Metal oxide nanoparticles as promising agents for triggering defense mechanisms in plants against bacterial diseases

Abstract

Metal oxide nanoparticles (MONPs) have received much attention in recent years because of their potential to improve plant defense mechanisms against bacterial infections. MONPs interact with plant tissues in a way that activates natural immune responses, making them an intriguing alternative to standard chemical pesticides. MONPs such as zinc oxide (ZnO), copper oxide (CuO), and titanium dioxide (TiO₂) can cause oxidative stress in plant cells and generate reactive oxygen species (ROS), which activate defense-related signaling pathways.Reactive oxygen species (ROS) can be directly scavenged by nanoparticles, which can also act as transporters to more efficiently deliver traditional antioxidants to target areas or mimic natural antioxidant enzymes.In addition to their ability to stimulate plant immune responses, MONPs have inherent antibacterial characteristics that can directly impede bacterial development. When applied to plants, MONPs penetrate the cell walls and membranes of both plant and bacterial cells, disrupting bacterial cell integrity and restricting pathogen growth. This dual effect, which stimulates plant defenses while directly targeting pathogens, improves the overall resistance of plants to bacterial infections. Furthermore, the ability of metal oxide nanoparticles to elicit systemic acquired resistance (SAR) makes them an appealing alternative for sustainable disease control, thereby reducing the reliance on chemical pesticides and minimizing their negative environmental consequences. MONPs have a promising future in plant protection, with continuing research aimed at optimizing their size, surface properties, and delivery techniques to improve their efficacy and durability.

Introduction

There are approximately 22,000 pathogens and pest species known to cause a variety of crop diseases that result in significant crop loss, both qualitatively and quantitatively [1]. Among plant diseases, bacterial diseases are the most contagious and spread through soil and other mechanisms. Among these diseases, tomato bacterial wilt caused by Ralstonia solanacearum is known to be dangerous [2]. Tomato bacterial wilt, caused mostly by R.solanacearum, is one of the most damaging diseases to tomato crops worldwide. This soil-borne disease causes large yield losses, which can range from 30 to 100% depending on environmental factors, host susceptibility, and pathogen virulence [3]. The illness is more severe in tropical, subtropical, and mild temperate regions, where it thrives because of favorable moisture and temperature conditions [4]. The economic impact of bacterial wilt is significant. In many underdeveloped nations, where tomatoes are important commercial crops and staples in local cuisines, the disease poses a significant danger to food security and income. In some parts of Asia and Africa, yield reductions have caused significant economic damage, with farmers frequently losing whole crops [5]. This has inspired substantial research into management measures, but effective control remains difficult because of the pathogen’s complex life cycle and persistence in soil. Cultural strategies such as crop rotation and the use of resistant varieties have been used to reduce the impact of bacterial wilt, but these methods have had limited efficacy in disease-prone areas [6]. Biological control and integrated disease management options are being investigated, but the worldwide agricultural community continues to face considerable challenges in managing this disease successfully. The persistent expansion and economic impact of tomato bacterial wilt highlight the importance of the ongoing study and development of sustainable management solutions [7]. At the national level, the impact of bacterial wilt on tomato crops can be significant. In countries where tomatoes are a key agricultural product, bacterial wilt can drastically diminish total yield. For example, bacterial wilt has been reported to cause yield losses of up to 80% in some locations in India [8]. Similar rates have been reported in portions of Africa and Southeast Asia, where tomato production is essential for both domestic consumption and export markets. The economic repercussions include not only lower yields but also higher disease management expenses and worse marketable crop quality. During the past few decades, plant diseases have been controlled via chemical methods that involve pesticides and antibiotics. However, controlling diseases via chemicals necessitates financial outlays and presents serious risks to agricultural output and environmental stability. Moreover, the consistent use of pesticides has resulted in resistance in microorganisms, increasing their difficulty in controlling and influencing output [9]. Therefore, innovative technologies are urgently needed to increase crop yields, as the world’s population is expected to surpass 10 billion people within 25 years [10]. MONPs are cutting-edge, promising remedies for addressing the problems of biotic and other plant stressors as well as disease prediction systems. New ideas and agricultural products have been developed as a result of nanotechnology. They have enormous potential to address the yield losses caused by bacterial disease.

MONPs represent a broad category of materials that encompass particulate substances having at least one dimension smaller than 100 nm [11, 12]. The significance of these materials became evident when scientists discovered that the size of a substance can affect its physiochemical properties. MONPs, also known as ultrafine particles, have dimensions of 1–100 nm and a high surface area: volume ratio, increasing their reactivity. Owing to their small size and large surface area, nanoparticles have unique size-dependent characteristics. When a particle approaches the nanoscale with a characteristic length scale close to or smaller than the de Broglie wavelength or light wavelength, the periodic boundary conditions of the crystalline particle are lost [13]. The harmful effects of nanoparticles, particularly copper-based nanomaterials, are mainly linked to the generation of lipid-based peroxides and DNA damage arising from oxidative stress generated by ROS [14]. These effects are thought to be caused by complicated toxicity pathways. Both physical stress-induced cell structural deformation and cell plasma membrane damage, which are linked to nanoparticle cell interactions, have also been identified as important drivers of toxicity [14].

