Expression of Genes Associated with Nickel Resistance in Red Oak (Quercus rubra) Populations from a Metal Contaminated Region

Nickel stress-tolerance in plant-bacterial associations

Nickel (Ni) is an essential element for plant growth and is a constituent of several metalloenzymes, such as urease, Ni-Fe hydrogenase, Ni-superoxide dismutase. However, in high concentrations, Ni is toxic and hazardous to plants, humans and animals. High levels of Ni inhibit plant germination, reduce chlorophyll content, and cause osmotic imbalance and oxidative stress. Sustainable plant-bacterial native associations are formed under Ni-stress, such as Ni hyperaccumulator plants and rhizobacteria showed tolerance to high levels of Ni. Both partners (plants and bacteria) are capable to reduce the Ni toxicity and developed different mechanisms and strategies which they manifest in plant-bacterial associations. In addition to physical barriers, such as plants cell walls, thick cuticles and trichomes, which reduce the elevated levels of Ni entrance, plants are mitigating the Ni toxicity using their own antioxidant defense mechanisms including enzymes and other antioxidants. Bacteria in its turn effectively protect plants from Ni stress and can be used in phytoremediation. PGPR (plant growth promotion rhizobacteria) possess various mechanisms of biological protection of plants at both whole population and single cell levels. In this review, we highlighted the current understanding of the bacterial induced protective mechanisms in plant-bacterial associations under Ni stress.

Organic and inorganic amendments for the remediation of nickel contaminated soil and its improvement on Brassica napus growth and oxidative defense

In-situ stabilization has been considered an effective way to remediate metal contaminated soil. Thus, pot experiments were undertaken to investigate the effectiveness of multiple stabilization agents such as biochar (BC), mussel shell (MS), zeolite (ZE) and limestone (LS) on the immobilization of Ni, physicochemical features and enzyme activities in polluted soil. Results showed that the sole application of Ni adversely affected the rapeseed growth, photosynthetic pigments, and antioxidative defense. However, the addition of amendments to the contaminated soil significantly reduced Ni bioavailability. The XRD analysis confirmed the formation of Ni related ligands and FTIR showed the presence of hydroxyl, carboxyl and sulfur functional groups, as well as complexation and adsorption of Ni on amendments. Among multiple amendments, biochar significantly enhanced plant biomass attributes and total chlorophyll content. Moreover, addition of amendments also strengthened the antioxidant defense by decreasing Ni induced oxidative stress (H₂O₂ and O₂^.-), increased macronutrient availability, reduced Ni uptake and improved soil health. The qPCR analysis showed that the Ni transporters were significantly suppressed by amendments, which is correlated with the lower accumulation of Ni in rapeseed. The present study showed that immobilizing agents, especially biochar, is an effective amendment to immobilize Ni in soil, which restricts its entry into the food chain.

Physiological and genetic effects of cadmium and copper mixtures on carrot under greenhouse cultivation

The exposure to combinations of heavy metals can affect the genes of vegetables and heavy metals would accumulate in vegetables and thereby indirectly affecting human health. Exploring the links between genetic changes and phenotypic changes of carrot under the combined pollution of Cd and Cu is of great significance for studying the mechanism of heavy metal pollution. Therefore, this study examined the effects of mixtures of cadmium (Cd) and copper (Cu) on physiological measures (malondialdehyde (MDA), proline, and antioxidant enzyme) and expression of growth-related genes (gibberellin gene, carotene gene, and glycogene) in carrot under greenhouse cultivation. The results showed in the additions with mixtures of Cd and Cu at higher concentration, the MDA content increased significantly (p < 0.05), whereas the proline content was not significantly different from those in the control. In the mixed treatments with high Cd concentrations, the activity of superoxide dismutase (SOD) was significantly lower than that in the control (p < 0.05); whereas the activity of peroxidase (POD) increased to different degrees compared to the control. In the additions with mixtures of Cd and Cu, compared with the control, the expression of the gibberellin gene was downregulated from 1.97 to 20.35 times (not including the 0.2 mg kg^-1 Cd and 20 mg kg^-1 Cu mixture, the expression of gibberellin gene in this treatment was upregulated 1.29 times), which lead to decreases in the length and dry weight of carrots. The expression of the carotene gene in mixed treatments downregulated more than that in single treatments, which could reduce the ability of carrots to resist oxidative damage, as suggested by the significant increase in the MDA content. In the addition with mixtures of Cd and Cu, compared with the control, the expression of the glycogene was downregulated by 1.42-59.40 times, which can cause a significant reduction in the sugar content in carrots and possibly further reduce their ability to resist heavy metal damage. A cluster analysis showed that in the additions with mixtures of Cd and Cu, the plant phenotype was affected first, and then with increases in the added concentration, the expression of genes was also affected. In summary, in the additions with mixtures of Cd and Cu, plants were damaged as Cd and Cu concentrations increased.

Metal Toxicity and Resistance in Plants and Microorganisms in Terrestrial Ecosystems

Metals are major abiotic stressors of many organisms, but their toxicity in plants is not as studied as in microorganisms and animals. Likewise, research in plant responses to metal contamination is sketchy. Candidate genes associated with metal resistance in plants have been recently discovered and characterized. Some mechanisms of plant adaptation to metal stressors have been now decrypted. New knowledge on microbial reaction to metal contamination and the relationship between bacterial, archaeal, and fungal resistance to metals has broadened our understanding of metal homeostasis in living organisms. Recent reviews on metal toxicity and resistance mechanisms focused only on the role of transcriptomics, proteomics, metabolomics, and ionomics. This review is a critical analysis of key findings on physiological and genetic processes in plants and microorganisms in responses to soil metal contaminations.