Absence of Nanoparticle-Induced Drought Tolerance in Nutrient Sufficient Wheat Seedlings.

Strategies to reduce crop losses due to drought are needed as climate variability affects agricultural productivity. Wheat (Triticum aestivum var. Juniper) growth in a nutrient-sufficient, solid growth matrix containing varied doses of CuO, ZnO, and SiO2 nanoparticles (NPs) was used to evaluate NP mitigation of drought stress. NP amendments were at fertilizer levels, with maxima of 30 Cu, 20 Zn, and 200 Si (mg metal/kg matrix). Seeds of this drought-tolerant cultivar were inoculated with Pseudomonas chlororaphis O6 (PcO6) to provide a protective root microbiome. An 8 day drought imposed on 14 day-old wheat seedlings decreased shoot and root mass, shoot water content, and the quantum yield of photosystem II when compared to watered plants. PcO6 root colonization was not impaired by drought or NPs. A dose-dependent increase in the Cu, Zn, and Si from the NPs was observed from analysis of the rhizosphere solution, and this process was not affected by drought. Consequently, fertilizer concentrations of the NPs did not further improve drought tolerance in wheat seedlings under the growth conditions of adequate mineral nutrition and the presence of a beneficial microbiome. These findings suggest that potential NP benefits in promoting plant drought tolerance occur only under certain environmental conditions.

Pesticidal activity of metal oxide nanoparticles on plant pathogenic isolates of Pythium.

CuO and ZnO nanoparticles (NPs) have antimicrobial effects that could lead to formulations as pesticides for agriculture or medicine. The responses of two soil-borne plant pathogenic Pythium isolates to the NPs were studied to determine the potential of these metal oxide NPs as pesticides. Growth of the P. ultimum isolate was more sensitive to CuO NPs than the P. aphanidermatum isolate. Growth in liquid medium with CuO NPs eliminated culturability whereas exposure to ZnO NPs resulted in stasis with growth resuming on transfer to medium lacking NPs. The citrate in the medium used for the growth assays was involved in enhanced release of the toxic metals from the NPs. Both CuO and ZnO NPs affected processes involved in Fe uptake. The NPs reduced levels of Fe-chelating siderophore-like metabolites produced by Pythium hyphae. CuO NPs inhibited, but ZnO NPs increased, ferric reductase activity detected at the mycelial surface. These findings illustrate that the toxicity of the metal oxide NPs towards Pythium was influenced by the medium, especially by the presence of a metal chelator. Environmental factors are likely to alter the pesticide potential of the metal oxide NPs when formulated for agricultural use in soils.

Changes in Metal-Chelating Metabolites Induced by Drought and a Root Microbiome in Wheat.

The essential metals Cu, Zn, and Fe are involved in many activities required for normal and stress responses in plants and their microbiomes. This paper focuses on how drought and microbial root colonization influence shoot and rhizosphere metabolites with metal-chelation properties. Wheat seedlings, with and without a pseudomonad microbiome, were grown with normal watering or under water-deficit conditions. At harvest, metal-chelating metabolites (amino acids, low molecular weight organic acids (LMWOAs), phenolic acids, and the wheat siderophore) were assessed in shoots and rhizosphere solutions. Shoots accumulated amino acids with drought, but metabolites changed little due to microbial colonization, whereas the active microbiome generally reduced the metabolites in the rhizosphere solutions, a possible factor in the biocontrol of pathogen growth. Geochemical modeling with the rhizosphere metabolites predicted Fe formed Fe-Ca-gluconates, Zn was mainly present as ions, and Cu was chelated with the siderophore 2'-deoxymugineic acid, LMWOAs, and amino acids. Thus, changes in shoot and rhizosphere metabolites caused by drought and microbial root colonization have potential impacts on plant vigor and metal bioavailability.

Rhizosphere interactions between copper oxide nanoparticles and wheat root exudates in a sand matrix: Influences on copper bioavailability and uptake.

The impact of copper oxide nanoparticles (CuONPs) on crop production is dependent on the biogeochemistry of Cu in the rooting zone of the plant. The present study addressed the metabolites in wheat root exudates that increased dissolution of CuONPs and whether solubility correlated with Cu uptake into the plant. Bread wheat (Triticum aestivum cv. Dolores) was grown for 10 d with 0 to 300 mg Cu/kg as CuONPs in sand, a matrix deficient in Fe, Zn, Mn, and Cu for optimum plant growth. Increased NP doses enhanced root exudation, including the Cu-complexing phytosiderophore, 2'-deoxymugineic acid (DMA), and corresponded to greater dissolution of the CuONPs. Toxicity, observed as reduced root elongation, was attributable to a combination of CuONPs and dissolved Cu complexes. Geochemical modeling predicted that the majority of the solution phase Cu was complexed with citrate at low dosing or DMA at higher dosing. Altered biogeochemistry within the rhizosphere correlated with bio-responses via exudate type, quantity, and metal uptake. Exposure of wheat to CuONPs led to dose-dependent decreases in Fe, Ca, Mg, Mn, and K in roots and shoots. The present study is relevant to growth of a commercially important crop, wheat, in the presence of CuONPs as a fertilizer, fungicide, or pollutant. Environ Toxicol Chem 2018;37:2619-2632. © 2018 SETAC.