The use of MONPs provides several options for protecting against plant disease. These nanoparticles can be used for structural reinforcement, antimicrobial activity, stimulation of defense responses, stress reduction, and improved nutrient intake [15]. The use of MONPs as promising agents to improve plant defense systems against bacterial infections aims to understand their potential role in the resilience and protection of plants from diseases [16]. This involves investigating the ways in which MONPs can increase resistance to bacterial infections by promoting physiological and biochemical alterations, inducing systemic acquired resistance, and stimulating plant innate immune responses [17]. Currently, there are multiple review articles on the use of nanomaterials in sustainable agriculture. However, these reviews generally focus on various types of nanomaterials (nanopolymers, nanocarbons, and metal nanoparticles) and their applications (fertilizers, nematicides, and fungicides) in agriculture, with no in-depth analysis of the use of metal oxide nanoparticles for controlling phytopathogenic bacteria [18]. In other cases, evaluations have focused on the utilization of MONPs in composite materials to suppress phytopathogenic bacteria [19]. This analysis aims to assess the role of metal oxide MONPs in plant defense against bacterial pathogens and explore the mechanisms of action and effects on various plant species to offer important insights into the use of MONPs to increase crop productivity and plant health to manage bacterial disease. The increasing frequency of bacterial infections in plants presents considerable challenges to agricultural production and food security. Traditional methods of disease control frequently involve the use of chemical pesticides, which can have negative environmental consequences and contribute to the development of pathogen resistance. As a result, there is increased interest in alternate crop disease management measures. One such option is the use of nanoparticles, namely, magnesium oxide (MgO) MONPs. Owing to their promising antibacterial capabilities, MgO (MONPs), which can successfully repel bacterial infections that harm plants, have attracted increasing interest.

Methodology of the study

Search strategy

Prisma standards were used for our literature search [20]. We analyzed the existing scientific literature on the use of MONPs as promising agents for triggering defense mechanisms in plants against bacterial diseases using databasesvia the Google Scholar, PubMed, Science Direct, Web of Science and Scopus databases under the following conditions. The following keywords were used: metal oxide nanoparticle, plant defense, bacterial disease and nanotechnology.

Selection criteria

When assessing publications on the use of MONPs to improve plant defense mechanisms against bacterial infections, inclusion criteria and exclusion criteria were used to determine fit with the eligibility criteria of this review (Fig. 1).

Fig. 1.

Flow diagram of study selection

Inclusion criteria

Nanoparticle Composition and Synthesis: MONPs, such as those made (ZnO), (CuO), (TiO₂), and (Fe₂O₃), must be synthesized via procedures that guarantee high purity, uniformity, and well-defined size and morphology. To establish their properties, these NPs should be characterized via techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction. To be useful in field applications, the composition of nanoparticles must remain stable under circumstances that replicate environmental exposure.

Antibacterial activity: MONPs clearly have antibacterial effects on a variety of harmful plant microorganisms. This involves determining their ability to limit bacterial growth in vitro, which is commonly accomplished via procedures such as the disc diffusion test or broth microdilution assays. MONPs should significantly reduce bacterial colony formation, indicating their potential for regulating bacterial infections in plants.

Mechanism of action: The inclusion criteria should include considering how metal oxide nanoparticles affect plant defense mechanisms. This involves determining their ability to elicit plant immunological responses such ROS generation and SAR activation defense-related gene activation, and secondary metabolite buildup. A thorough understanding of how these nanoparticles interact with plant cells and tissues is needed to ensure that they produce protective responses that are effective and consistent [20].

Exclusion criteria

The exclusion criteria for studies investigating the use of MONPs as prospective agents for activating plant defense mechanisms against bacterial illnesses should consider many critical parameters to guarantee that the results are reliable and relevant. Studies that lack a clear and consistent technique for assessing the effects of MONPs on plant defense responses should be omitted. Studies that do not adequately characterize MONPs, such as their size, surface charge, and dispersion properties, should be avoided, as these factors are crucial in assessing MONPs effectiveness in plant defense. Studies that focus solely on the physical features of MONPs without considering their biological interactions with plant defense mechanisms or bacterial pathogens should also be eliminated. Finally, studies that focus on nonplant systems or use ineffective bacterial strains that do not mimic typical agricultural pathogens should be omitted from the assessment of the potential of MONPs in plant disease control (Fig. 1).

Data extraction

Information on the use of metal oxide nanoparticles as promising agents for triggering defense mechanisms in plants against bacterial diseases was included in the original publication.

Data analysis

Data obtained from the literature search were maintained and organized in Microsoft Excel, analyzed and visualized via SPSS and presented in graphs and tables.

Results

Metal oxide nanoparticles related to plant defense against bacterial pathogens

The increasing use of nanoparticles in agricultural techniques has stimulated substantial research into the characteristics and functions of (MONPs) in terms of plant defense systems [21]. MONPs, including (ZnO), (TiO₂), and (CuO), have distinct physical and chemical features that can improve plant tolerance to biotic and abiotic stresses [21]. These nanoparticles increase nutrient intake, photosynthetic efficiency, and systemic resistance to infections. For example, ZnO MONPs have been found to increase the production of phytohormones and secondary metabolites, strengthening the defense system of plants against pests and diseases [22].

Antimicrobial properties of the metal oxide nanoparticles

MONPs have received much interest in recent years because of their extraordinary antibacterial characteristics, especially against bacterial infections that harm plant health. The increasing occurrence of bacterial infections in crops has forced the development of novel disease management measures, as traditional chemical pesticides frequently cause environmental damage and the emergence of resistant strains. MONPs (ZnO, CuO, TiO₂, and AgO) effectively suppress bacterial infections, promote plant development, and improve stress resistance. ZnO NPs have been recognized for their broad-spectrum antibacterial activity. Several studies have demonstrated that ZnO NPs can successfully inhibit bacterial pathogens such as E. coli, Pseudomonas syringae, and X. campestris [23]. When exposed to light, ZnO NPs generate (ROS), which are principally responsible for their antibacterial properties. In addition to direct bacterial suppression, ZnO NPs can activate plant defense mechanisms by increasing the expression of defense-related genes and generating systemic acquired resistance [21]. ZnO NPs have been highlighted for their broad-spectrum antibacterial activity. ZnO NPs have been proven in several studies to successfully inhibit bacterial pathogens such as E. coli, P. syringae, and X. campestris [23]. When exposed to light, ZnO NPs generate reactive oxygen species, which is thought to be their primary antibacterial mechanism. These ROS can cause oxidative stress in bacterial cells, resulting in lipid peroxidation, protein denaturation, and cell death [24]. In addition to direct bacterial suppression, ZnO NPs can promote plant defense systems by increasing the expression of defense-related genes and generating systemic acquired resistance [21].

