Remodeling of root morphology by CuO and ZnO nanoparticles: effects on drought tolerance for plants colonized by a beneficial pseudomonad

Abstract

Formulations that include nanoparticles of CuO and ZnO are being considered for agricultural applications as fertilizers because they act as sources of Cu or Zn. Currently, few studies of the effects of these nanoparticles (NPs) consider the three-way interactions of NPs with the plant plus its microbiome. At doses that produced root shortening by both nanoparticles (NPs), CuO NPs induced the proliferation of elongated root hairs close to the root tip, and ZnO NPs increased lateral root formation in wheat seedlings (Triticum aestivum L.). These responses occurred with roots colonized by a beneficial bacterium, Pseudomonas chlororaphis O6 (PcO6), originally isolated from roots of wheat grown under dryland farming in calcareous soils. The PcO6-induced tolerance to drought stress in wheat seedlings was not impaired by the NPs. Rather, growth of the PcO6-colonized plants with NPs resulted in systemic increases in the expression of genes associated with tolerance to water stress. Increased expression in the shoots of other genes related to metal stress was consistent with higher levels of Cu and Zn in PcO6-colonized shoots grown with the NPs. This work demonstrates that plants grown with CuO or ZnO NPs showed cross-protection from different challenges such as metal stress and drought.

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

“…The shoot tissues had decreased chlorophyll contents and increased oxidative stress in the roots, as determined by increased lipid peroxidation and oxidized glutathione (Dimkpa et al 2012). Molecular analysis confirmed increased expression for genes functioning in Cu chelation (Wright et al 2016;Yang et al 2018) as examples for activated mechanisms for tolerance to Cu, such as limiting Cu translocation into shoot tissue ( Figure 1E).…”

“…Copper oxide NPs are phytotoxic to lettuce and maize in hydroponic conditions with application of 10 mg NP/L (the lowest concentration tested; Wang et al ; Trujillo‐Reyes et al ) and to beans in a sand matrix at the test concentration of 250 mg Cu/kg with no effect at 100 mg Cu/kg dosing (Dimkpa et al ). Sublethal doses of CuONPs resulted in visible effects on root morphology, with shortening, fattening, and root hair changes at concentrations as low as 10 mg Cu/kg (Wright et al ; Yang et al ). When added to soil (loamy sand, 1.6% organic carbon, pH 6–6.5) CuONPs at 500 mg Cu/kg also showed toxicity via decreased root length in wheat (Gao et al ) but had little effect on green peppers grown in a silty loam soil (pH 7.5; Rawat et al ).…”

Facile Coating of Urea With Low-Dose ZnO Nanoparticles Promotes Wheat Performance and Enhances Zn Uptake Under Drought Stress

“…The mechanism surrounding ZnO-NP effects on plant development under drought stress may be related to Zn effecting hormonal induction to regulate root growth for improved adaptation to limited water supply. Indeed, expression of genes related to hormones such as ABA and cytokinins was enhanced by ZnO-NPs in droughted wheat plants, concomitant with modulation of root architecture that helped tolerate the stress (Yang et al, 2018). In addition, soil microbes can facilitate hormonal activity in plants, as microbially-produced hormones can be accessed by plants, contributing to tolerance to drought stress (Defez et al, 2017;Yang et al, 2018).…”

“…Mechanistically, Zn can mitigate drought effects in crops (Karim et al, 2012;Dimkpa et al, 2017a;Dimkpa et al, 2019b), due to the role of Zn in metabolic processes regulating water dynamics. For example, under water stress, plants produce elevated amounts of abscisic acid (ABA) to optimize stomatal closure so as to conserve water (Karim and Rahman, 2015;Yang et al, 2018). Zn is known to increase ABA production in plants (Zengin, 2006;Yang et al, 2018), thereby enhancing stomatal regulation by ABA under water limitation.…”

“…For example, under water stress, plants produce elevated amounts of abscisic acid (ABA) to optimize stomatal closure so as to conserve water (Karim and Rahman, 2015;Yang et al, 2018). Zn is known to increase ABA production in plants (Zengin, 2006;Yang et al, 2018), thereby enhancing stomatal regulation by ABA under water limitation.…”