Chromium is naturally found in the soil, air, and water. The trivalent chromium (Cr⁺³) and hexavalent chromium (Cr⁺⁶) are two forms of chromium (Yoshinaga et al., 2018). The hexavalent chromium is considered the most toxic form that mainly exits as chromate or dichromate. Human activities and industrialization have increased the addition of chromium in the environment, and it has become a significant concern for plants, animals, and humans due to its lethal effects. The primary source of chromium pollution is the leather industry, followed by the electroplating industry textile and chromate mining (Belay, 2010; Tumolo et al., 2020). Chromium is a toxic and non-essential element for plants, and plants do not have a specific mechanism for its uptake (Shanker et al., 2005). Chromium is highly toxic for plants due to its detrimental effect on plants’ growth and development. The most common effect of chromium on plants is reduced leaf and root growth, reduced yield, inhibition of enzymatic activity, and mutagenesis (Singh et al., 2013).

Sunflower (Helianthus annuus L.) is one of the essential vegetable oil sources, with an annual production of about 9 million tonnes. The sunflower oil is highly healthy due to its high concentration of mono-saturated and poly-saturated fatty acid and high contents of vitamin E (Kaya et al., 2012). Sunflower has been widely studied due to its unique ability to uptake and accumulate various heavy metals (Alaboudi et al., 2018; Chen et al., 2020). Sunflower can uptake and accumulate metals such as Pb, Cd, and Cr in its roots and shoots from contaminated soil (Chen et al., 2020). Due to its high biomass, sunflower is recommended to remediation soil contaminated with heavy metals. However, the high concentration of chromium has a detrimental effect on sunflower productivity and enzymatic activity (Fozia et al., 2008).

Phytoremediation is a green technology in which plants remove toxic pollutants and heavy metals from contaminated soil and water. In this technology, the potential of plants is applied to uptake, accumulate, and degrade pollutants from the contaminated medium (Hauptvogl et al., 2019). The ability of plants to remediate the pollutants depends upon the type of pollutants, concentration of pollutant and plant biomass production, and ability to accumulate metals (Ashraf et al., 2019). Hyperaccumulator plants have tremendous ability to accumulate particular metals and metalloids, hundred or thousand times greater than typical plants in the same environmental condition (Peer et al., 2005). These plants are most tolerant to high concentrations of heavy metals, can uptake these pollutants through their roots system, and accumulate high levels of these metals in their living tissue such as roots, stems, and leaves (Peer et al., 2005; Baker et al., 2020). Plants also have different metabolic processes to alleviate the metal stress, such as producing organic acids that can chelate the heavy metals and covert these metals into not toxic form and prevent the uptake of heavy metal by plants (Shahid et al., 2017, 2019). Other methods used by the plant to minimize the metal toxification include extracellular complexation and cytoplasmic complexation. The metal stress in the plant causes severe damage to the plant’s metabolic process. To tolerate the metals stress, the plant should activate the antioxidant enzymes system and repair the oxidative stress and damage caused by metal stress (Sharma et al., 2016).

The microorganism can be applied as a biological tool to remove the heavy metals from the polluted medium due to their ability to concentrate and recover heavy metals (Devi et al., 2021). Bioremediation is the application of microorganisms to remediate polluted sites contaminated with heavy metals and other organic/inorganic pollutants (Shahid et al., 2020). Bacteria have evolved efficient mechanisms to bio-remediate heavy metals. These mechanisms include detoxification, adsorption, oxidation, and reduction of heavy metals (Kang et al., 2016; Naik and Kumar, 2020). The versatility of bacteria to degrade various pollutants and heavy metals has emphasized their application in the remediation of different types of soils. The bioremediation has shown promising results in the remediation of heavy metals such as lead, chromium, and cadmium, even in deficient concentrations in soil and water where other processes failed (Tiquia-Arashiro, 2018; Pushkar et al., 2021). The phytoremediation process can be boosted by applying selective bacteria resistant to specific metals. It has been widely proven that the application of bacteria in phytoremediation increases heavy metals such as lead, chromium, and iron uptake and removal from the contaminated soils (Kong et al., 2019; Ma et al., 2019).