TiO₂ NPs have gained popularity because of their unique photocatalytic characteristics. When exposed to UV radiation, TiO₂ NPs produce hydroxyl radicals and other reactive species, which have strong antibacterial properties. TiO₂ NPs have been shown to drastically diminish the viability of P. aeruginosa and E. coli in culture [25]. Activating TiO₂ NPs under light is useful because it allows focused activation during sunlight, potentially reducing their toxicity to nontarget organisms. TiO₂ NPs have been shown to increase plant photosynthesis and growth, potentially leading to increased resistance to bacterial infections [26]. AgO NPs are highly antimicrobial and have been extensively examined for their possible use in agriculture. AgO NPs exert their antibacterial activities by disrupting bacterial cell walls, inhibiting DNA replication, and generating ROS [27]. Several studies have shown that AgO NPs can successfully suppress bacterial diseases, including B. subtilis and K. pneumoniae, resulting in improved plant health and resilience [27]. The use of AgO NPs can also reduce the dependency on chemical pesticides, coinciding with sustainable farming methods that aim to minimize environmental impacts. MgO NPs have multiple mechanisms that contribute to their antibacterial action. Their distinct physicochemical features, such as large surface area and reactivity, enable interactions with microbial cells. The principal mode of action is the production of (ROS), which can cause oxidative stress in bacterial cells and damage biological components such as DNA, proteins, and membranes [27]. This oxidative damage eventually causes bacterial cell death or reduced growth. Furthermore, MgO NPs can release magnesium ions in aqueous environments. This ion release may lead to breakdown of the microbial cell membrane, limiting bacterial function [28]. The incorporation of MgO NPs into plant care procedures may improve crop resistance to bacterial diseases not only by directly attacking pathogens but also by enhancing the intrinsic defense mechanisms of plants. Recent research has yielded promising results in the use of MgO nanoparticles for the treatment of a variety of bacterial diseases that harm plants. For example, [29] reported that MgO NPs strongly prevented the growth of P. syringae, a prevalent bacterial pathogen that causes illnesses in a variety of crops, including tomatoes and beans. When administered as a foliar spray at concentrations ranging from 100 to 250 mg/L, MgO NPs substantially reduced disease incidence and severity while promoting healthier plant growth. In addition to P. syringae, MgO NPs have been proven to be active against X. campestris, which causes bacterial blight in crops such as cabbage and mustard. The results showed that MgO NPs treatment considerably reduced the bacterial population, which led to fewer lesions on affected plant tissues. This bactericidal activity shows that MgO NPs can serve as a viable environmentally acceptable alternative for reducing bacterial infections in agriculture. MONPs offer a viable avenue for the development of environmentally friendly antibacterial drugs against bacterial infections in plants. Their distinct features not only enable direct bacterial suppression but also encourage improved plant development and defense systems. As research continues to uncover the complex interactions between MONPs and plant systems, these nanoparticles have tremendous promise for incorporation into agricultural practices, paving the way for creative ways to address increasingly widespread bacterial pathogens.

Antioxidant properties of metal oxide nanoparticles against pathogenic bacteria

MONPs have attracted widespread attention in agricultural research because of their antioxidant qualities, which play an important role in strengthening plant defenses against harmful microorganisms. When plants are exposed to biotic stress, such as bacterial infections, they often experience oxidative stress, which is characterized by an excess of (ROS). These ROS can cause cellular damage, impair metabolic activities, and eventually contribute to plant diseases. MONPs such as (ZnO), (CuO), and (TiO₂) can reduce oxidative stress by stimulating the production of antioxidant enzymes and ROS-neutralizing chemicals. ZnO NPs, for example, have been demonstrated to increase the activity of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in plants exposed to bacterial stress. Increased levels of these enzymes aid in scavenging ROS and reducing oxidative damage, hence preserving cellular homeostasis [30]. Similarly, CuO NPs can activate the antioxidant defense system, leading to increased tolerance and resistance to harmful bacteria such as X. campestris [31]. MONPs stimulate these antioxidant pathways, which not only strengthen the plant’s immediate response but also contribute to long-term resilience against recurring infections. Furthermore, titanium dioxide nanoparticles have been shown to stimulate plant development and increase the accumulation of various phytohormones and antioxidants, providing an extra layer of protection against infections. TiO₂ application has been associated with increased levels of phenolic chemicals, which are antioxidants and can limit pathogenic bacterial growth [32]. The overall consequence of MONPs application is a significant improvement in plant health, as reflected by decreased disease incidence and severity. In conclusion, the antioxidant properties of MONPs play an important role in improving plant defense systems against harmful bacteria. These MONPs can efficiently reduce oxidative stress by triggering the plant’s antioxidant response, resulting in increased resistance and overall plant health.

By generating reactive oxygen species (ROS) that function as signalling molecules, metal oxide nanoparticles (MONPs) can effectively strengthen plant defences [24]. However, it is important to recognise that excessive ROS creation might result in phytotoxicity and jeopardise plant health. When MONP concentrations are high, they can overpower the plant’s natural antioxidant defences, causing oxidative stress that harms DNA, proteins, and lipids in cells. This manifests as visible symptoms like stunted growth, chlorosis, and reduced photosynthetic efficiency, ultimately making the plant more vulnerable to both abiotic and biotic stresses. Therefore, dose optimization is paramount for the safe and effective application of MONPs, ensuring that their beneficial elicitor effects are harnessed without inducing detrimental phytotoxic responses that could undermine disease resistance and crop yield [30].

