Advances in the Involvement of Metals and Metalloids in Plant Defense Response to External Stress

Plants, as sessile organisms, uptake nutrients from the soil. Throughout their whole life cycle, they confront various external biotic and abiotic threats, encompassing harmful element toxicity, pathogen infection, and herbivore attack, posing risks to plant growth and production. Plants have evolved multifaceted mechanisms to cope with exogenous stress. The element defense hypothesis (EDH) theory elucidates that plants employ elements within their tissues to withstand various natural enemies. Notably, essential and non-essential trace metals and metalloids have been identified as active participants in plant defense mechanisms, especially in nanoparticle form. In this review, we compiled and synthetized recent advancements and robust evidence regarding the involvement of trace metals and metalloids in plant element defense against external stresses that include biotic stressors (such as drought, salinity, and heavy metal toxicity) and abiotic environmental stressors (such as pathogen invasion and herbivore attack). We discuss the mechanisms underlying the metals and metalloids involved in plant defense enhancement from physiological, biochemical, and molecular perspectives. By consolidating this information, this review enhances our understanding of how metals and metalloids contribute to plant element defense. Drawing on the current advances in plant elemental defense, we propose an application prospect of metals and metalloids in agricultural products to solve current issues, including soil pollution and production, for the sustainable development of agriculture. Although the studies focused on plant elemental defense have advanced, the precise mechanism under the plant defense response still needs further investigation.

Alkaline industrial wastes - Characteristics, environmental risks, and potential for mine waste management

The large quantities of alkaline industrial wastes that are generated globally have the potential to be valorized in various applications instead of being landfilled. This study evaluated the potential reuse of green liquor dregs (GLD), wood ashes, coal ash, red mud, mussel, scallop, and oyster shells to control acid and metalliferous drainage (AMD). Low hydraulic conductivities (10^-7 to 10^-9 m/min) suggest that covers constructed from fine-grained GLD, red mud, coal ash and wood fly ash can limit the formation of AMD. Static and kinetic test leachates of pH 5.8 to 10.6 indicate that the tested materials can neutralize acidic drainage and immobilize metal(loid)s by precipitation. The alkalinity is proportional to the amount and reactivity of carbonate and hydroxide fractions with red mud followed by coal ash being the most alkaline over 100 weeks and wood ashes the least. The tested industrial wastes generate leachates with a low metal(loid) risk when screened against the Australian freshwater guidelines. However, oxyanions including Al, Cr, Cu, Se, and V were leached in deleterious concentrations ≤100 times more than the guidelines because of their mobility in alkaline conditions. The outcomes of this study highlighted that alkaline industrial wastes can be potentially used in the long-term remediation of AMD as part of an environmentally sustainable and cost-effective integrated mine waste management strategy.

Rice waste biochars produced at different pyrolysis temperatures for arsenic and cadmium abatement and detoxification in sediment

The effectiveness of rice waste biochars on heavy metal and metalloid abatement and detoxification was investigated using comprehensive studies based on As and Cd immobilization, bioaccumulation in tubifex, and microbial community changes in contaminated sediment. The remediation effects of biochars produced at different pyrolytic temperatures (400-700 °C) were evaluated. Bioaccumulation of heavy metal and metalloid in the tubifex tissue and change of indigenous microbial community under treatment of different biochars were assessed. Biochars produced at 700 °C exhibited greater effect on decreasing the concentrations of As and Cd in aqueous phase, and TCLP extractable and bioavailable metal(loid) in solid phase of sediment. The concentration of As and Cd in water phase decreased by 26%-89% and 22%-71% under the treatment of straw biochar, and decreased by 13%-92% and 5%-64% under the treatment of rice husk biochar, respectively. As and Cd contents in the tubifex tissue were positively correlated with their concentrations in aqueous phase. High-temperature biochars significantly reduced metal(loid) bioaccumulation in tubifex. The richness and biodiversity of microbial community were both greater in all biochars remediated sediment compared to non-treated sediment. These results indicated that rice waste biochars could effectively inhibit the bio-availability and toxicity of heavy metal and metalloid in sediment, and the higher-temperature biochar exhibited better performance.