Nanotechnology has grown remarkably in the last two decades with its wide application in agriculture and environmental remediation (Song et al., 2019; Zand et al., 2020). Nanoparticle-assisted phytoremediation has emerged as a reliable technology for removing contaminated soil with metals and metalloids. The nanoparticle has been widely applied from remediation of lead, cadmium, and arsenic from contaminated soil (Song et al., 2019; Zhu et al., 2019). The nanoparticle may improve the phytoremediation by a direct effect such as directly removing the pollutant from the soil by adsorption and immobilizing the metals, and ultimately reducing the concentration of metals in the soil (Omara et al., 2019). The nanoparticle can also improve phytoremediation by promoting plant growth, such as carbon nanotubes, Ag nanoparticles, and ZnO nanoparticles (Sabir et al., 2014). The nanoparticle may also increase the plant tolerance to abiotic stress by regulating the gene expression of enzymes, such as application of silicon nanoparticles improved the phytoextraction capacity of pea plant for chromium (VI; Tripathi et al., 2015). The applied nanoparticle may also improve plant growth by increasing the availability and absorption of nutrients and water (Kale and Gawade, 2016).

Considering the influential role of nanoparticles and bacteria, a field study was conducted to analyze the potential of Cerium Dioxide Nanoparticles (CeO₂ NPs) and a chromium-resistant bacteria Staphylococcus aureus in remediating the chromium from contaminated soil through sunflower plants. It was assumed that combined application of CeO₂ NPs and chromium-resistant bacteria will alleviate the chromium toxicity by their mutual role and enhance plant tolerance to metal toxicity. The augmented and individual role of CeO₂ NPs and S. aureus on the performance of chromium-stressed plants was evaluated by analyzing the changes in growth parameters, enzymatic activity, and oxidative stress. The finding from this research will provide valuable information about the application of nanoparticles and bacteria in remediating heavy metals from the contaminated medium.

In this study, for the first time, we showed that the application of CeO₂ NPs and bacteria S. aureus could alleviate chromium-induced toxicity in the sunflower plants at an optimal concentration of chromium (Cr50). The combined application of CeO₂ nanoparticles and S. aureus efficiently ameliorated Cr-induced oxidative stress by promoting antioxidant enzymes activities and improved the photosynthetic activity by promoting chlorophyll contents in plants. In addition, CeO₂ NPs and S. aureus decreased the Cr accumulation in roots and shoots in plants tolerating the highest chromium concentration (Cr50). The high concentration of chromium negatively affects morphological and agronomical parameters of plants through altering the water use efficiency, stomatal conductance, and photosynthesis process (Figures 1A–F, 2A–D; Ertani et al., 2017). In the present study, we evaluated that the application of CeO₂ NPs alleviated the adverse impacts of chromium and improved plant agronomic characteristics (Figures 1A–F). Similar findings were reported by Gohari et al. (2020), where TiO₂ NPs improved the agronomic traits in Moldavian balm under stress (Gohari et al., 2020). The large surface area and small size of the nanoparticles allow nanoparticles to penetrate in plant cells and then alleviate the negative effect of metal stress by improving plant growth (Singh et al., 2019). This increase in plant growth might be attributed by increases in water use efficiency, transpiration rate, stomatal conductance, and photosynthetic rate (Figures 2A–D; Hezaveh et al., 2019). The combined application of S. aureus and CeO₂ NPs further improved the plant morphological and agronomic parameters as compared to the only application of NPs. It is well established that bacteria can increase essential nutrient availability to the plant (Ren et al., 2019). The bacteria also can degrade and transform the metals into less toxic forms and decrease the stress on the plant due to metal toxicity. Similarly, in this study, the application of S. aureus improved plant growth and biomass production due to its ability to detoxify the high concentration of chromium. The bacteria can improve plant nutrition through phosphorus solubilization, nitrogen fixation, and secretion of hormones and enzymes essential for plant growth and ultimately enhance the phytoremediation potential of plants (Marciano Marra et al., 2012; Matse et al., 2020).