Impact and individual efficacy of of MONPs against bacterial pathogens

The efficacy of MONPs against bacterial plant pathogens has been extensively studied in recent years. MONPs such as (ZnO), (CuO), and (TiO₂) have shown potential as antimicrobial agents against a wide range of bacterial plant pathogens [33]. For example, ZnO NPs have been shown to inhibit the growth of P. syringae, a bacterial pathogen that causes bacterial leaf spot in plants [34]. Similarly, CuO NPs have been found to be effective against X. campestris, a bacterial pathogen that causes black rot in plants [35]. The bacterial cell membrane physically damages and compromises cell integrity [36]. MONPs have been shown to be effective against bacterial plant pathogens in a variety of investigations, both in vitro and in vivo [37]. For example, [38] reported that TiO₂ NPs efficiently lowered the severity of bacterial leaf spot in tomato plants caused by P. syringae (Table 1). ZnO NPs is well known and frequently ascribed to both the production of ROS and the release of Zn² + ions. Because ZnO NPs can directly limit bacterial growth and possibly trigger host defence responses, they have demonstrated promise in plant systems against bacterial blight caused by X. oryzae pv. oryzae in rice [21]. On the other hand, TiO₂ NPs are more well-known for their function in promoting plant growth and stress tolerance, which can tangentially aid in disease resistance, even though they also have photocatalytic activity and produce ROS when exposed to UV radiation. These MONPs’ relative effectiveness varies greatly depending on the context. For example, ZnO NPs may be selected because of their lesser phytotoxicity concerns in specific crops, but CuO NPs may have higher direct bactericidal effects against a variety of plant microorganisms [33].

Table 1.

Efficacy of various metal oxides against bacterial plant pathogens along with their minimum inhibitory concentrations (MICs) based on available studies

Metal oxide	Bacterial pathogen	Plant species	Efficacy	Minimum inhibitory concentration (MIC)	References
ZnO	P. syringae	Tomato	Inhibitory effect	200 mg/L	[39]
ZnO	X. campestris	Cauliflower	Antimicrobial activity	150 mg/L	[40]
CuO	X. campestris	Cabbage	Effective against growth	100 mg/L	[41]
CuO	P. syringae	Lettuce	Inhibitory effect	125 mg/L	[41]
TiO2	Erwinia carotovora	Potato	Strong antimicrobial activity	50 mg/L	[42]
ZnO	R. solanacearum	Tomato	Significant growth inhibition	250 mg/L	[43]
Ag2O	Pectobacterium carotovorum	Potato	High efficacy	15 mg/L	[44]
CuO	Xanthomonas vesicatoria	Pepper	Effective antimicrobial activity	125 mg/L	[45]
TiO2	P. syringae	Eggplant	Moderate inhibition	75 mg/L	[46]
ZnO	Erwinia amylovora	Apple	Inhibitory effect	180 mg/L	[47]
ZnO	X. campestris	Cabbage	Induces systemic resistance; membrane damage	250 mg/L	[39, 48]
ZnO	P. syringae	Spinach	ROS-mediated bacterial inhibition	200 mg/L	[42, 49]
CuO	Xanthomonas oryzae	Rice	Cell wall rupture, ROS production	100 mg/L	[36, 50]
CuO	P. syringae	Tomato	Membrane damage, enzyme inhibition	125 mg/L	[36, 51]
CuO	E. amylovora	Apple	Inhibition of bacterial growth, membrane disruption	150 mg/L	[29, 51]
TiO2	P. syringae	Tomato	Photocatalytic ROS production	50 mg/L	[52, 53]
Ag2O	X. campestris	Cabbage	Membrane disruption, DNA damage	15 mg/L	[40, 54]
Ag2O	P. s syringae	Tomato	ROS generation, cell membrane damage	15 mg/L	[40, 54]
Fe2O3	Xanthomonas oryzae	Rice	Induces oxidative stress in bacteria, enhances plant defenses	100 mg/L	[49, 55]
Fe2O3	R. solanacearum	Tomato	Disrupts bacterial cell integrity	100 mg/L	[49, 56]
CeO2	X. campestris	Cabbage	Antibacterial via ROS modulation	50 mg/L	[57, 58]
CeO2	P. syringae	Tomato	Enhances plant antioxidant defenses	50 mg/L	[59, 60]
MnO2	Xanthomonas citri	Citrus	Produces ROS, activates plant defenses	80 mg/L	[61]
MnO2	P. syringae	Tomato	Antimicrobial, ROS-mediated	80 mg/L	[61]

Synergistic effects and formulation strategies of metal oxide nanoparticles against pathogenic plant bacteria

MONPs have sparked widespread interest in plant pathology because of their potential as antibacterial agents against plant pathogenic microorganisms. The distinctive features of MONPs, such as their large surface area, reactivity, and functionalization potential, all contribute to their efficacy. Certain MONPs, including (ZnO), (CuO), and (TiO₂), have shown promising antibacterial action against several plant diseases. Their modes of action often include the production of (ROS), which break bacterial cell membranes and eventually cause cell death [33]. When coupled with other antimicrobial drugs or packaged via specialized delivery methods, MONPs can have synergistic effects that improve their antibacterial activities. For example, coencapsulating MONPs with organic fungicides has been demonstrated to increase the solubility and stability of active compounds, hence increasing their overall efficiency against bacteria such as P. syringae [32]. Furthermore, modifying MONP surfaces with biopolymers or surfactants can improve adhesion to plant surfaces and enhance nanoparticle retention time, resulting in prolonged antibacterial action [41].

Nanocomposites, which combine MONPs with other nanomaterials or natural substances, constitute another formulation strategy that can improve antibacterial effectiveness. For example, nanocomposites of ZnO and chitosan preserve the antibacterial properties of metal oxides while also benefiting from the biocompatibility and biodegradability of chitosan [42]. This method not only helps manage pathogenic germs but also reduces the risk of phytotoxicity and environmental damage. Furthermore, controlled release systems have been devised to allow for the progressive release of MONPs, resulting in persistent antibacterial action against pathogens over time [43]. In conclusion, the use of metal oxide nanoparticles to combat plant pathogenic bacteria shows significant promise, especially when combined with synergistic tactics and new formulation processes. Continued study on this subject is critical for optimizing these tactics and striking a balance between efficacy, environmental safety, and economic viability in agricultural applications (Table 2).

Table 2.

