Exogenous TiO2 Nanoparticles Alleviate Cd Toxicity by Reducing Cd Uptake and Regulating Plant Physiological Activity and Antioxidant Defense Systems in Rice (Oryza sativa L.)

Oryza glumaepatula and calcium oxide nanoparticles enhanced Cr stress tolerance by maintaining antioxidant defense, chlorophyll and gene expression levels in rice

Chromium (Cr), a potent heavy metal, threatens rice cultivation due to its escalating presence in soil from human activities. Wild rice contains useful genes for phytoremediation; however, it is difficult to use directly for metal mitigation. Here, a single segment substitution line (SSSL), SG001, was developed by crossing O. glumaepatula and Huajingxian74 (HJX) to evaluate the survival ability of plants against Cr. Further, we explored the potential effect of calcium oxide nanoparticles (CaO-NPs) (50 μM) to minimize the toxic effect of Cr (100 μM) in rice cultivars, SG001 and HJX. The findings of this study indicated that Cr toxicity led to increased oxidative stress. This was shown by higher levels of hydrogen peroxide (H2O2), which was increased by 104% in SG001 and 177% in HJX, and malondialdehyde (MDA) increased by 79% in SG001 and 135% in HJX. Furthermore, it also depicted that Cr toxicity considerably declined shoot and root length, shoot and root fresh weight by 30%, 27%, 25%, and 20% in SG001 and 44%, 51%, 42%, and 45% in HJX, respectively. This mitigation was evidenced by decreased Cr contents, increased calcium (Ca) levels in SG001, and the maintenance of chlorophyll, antioxidant defense, and gene expression levels. Moreover, there was a notable reduction in MDA and H2O2, while the defense mechanisms of key antioxidants, including ascorbate peroxidase, superoxide dismutase, glutathione, catalase, and peroxidase were upregulated, along with an increase in soluble protein contents in both rice cultivars after applying CaO-NPs. CaO-NPs effectively restored cellular and subcellular structural integrity and growth in both lines, which had been seriously disrupted by Cr toxicity. Overall, our findings suggest that SG001, in combination with CaO-NPs, could serve as an effective strategy to mitigate Cr toxicity in plants.

Nanosized-Selenium-Application-Mediated Cadmium Toxicity in Aromatic Rice at Different Stages

Cadmium (Cd) pollution restricts the rice growth and poses a threat to human health. Nanosized selenium (NanoSe) is a new nano material. However, the effects of NanoSe application on aromatic rice performances under Cd pollution have not been reported. In this study, a pot experiment was conducted with two aromatic rice varieties and a soil Cd concentration of 30 mg/kg. Five NanoSe treatments were applied at distinct growth stages: (T1) at the initial panicle stage, (T2) at the heading stage, (T3) at the grain-filling stage, (T1+2) at both the panicle initial and heading stages, and (T1+3) at both the panicle initial and grain-filling stages. A control group (CK) was maintained without any application of Se. The results showed that, compared with CK, the T1+2 and T1+3 treatments significantly reduced the grain Cd content. All NanoSe treatments increased the grain Se content. The grain number per panicle, 1000-grain weight, and grain yield significantly increased due to NanoSe application under Cd pollution. The highest yield was recorded in T3 and T1+3 treatments. Compared with CK, all NanoSe treatments increased the grain 2-acetyl-1-pyrroline (2-AP) content and impacted the content of pyrroline-5-carboxylic acid and 1-pyrroline which are the precursors in 2-AP biosynthesis. In conclusion, the foliar application of NanoSe significantly reduced the Cd content, increased the Se content, and improved the grain yield and 2-AP content of aromatic rice. The best amendment was applying NanoSe at both the panicle initial and grain-filling stages.

Silicon and selenium alleviate cadmium toxicity in Artemisia selengensis Turcz by regulating the plant-rhizosphere

Soil cadmium (Cd) pollution has emerged as a pressing concern due to its deleterious impacts on both plant physiology and human well-being. Silicon (Si) is renowned for its ability to mitigate excessive Cd accumulation within plant cells and reduce the mobility of Cd in soil, whereas Selenium (Se) augments plant antioxidant capabilities and promotes rhizosphere microbial activity. However, research focusing on the simultaneous utilization of Si and Se to ameliorate plant Cd toxicity through multiple mechanisms within the plant-rhizosphere remains comparatively limited. This study combined hydroponic and pot experiments to investigate the effects of the combined application of Si and Se on Cd absorption and accumulation, as well as the growth and rhizosphere of A. selengensis Turcz under Cd stress. The results revealed that a strong synergistic effect was observed between both Si and Se. The combination of Si and Se significantly increased the activity and content of enzymes and non-enzyme antioxidants within A. selengensis Turcz, reduced Cd accumulation and inhibiting its translocation from roots to shoots. Moreover, Si and Se application improved the levels of reducing sugar, soluble protein, and vitamin C, while reducing nitrite content and Cd bioavailability. Furthermore, the experimental results showed that the combination of Si and Se not only increased the abundance of core rhizosphere microorganisms, but also stimulated the activity of soil enzymes, which effectively limited the migration of Cd in the soil. These findings provided valuable insights into the effective mitigation of soil Cd toxicity to plants and also the potential applications in improving plant quality and safety.

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.