The increased metal stress in various plant species resulted in a decrease in chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid content, ultimately disrupting of whole photosynthesis process (Chandra and Kang, 2016; Sytar et al., 2019). The reduction in photosynthesis under metal stress is considered a sign of oxidative stress due to the rapid decrease in photosynthetic pigments and reduction in light absorbance capacity (Ashfaque et al., 2017). It was evident that the application of CeO₂ NP improved the chlorophyll a, chlorophyll b, and total carotenoid content in plants under chromium stress and reduced the oxidative stress (Figures 3A–D). The beneficial role of many nanoparticles in enhancing the chlorophyll contents and improving the plant photosynthetic activity has been reported by many researchers. Such as application of CeO₂ NPs acted as a catalyst in the production of Chl a and Chl b contents and increased the production of carotenoid contents in plant (Kataria et al., 2019; Etesami et al., 2021). Similarly, the foliar application of cerium oxide nanoparticles improved the photosynthesis pigment in Calendula officinalis (Jahani et al., 2019). The decline in antioxidant enzyme activity plays a crucial role in producing oxidative stress in plants suffering from chromium stress (Etesami, 2018). The significant increase in the chlorophyll contents and photosynthesis activity due to combined application of S. aureus and CeO₂ NPs might be attributed to the ability of bacteria to increase the availability of micronutrients, decrease in bioavailability of chromium, and increase the ability of the plant to tolerate metal stress (Rizwan et al., 2019; Etesami et al., 2021). Similar to our findings, the application of bacteria improved the chlorophyll a, chlorophyll b, and carotenoid contents in the sunflower plant under Pb stress (Saleem et al., 2018). In another study, the TiO₂ NPs improved the photosynthetic activity in soybean plants under Cd stress by entering the chloroplast and enhancing light adaption and electron transfer (Singh and Lee, 2016).

The increasing concentration of chromium declined the enzyme activity in the plants (Figures 4A–D). At the highest concentration of Cr50, the SOD activity, POD activity, CAT activity, and APX activity reduced sharply due to damage to the antioxidant enzymes metabolism induced by chromium toxicity. It is well reported that metal toxicity decreases the catalytic activities in the plant which ultimately decrease the activity of the antioxidant enzymes (Bhaduri and Fulekar, 2012). The production of ROS attacks antioxidants enzymes, consumes a huge concentration of antioxidant enzymes, and generates oxidative stress by disturbing the redox homeostasis in plants (He et al., 2017; Ahmad et al., 2019). This oxidative stress can cause severe damage to specific proteins and destruction to the cell structure and function (Ahmad et al., 2019). The application of nanoparticles has been reported to improve SOD, CAT, and POD activity without producing the hydroxyl radicals and improving the plants’ overall growth.

The chromium stress in plants leads to the production and accumulation of toxic substances such as reactive oxygen species (ROS) that initiate the lipid peroxidation and damage to cell structure (Figures 5A–C; Ghori et al., 2019; Jahani et al., 2019). The production of ROS initiates the death of plant cells by damaging specific proteins, lipids, and nucleic acids and decreases the chlorophyll contents (Sytar et al., 2019). This damage can be identified by increased production and accumulation of MDA, and increased concentration of electrolytes leakage (EL) and high content of H₂O₂ (Mallhi et al., 2019). Under metals stress, the concentrations of MDA, EL, and H₂O₂ increased in the plants (Berni et al., 2019; Ghori et al., 2019). The nanoparticle has been found to reduce ROS production in plants under metal stress. TiO₂ NPs and Fe NPs have been found effective in reducing the abiotic stress in plants by reducing ROS production in Zea mays and grape, respectively (Mozafari and Ghaderi, 2018; Singh et al., 2021). In the current study, the increased production of antioxidants enzymes due to the positive effect of CeO₂ NPs ultimately decreased the production of EL, MDA, and H₂O₂ contents in plants under chromium stress. In the plants, ROS production is initiated in response to stress, and antioxidants act as a defense mechanism against stress condition and act as a front line to cope with and protect the plant cell from free radicals and minimize the damage initiated by ROS in response to stress (Kohli et al., 2017). Antioxidant enzymes scavenge the H₂O₂ contents APX and SOD cope with ROS and neutralize the superoxidase radicals. Our study showed that CeO₂ NPs increased the production of antioxidants enzymes, which resulted in decreased activity of EL, MDA, and H₂O₂ and alleviated the metal stress in the plant. Our findings agree with the aforementioned studies where the application of nanoparticles enhanced antioxidant enzyme activities in response to oxidative stress (Hussain et al., 2019b; Sun et al., 2019). The increased level of antioxidant enzyme activity might be linked to the antioxidant ability of nanoparticles, which enhances the plants’ activities (Jahani et al., 2019). It was found that the increasing concentration of Ag NPs increased the catalase and peroxidase activity in the Lycopersicon esculentum (Karami Mehrian et al., 2016). Similarly, wheat plants treated with silver and gold NPs significantly improved the plant dry biomass and improved the SOD, CAT, and APX activity in plants under abiotic stress (Manaf et al., 2021). The increase in antioxidant enzymes activity and decrease in ROS contents with the application of S. aureus may be attributed to bacteria’s ability to increase mRNA/gene expression of antioxidants in inoculated plants compared to non-inoculated plants (Gururani et al., 2013; Khan et al., 2022). The inoculation of bacteria enhanced the expression level of various ROS scavenging enzymes and increased the proline contents in potatoes plants under stress (Gururani et al., 2013).