Synergistic effects and formulation strategies of metal oxide nanoparticles against plant pathogenic bacteria

Metal oxide nanoparticle	Pathogen targeted	Plant species	Formulation strategy	Synergistic effects	References
ZnO	P. syringae	Tomato (Solanum lycopersicum)	ZnO + Chitosan Nanocomposite	Improved antimicrobial efficacy and biocompatibility, more effective at lower concentrations	[42]
CuO	X. campestris	Lettuce (Lactuca sativa)	CuO with Synthetic Surfactants	Enhanced solubility and stabilization leading to increased bacterial inhibition	[32]
TiO2	F. oxysporum	Banana (Musa spp.)	TiO2 + Essential Oils	Combined effect of ROS production from TiO2 and the antibacterial properties of oils	[43]
Ag2O	Rhizoctonia solani	Wheat (Triticum aestivum)	Ag2O loaded on Biopolymer Matrices	Sustained release effect leading to prolonged antibacterial activity	[41]
MgO	Alternaria solani	Potato (Solanum tuberosum)	MgO + Plant Extracts	Enhanced bioavailability and efficacy due to the complementary action of the extracts	[33]
ZnO	Xanthomonas oryzae pv. oryzae	Rice (Oryza sativa)	ZnO + Gum Arabic Coating	Increased stability and enhanced antibacterial effect	[44]
CuO	E. amylovora	Apple (Malus domestica)	CuO Nanorods with Organic Surfactants	Improved bacterial inhibition and plant safety	[45]
TiO2	Phytophthora infestans	Potato (Solanum tuberosum)	TiO2 + Green Tea Extracts	Synergistic effect of ROS and antioxidants leading to pathogen suppression	[46]
Ag2O	Botrytis cinerea	Grape (Vitis vinifera)	Ag2O Nanoparticles with Stabilizing Agents	Enhanced stability and antifungal activity	[47]
MgO	Sclerotinia sclerotiorum	Sunflower (Helianthus annuus)	MgO + Neem Extracts	Increased efficacy and reduced phytotoxicity	[48]

Mode of application of metal oxide nanoparticles in plants against bacterial pathogens

MONPs are commonly used in plant disease management via a variety of delivery strategies designed to maximize antibacterial efficacy while minimizing detrimental effects on plant health and the environment. Foliar spray is a popular mode of application in which MONPs are suspended in water or a suitable solvent, together with surfactants and/or stabilizers, to improve dispersion and adherence to plant surfaces. This approach is especially effective against infections that enter through stomata or plant wounds. For example, ZnO NPs have been used as foliar sprays against P. syringae, resulting in an effective bacterial population decrease due to their ability to generate (ROS) on the leaf surface [44]. Another option is to incorporate MONPs into soil or nutrient solutions, which aids in root uptake by plants.

TiO₂ NPs can be integrated into irrigation systems, resulting in increased plant development and reduced root pathogen populations due to their antibacterial characteristics [43]. Soil application provides a more systemic effect, allowing nanoparticles to exert their antibacterial effects not only in the rhizosphere but also throughout the plant, potentially improving overall plant health. Additionally, seed treatment with MONPs has emerged as a viable strategy for increasing seedling resistance to bacterial infections. Seeds can be coated with MONPs or submerged in nanoparticle solutions prior to sowing. This approach can protect plants during their key early phases of development(Table 3).

Table 3.

Mode of application of metal oxide nanoparticles against bacterial pathogens in plants

Metal oxide nanoparticle	Plant disease pathogen	Mode of application	Application method	Plant species	References
ZnO	P. syringae	Foliar Spray	Aqueous dispersion; applied directly to plant leaves	Solanum lycopersicum (Tomato)	[39]
CuO	Xanthomonas campestris	Soil Amendment	Mixed into the soil to enhance plant root uptake	Capsicum annuum (Bell Pepper)	[32]
TiO2	Fusarium oxysporum	Seed Treatment	Soaking seeds in TiO2 suspension before planting	Glycine max (Soybean)	[43]
Ag2O	Rhizoctonia solani	Controlled Release Systems	Incorporation into biodegradable films for gradual release	Zea mays (Maize)	[41]
MgO	Alternaria solani	Soil Drench	Applied in liquid form directly to the root zone	Solanum melongena (Eggplant)	[33]
Fe2O3	P. carotovorum	Hydroponic Culture	Incorporation into hydroponic nutrient solutions	Lactuca sativa (Lettuce)	[45]
ZnO	Xanthomonas campestris	Foliar Spray	Aqueous dispersion; applied directly to plant leaves	Brassica oleracea (Cabbage)	[50]
CuO	P. syringae	Soil Amendment	Mixed into the soil to enhance plant root uptake	Solanum tuberosum (Potato)	[29]
TiO2	Clavibacter michiganensis	Seed Treatment	Soaking seeds in TiO2 suspension before planting	Cucumis sativus (Cucumber)	[54]

CuO NPs have been demonstrated to improve disease resistance in seedlings of several crops, resulting in lower infection rates from pathogens such as X. campestris [32]. This treatment strategy, which provides seedlings with protective characteristics from the start, can considerably minimize early-stage infections and increase crop establishment. In summary, the mechanism by which MONPs can be applied to plants to prevent bacterial diseases, including foliar sprays, soil incorporation, and seed treatments, can vary greatly. Each strategy has advantages and may be adapted to specific diseases and crop circumstances to improve agricultural efficiency and sustainability.

Types and effects of metal oxide nanoparticles

MONPs are considered among the most promising nanomaterials because of their special chemical and physical characteristics, such as heat transfer and thermal conductivity [46]. The most common types of MONPs include Fe₂O₃, SiO₂, Al₂O₃, MgO, ZrO₂, CeO₂, TiO₂, and ZnO (Fig. 2). MONPs are a subfield of materials chemistry that is of great interest because of the possible technological uses of these compounds [47]. MONPs have emerged as attractive agents for bacterial disease control in plants because of their unique antibacterial characteristics and ability to improve plant development and stress resistance (Table 4). Among the numerous varieties of MONPs, ZnO NPs are particularly notable for their broad-spectrum antibacterial action, successfully suppressing various plant pathogenic bacteria, such as P. syringae and Xanthomonas species [39]. TiO₂ NPs, known for their photocatalytic capabilities, demonstrate antimicrobial effects, improve soil quality, and promote plant development when applied to crops [48]. Moreover, NPs of iron oxide have been found to enhance plant antioxidant defense mechanisms, resulting in resistance against harmful microorganisms [49] (Fig. 3).

