Soil Bacteria Override Speciation Effects on Zinc Phytotoxicity in Zinc-Contaminated Soils

Abstract

The effects of zinc (Zn) speciation on plant growth in Zn-contaminated soil in the presence of bacteria are unknown but are critical to our understanding of metal biodynamics in the rhizosphere where bacteria are abundant. A 6-week pot experiment investigated the effects of two plant growth promoting bacteria (PGPB), Rhizobium leguminosarum and Pseudomonas brassicacearum, on Zn accumulation and speciation in Brassica juncea grown in soil amended with 600 mg kg elemental Zn as three Zn species: soluble ZnSO and nanoparticles of ZnO and ZnS. Measures of plant growth were higher across all Zn treatments inoculated with PGPB compared to uninoculated controls, but Zn species effects were not significant. Transmission electron microscopy identified dense particles in the epidermis and intracellular spaces in roots, suggesting Zn uptake in both dissolved and particulate forms. X-ray absorption near-edge structure (XANES) analysis of roots revealed differences in Zn speciation between treatments. Uninoculated plants exposed to ZnSO contained Zn predominantly in the form of Zn phytate (35%) and Zn polygalacturonate (30%), whereas Zn cysteine (57%) and Zn polygalacturonate (37%) dominated in roots exposed to ZnO nanoparticles. Inoculation with PGPB increased (>50%) the proportion of Zn cysteine under all Zn treatments, suggesting Zn coordination with cysteine as the predominant mechanism of Zn toxicity reduction by PGPB. Using this approach, we show, for the first time, that although speciation is important, the presence of rhizospheric bacteria completely overrides speciation effects such that most of the Zn in plant tissue exists as complexes other than the original form.

Evaluation of cadmium and arsenic effects on wild and cultivated cardoon genotypes selected for metal phytoremediation and bioenergy purposes

“…Different lowercase letters indicate statistically significant differences among the roots and shoots concentration on different levels contaminations among the same genotype (p≤0.05) sulphur-containing ligands to complex and translocate Cd into shoots. Such a mechanism is consistent with our previous studies on Zn toxicity and uptake in Brassica juncea when inoculated with bacteria (Adediran et al 2015;Adele et al 2018).…”

Rhizospheric Phosphate Solubilizing Bacillus atrophaeus GQJK17 S8 Increases Quinoa Seedling, Withstands Heavy Metals, and Mitigates Salt Stress

“…The use of zinc fertilizers overcomes this problem [76][77][78]. Zn predominantly occurs in soil as zincite (zinc oxide, ZnO) and sphalerite (zinc sulfide, ZnS), which may alter the plant−soil ecosystem [79,80]. Here, we demonstrated a heterogenous Zn solubilization efficiency by the studied strains.…”

Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements

“…The toxicity of high Zn concentration (>25 mg kg −1 ) in soil has been well documented in various species of flora and microbial biomass [ 167 , 168 , 169 , 170 ]. It has also been established that it may act synergistically in the co-presence of other metals and still show phytotoxic effects in coarse- and fine-textured soils after several years [ 171 , 172 ].…”