The application of CeO₂ nanoparticles reduces the uptake and accumulation of Cr⁺⁶ and Cr⁺³ in the sunflower root and shoots despite increasing chromium concentration (Figures 6A–D). Many studies reported that exogenous application of nanoparticles could reduce the uptake of toxic metals by plants (Hussain et al., 2019b; Rizwan et al., 2019). The use of TiO₂ NPs in rice plants grown hydroponically reduced the concentration of Pb in roots and shoots and improved plant growth (Cai et al., 2017). Similarly, ZnO NPs decreased the arsenic concentration in the rice plat’s roots and leaves suffering from arsenic stress (Wu et al., 2020). It is well reported that nanoparticles can absorb and transform the heavy metals in the soils by reducing their mobility and bioavailability such as Fe₃O₄ NPs reduced the mobility of Cd in the soil and reduced its bioavailability to the plants (Chen et al., 2018; Hussain et al., 2019a). Thus, the decreased accumulation of chromium in the sunflower plant can be attributed to the ability of the nanoparticle to decrease the bioavailability of chromium. Further, most of the NPs accumulate in the cell wall, bind with the heavy metals, make them unavailable, and hinder the migration of heavy metals in the plant (Molnár et al., 2020). The bacteria can tolerate metal toxicity and break down and remove the heavy metal through their metabolic process. Bacteria can reduce the bioavailability of heavy metals through biosorption, bioaccumulation, biotransformation, bio-precipitation, and bio-crystallization in contaminated soil (Medfu Tarekegn et al., 2020). In this study, the application of bacteria transformed the Cr⁺⁶ into Cr⁺³ through their metabolic process. It reduced the bioavailability of the chromium to the plant, which ultimately reduced its bioaccumulation in plant roots and shoots. Similarly, the application of S. aureus in wheat plants reduced the uptake and accumulation of chromium in plant roots and shoots grown in chromium-contaminated soil (Zeng et al., 2020). The nanoparticles can improve bacterial growth when applied in small amounts; for example, the use of TiO₂ NPs improved the performance of plant growth-promoting rhizospheric bacteria in plants under stress (Timmusk et al., 2018).

The application of nanoparticles is a promising approach that has the tremendous potential to protect the plant from metal-induced stress. The CeO₂ NPs improved the morphological, physiological, and biochemical properties and overall growth and biomass production of sunflower plants grown in the high chromium concentration. The CeO₂ NPs reduce the H₂O₂ stress, enhance the antioxidant enzymatic activities, and ultimately alleviate plants’ oxidative stress due to chromium toxicity. Along with nanoparticles, bacteria also have a prominent role in alleviating the metal-induced toxicity through their metabolic process and reducing the bioavailability of Cr to the plant. The combined application of S. aureus and CeO₂ nanoparticles improved the metabolic process, triggered the activation of the enzymatic defense system, and thus enhanced the plant performance in sunflower plants under chromium stress. The use of nanoparticles and bacteria in combination could be a novel way to clean up contaminated soil and strengthen plants to withstand metal-induced stress. The novel application of nanoparticles in agriculture may aid in meeting rising food demand while also ensuring environmental sustainability. However, there is still needed to further explore about action mechanism of nanoparticles, permissible limit and ecotoxicity in edible crops.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

JM: conceptualization and project administration. HA: data curtain and investigation. MO: funding and resources allocation. YA: formal analysis and software. FC: methodology and data curtain. MA: formal analysis and project administration. AH: review and editing and formal analysis. SH: investigation and formal analysis. MS: writing an original draft, methodology, and review and editing. All authors contributed to the article and approved the submitted version.

This work was supported by the National Natural Science Foundation of China (no. 51974313) and the key project of Jiangsu Key Laboratory of Coal-Based Greenhouse Gas Control and Utilization (2020ZDZZ03). The authors extend their appreciation to the Researchers Supporting Project number (RSP-2021/374) King Saud University, Riyadh, Saudi Arabia.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.