Fig. 2.

Schematic illustration of nanoparticle-mediated protection against bacteria [52]

Table 4.

Effects of metal oxide nanoparticles on bacterial cells and plant defense mechanism

Metal oxide NP	Target pathogen	Plant species	Mechanism of action	References
ZnO	P. syringae	Tomato (Solanum lycopersicum)	Disruption of bacterial cell membrane, ROS generation	[42]
4CuO	X. oryzae	Rice (Oryza sativa)	Interaction with bacterial cell wall, ROS generation	[62]
TiO2	E. coli	Arabidopsis thaliana	ROS generation upon UV exposure, antibacterial activity	[63]
ZnO	E. coli	Maize (Zea mays)	Antibacterial activity via ROS and membrane disruption	[43]
CuO	Pseudomonas aeruginosa	Wheat (Triticum aestivum)	Generation of ROS, leakage of intracellular components	[64]
ZnO, MnO2, and MgO	P. syringae	Tomato (Solanum lycopersicum)	Antibacterial Activity	[65]
ZnO	R. solanacearum	Eggplant (Solanum melongena)	Strong antibacterial action against both gram-positive and gram-negative bacteria	[66]
CuO	E. coli	Lettuce (Lactuca sativa)	Effective against a wide range of pathogens including E. coli, Pseudomonas	[67]
TiO2	E. coli	Cucumber (Cucumis sativus)	Broad-spectrum antibacterial effects, with enhanced activity under UV light	[68]
Ag2O	R. solanacearum	Tomato	Highly potent against bacteria and fungi	[69]
Fe2O3	R. solanacearum	Pepper (Capsicum annuum)	Mild to moderate antibacterial activity	[70]
MgO	X. oryzae	Rice	Moderate antibacterial effects	[34]
CeO2	X. campestris	Wheat	Strong antibacterial properties	[71]
ZnO	X. campestris	Broccoli	Induces systemic resistance, increases antioxidant activity, disrupts bacterial cell walls	[72]
CuO	R. solanacearum	Tomato	Disruption of bacterial membrane integrity, activates plant immune responses	[73]
TiO2	P. syringae, E. carotovora	Potato	Antibacterial properties, induction of defense-related genes, enhancement of plant growth	[74]
MgO	E. coli, P. syringae	Maize	Induces plant systemic acquired resistance (SAR), antibacterial effects via ROS production	[11, 12]
Fe2O3	X.oryzae, P. aeruginosa	Rice, Tomato	Enhances plant immune system, triggers oxidative stress in pathogens, improves plant defense signaling	[30]
Ag2O	P. syringae, F. oxysporum	Tomato	Nanoparticle-induced oxidative stress, antibacterial effects, disruption of bacterial biofilm	[75]
MnO2	Xanthomonas citri, P. syringae	Citrus (Citrus sinensis)	Enhances ROS production, activates plant defense pathways, antimicrobial activity	[2]
ZnO	P. syringae	Tomato	Disruption of bacterial cell membrane, ROS generation	[42]
CuO	X. oryzae	Rice	Interaction with bacterial cell wall, ROS generation	[76]
TiO2	E. coli	Arabidopsis thaliana	ROS generation upon UV exposure, antibacterial activity	[63]
ZnO	E.coli	Maize	Antibacterial activity via ROS and membrane disruption	[43]
CuO	P. aeruginosa	Wheat	Generation of ROS, leakage of intracellular components	[64]
ZnO	P. aeruginosa	Cabbage (Brassica oleracea)	Strong antibacterial action against both gram-positive and gram-negative bacteria	[66]
CuO	E. carotovora	Potato	Effective against a wide range of pathogens including E. coli, Pseudomonas	[67]
TiO2	E. carotovora	Lettuce	Broad-spectrum antibacterial effects, with enhanced activity under UV light	[68]
Ag2O	R. solanacearum	Tomato	Highly potent against bacteria and fungi	[69]
Fe2O3	R. solanacearum	Pepper	Mild to moderate antibacterial activity	[72]
MgO	R. solanacearum	Eggplant	Moderate antibacterial effects	[71]
CeO2	X. campestris	Wheat	Strong antibacterial properties	[15]
ZnO	X. campestris	Broccoli	Induces systemic resistance, increases antioxidant activity, disrupts bacterial cell walls	[77]
TiO2	P. syringae	Tomato	Antibacterial properties, induction of defense-related genes, enhancement of plant growth	[78]

Fig. 3.

NP’s diferent mode of penetrations to plant cells and signifcant efect on morphological and physiological alterations [50]

Silver-based nanoparticles

The antibacterial properties of metal-based nanoparticles have been documented by numerous researchers (Table 1). When bacteria come into contact with silver nanoparticles, they produce silver ions, which deactivate cellular enzymes, impair membrane permeability, and cause cell death [50]. Silver nanoparticles can enter bacterial cell walls and cause structural damage as well as cell death [51]. Free radicals generated by silver nanoparticles damage cell membranes and cause cell death [29]. Silver nanoparticles interact with the phosphorus and sulfur atoms in DNA to prevent bacterial DNA replication, which ultimately results in cell death. The phosphotyrosine profile of bacterial peptides is altered by silver nanoparticles, which inhibit signal transmission and cell proliferation (Table 3). Silver-based oxide nanoparticles have received substantial research attention for their antibacterial capabilities, especially in the context of treating bacterial plant diseases. The distinctive physical and chemical features of these materials, including a high surface area-to-volume ratio and the release of silver ions, contribute to their efficiency as antibacterial agents. Numerous studies have demonstrated the antibacterial activity of silver nanoparticles against a variety of plant diseases, including P. syringae, X. campestris, and E. amylovora [53].. Silver metal oxides have been demonstrated to damage bacterial cell membranes, impede enzyme activity, and create reactive oxygen species, which all lead to bacterial cell death (Table 4).

