biomass being affected only at the highest doses applied. Metals accumulated mainly in roots (up to 6,537 mg kg-1 Cd and 21,500 mg kg-1 Zn), root-to-shoot translocation peaked at the 9 mg kg-1 dose for Cd (41%) and 90 mg kg-1 for Zn (40%). At high Cd/Zn applications, expression of Pt-HMA4 in roots decreased significantly. Contrary to the initial presumption, Zn addition increased Cd uptake, reaching hyperaccumulator-like concentrations in shoots (≥ 100 mg kg-1 Cd). Differential root-to-shoot partitioning has a major role in Cd tolerance in P. trichocarpa; partly by down-regulating the Pt-HMA4 gene in roots. Zn addition promoted high Cd uptake without any detriment to plant growth. P. trichocarpa was tolerant to extreme Cd concentrations, offering a great potential to be used in phytoremediation techniques for stabilization/extraction of Cd from soils contaminated by both Cd and Zn.

Bioremediation potential of Cd by transgenic yeast expressing a metallothionein gene from Populus trichocarpa

“…Similarly, we have recently demonstrated that the gene PtMT2b from tree species Populus trichocarpa cv 'Trichobel' was able to reduce Cd toxicity when expressed in S. cerevisiae (De Oliveira et al, 2020). This poplar clone is particularly tolerant to elevated Cd concentrations (De Oliveira and Tibbett, 2018), whose high expression of MT2b in roots was shown to be correlated to enhanced Cd sequestration (De Oliveira et al, 2020). Moreover, those yeasts expressing poplar MT2b may effectively remove Cd from contaminated water by preventing the excretion of metals back to the medium through chelation (Ruta et al, 2017).…”

The role of roots and rhizosphere in providing tolerance to toxic metals and metalloids

“…Decreasing HMA function could provide avenues to prevent toxic build-up of metal(loid)s in aboveground organs. For example, the presence of metals such as Cd and Zn leads to downregulation of HMA4 in the pericycle and was shown to be accompanied by lower accumulation of Cd and Zn in leaves of Thlaspi arvense (Hammond et al, 2006),Populus trihocarpa (Hammond et al, 2006, De Oliveira andTibbett, 2018) and of P. nigra (Adams et al, 2011). Similarly, overexpression in rice of the tonoplast located OsHMA3 (Ueno et al, 2010) greatly enhances Cd sequestration in the root and hence drastically reduced grain Cd levels under field conditions (Lu et al, 2019).…”

Combining mercapto‐functionalized palygorskite with zinc affect cadmium phytoavailability and soil microbial activity in rhizosphere soil

“…Although, (Lu & Xu, 2009) found that application of Zn 2+ could compete with Cd 2+ for ion adsorption sites in soil, which could cause the dissociation of Cd 2+ on the surface of soil particles and increase the availability of Cd 2+ . Studies have also shown that the application of cations (270 mg Á kg -1 Zn) can significantly reduce the soil pH, particularly at extreme cation concentrations (2430 and 7420 mg Á kg -1 Zn) (Oliveira & Tibbett, 2018). The EC is also a main index of the presence of soil soluble ions and reflects the available ion content in the soil to some extent.…”