Iron oxide nanoparticles

Plants are sessile organisms that are forced to conceal from unfavorable environmental conditions, including salinity, drought, water logging, radiation, and severe temperatures. Reactive oxygen species, including hydroxyl radicals, hydrogen peroxide, hydroperoxy radicals, superoxide radicals, and singlet oxygen, are produced under these circumstances [54]. Excessive ROS can damage proteins, lipid membranes, and nucleic acids (Table 3). Reactive oxygen species are cytotoxic and nontoxic [55, 56]. ROS that reduce ant oxidative enzymatic activity improve plant development and play a significant role in protecting plants against a variety of biotic stresses [57]. FeO-NPs have unique properties, including nanometer size, high permeability, low cost, surface modification, good chemical stability, easy synthesis, colloidal stability, and dispersion in aqueous media [58]. NPs, including nanoiron, can target the delivery of antibacterial agents to the infected site, thereby increasing the effective drug concentration at the infected site, producing antibacterial effects, reducing the dosage of drugs and overcoming bacterial resistance [58]. The chitosan-coated iron oxide nanocomposite has antioxidant activity and potential antibacterial activity against both gram-positive and gram-negative pathogens. It is a promising bionanomaterial. FeO-NPs are toxic to bacterial strains because they bind to sulfhydryl compounds in the respiratory group of bacterial cells, thereby exerting antibacterial effects. FeO-NPs synthesized from guava leaf extract and FeO-NRs synthesized with Moringa (MO)-coated FeCl3 are promising antibacterial agents(Table 4).

Copper oxide nanoparticles

The geometric shapes of the copper oxide nanoparticles demonstrated strong antibacterial activity (Table 2). The tapered spears of the multiarmed nanoparticles indicated that they had come into contact with the bacterial cell and that they could readily enter it and kill the cell [59]. The copper oxide nanoparticles ruptured the cell membranes and destroyed the cells; 2.3 × 107 CFU/mL cells were killed after 2 h of exposure to 1 mg/mL copper oxide nanorods, and 1.4 × 107 CFU/mL cells were killed after 30 min of exposure to 0.5 mg/mL, demonstrating excellent bactericidal activity [60].

Cerium oxide nanoparticles

CeO₂ metal oxide nanoparticles are naturally found in the soil. The effects of CeO₂ nanoparticles on the growth and production of soybeans, a significant crop that contains protein, have been investigated [79]. Soybean plants cultivated hydroponically accumulate metal oxide nanoparticles. CeO₂ nanoparticles affect plant biomass and beneficial microbes, particularly nitrifying symbiotic bacteria found in the root nodules of soybeans and many other Fabaceae plants [79].

Zinc oxide nanoparticles

ZnO-NPs are a sustainable solution for combating dangerous plant diseases and ensuring global food security. According to the literature, they are efficient against bacterial infections such as P. syringae [80]. At the molecular level, ZnO NPs can interact with many biological components in plant cells to improve disease resistance. These interactions include those with plant cell membranes, proteins, DNA, and other cellular components that can prevent disease-causing bacteria from growing and reproducing. Mechanically, ZnO NPs can cause the generation of reactive oxygen species (ROS) within plant cells, inhibiting pathogen growth (Fig. 4).

Fig. 4.

Types of metal oxide nanoparticles

The molecular mechanisms underlying how metal oxide nanoparticles affect interactions between plant and bacterial pathogens

Through the modulation of plant–microbe interactions, the growing use of metal oxide nanoparticles (MONPs) in agriculture presents interesting approaches to disease management[14]. Through a variety of complex methods, including direct antimicrobial actions, modification of plant defense pathways, and interference with pathogen virulence factors, MONPs affect plant-bacterial pathogen dynamics at the molecular level. Both sides have evolved complex defense and assault systems in the ongoing struggle for survival between bacterial pathogens and plants. Metal oxide nanoparticles (MONPs) are a new player in this field that has emerged in recent years due to the development of nanotechnology[17]. These materials’ distinct physicochemical characteristics, which result from their nanoscale dimensions, are being investigated more and more for their potential applications in agriculture, including their effects on disease resistance and plant health. To maximize MONPs’ advantages while reducing any potential hazards, it is essential to comprehend the molecular mechanisms via which they alter plant-bacterial interactions. The outcome of the host–pathogen interaction is ultimately shaped by this intricate interaction, which can appear on several levels and affect the pathogen’s virulence as well as the plant’s immune responses [15].

MONPs generate reactive oxygen species (ROS) as one of the main molecular mechanisms by which they carry out their antibacterial action. For example, ZnO and TiO₂ nanoparticles catalyse the creation of ROS, such as superoxide anions, hydrogen peroxide, and hydroxyl radicals, especially when exposed to UV light. These ROS break cell membranes, deactivate enzymes, and fragment DNA by oxidatively damaging bacterial cellular constituents such lipids, proteins, and nucleic acids [25]. In addition to killing bacterial pathogens directly, this oxidative stress also makes it more difficult for them to successfully infect host plants. In addition to their direct bactericidal actions, MONPs have the ability to prepare plant immune responses by triggering intricate biochemical pathways that improve disease resistance. Research has demonstrated that MONPs trigger the expression of defense-related genes that encode phenylalanine ammonia-lyase (PAL), pathogenesis-related (PR) proteins, and enzymes involved in the manufacture of secondary metabolites. One important signalling mechanism in systemic acquired resistance (SAR), the salicylic acid (SA) pathway, has been shown to be upregulated by TiO₂ nanoparticles. In a similar vein, several MONPs activate the ethylene and jasmonic acid (JA) pathways, which are essential for protection against necrotrophic infections (Fig. 5).

Fig. 5.

ROS play a dual role in SAR. Pathogen infection leads to accumulation of SA, ROS, and NO. ROS and NO are produced in multiple cellular compartments including the plasma membrane, chloroplasts, and mitochondria [60]

Green synthesis: a link between organic farming methods and biocontrol

A major step towards smoothly incorporating nanotechnology into sustainable agricultural systems is the emerging topic of green production of metal oxide nanoparticles (MONPs), especially in the context of organic farming and biocontrol techniques [81]. By using biological entities like plant extracts, microorganisms, or biopolymers as ecologically benign reducing and capping agents, green methods as opposed to conventional chemical synthesis avoid hazardous chemicals and reduce ecological footprints [81]. Because of their intrinsic environmental friendliness, green-synthesized MONPs are more consistent with the fundamental principles of organic farming, which place a higher priority on biodiversity, ecological balance, and the avoidance of synthetic agrochemicals. Green-synthesized MONPs present intriguing opportunities for a multifaceted defence strategy when combined with well-established biocontrol agents and organic farming methods [73]. First of all, because of their safe manufacturing, these nanoparticles often cause less harm to non-target creatures (such as pollinators and natural predators) and the beneficial soil microbiota, both of which are essential to the health of organic ecosystems. This stands in stark contrast to the wide-ranging impacts of traditional insecticides. Second, MONPs that have been green-synthesised have the potential to enhance biocontrol agents. By improving their adherence to plant surfaces, modifying the plant’s local immune response to support the establishment of the biocontrol agent, or even synergistically increasing the antimicrobial activity of microbial metabolites, they may, for example, increase the effectiveness of beneficial microorganisms on antioxidant properties, which could be extended to synergistic defence). These combinations could overcome some of the drawbacks of individual biocontrol approaches and result in a more strong and long-lasting level of disease suppression.

However, focused research is necessary to fully realise this integration’s potential. There are still important questions about how green-synthesized MONPs interact with the complex network of soil microbes and how they affect the long-term viability and effectiveness of certain biocontrol agents. In order to guarantee that nano-enabled solutions actually improve, rather than unintentionally upset, the ecological balance promoted by organic farming and biocontrol techniques [73].

Discussion

The ability of MONPs to elicit plant defense responses is mostly due to their interaction with plant cells and tissues. MONPs cause a variety of physiological and molecular reactions in plants, which serve to increase immunity and resist bacterial infections. MONPs increase plant defense by generating ROS, activating immunological receptors, and modulating defense-related gene expression. Studies have revealed that exposure to MONPs increases ROS generation in plant cells, resulting in the activation of signaling pathways implicated in disease resistance [82]. Bajpai et al. [77] reported that ZnO NPs promoted ROS generation in A. thaliana and S. lycopersicum (tomato), which then activated the plant’s defense system against bacterial diseases such as P. syringae. Similarly, Hussain et al. [17] reported that CuO nanoparticles increased ROS production in C. sativus (cucumber), improving resistance to X. campestris bacterial wilt. MONPs generate ROS, which can directly impede bacterial development by damaging bacterial cellular components such as lipids, proteins, and DNA. ROS-mediated oxidative stress generates an unfavorable environment for bacterial life while simultaneously activating plant defense mechanisms. Hussain et al. [17] reported that ZnO NPs stimulated the production of salicylic acid (SA) and other defense-related signaling molecules in Cucumis sativus, leading to systemic acquired resistance (SAR). SAR is a long-term defense system that helps plants fight a variety of diseases, including bacteria. Metal oxide nanoparticles can also trigger plant immune responses by engaging with pathogen recognition receptors. Pan et al. [83] reported that TiO₂ NPs increased the expression of PR proteins in S. lycopersicum, improving resistance to bacterial infections such as bacterial wilt caused by R. solanacearum. Furthermore, metal oxide nanoparticles can affect the expression of critical defense genes involved in numerous plant immune processes. These genes are involved in the manufacture of secondary metabolites such as alkaloids, phenolics, and flavonoids, which have antibacterial characteristics and serve to inhibit the proliferation of bacterial infections. Recent research has investigated the synergistic effects of metal oxide nanoparticles in conjunction with other plant protection techniques, such as biological control agents or organic fungicides. The combination of nanoparticles with biocontrol agents improves plant resistance to bacterial infections, resulting in a more integrated and sustainable strategy for disease management [84]. Emerging research emphasises the synergistic effects that occur when metal oxide nanoparticles (MONPs) are mixed with plant extracts, even though MONPs alone show promise in strengthening plant defence against bacterial pathogens and demonstrating direct antibacterial characteristics. By utilising the distinct qualities of both elements, this combination may result in a more powerful and comprehensive defence reaction [71]. They specifically looked at how certain nanoparticles and plant extracts increased the production of antioxidant enzymes in plants. This implies that the combined application can enhance the plant’s defence mechanisms and help it better control oxidative stress caused by the pathogen and the nanoparticles, therefore maximising the positive signalling effects of ROS. The methods by which plant secondary metabolites from the extracts can stabilise nanoparticles, lessen their potential phytotoxicity at greater concentrations, and directly support defence processes like ROS scavenging are explained in detail in their paper. A major breakthrough, this dual action of plant extract-enhanced host immunity and nanoparticle-mediated direct antimicrobial effects may enable lower effective doses of MONPs, minimising environmental accumulation and possible phytotoxicity while optimising disease suppression [73]. The development of extremely efficient and ecologically friendly plant disease management techniques is made possible by this synergistic approach.

Conclusion

In conclusion, MONPs have strengthened plant defences against bacterial diseases, indicating their revolutionise agricultural practices. These MONPs, which include zinc oxide, copper oxide, and titanium dioxide, have unique properties that facilitate the induction of plant immune responses and boost infection resistance. Through their impact on a range of biochemical processes, metal oxide nanoparticles can enhance the production of reactive oxygen species and the expression of genes associated with defence, so fortifying plants against harmful agents. Metal oxide nanoparticles offer a novel and environmentally friendly way to address the growing issues facing the agriculture industry, such as antibiotic-resistant illness and the need for sustainable crop protection methods.

Future forecasting

MONPs have a promising future in plant disease management, particularly as environmentally acceptable alternatives to chemical pesticides. As research progresses, there is increasing evidence that these nanoparticles can not only increase plant immunity but also act as direct antibacterial agents, lowering the overall bacterial disease burden on crops. MONPs are promising candidates for integrated disease management techniques because of their capacity to stimulate localized defense responses and trigger systemic acquired resistance (SAR). Furthermore, as nanotechnology advances, future innovations may allow for the fine-tuning of nanoparticle features, increasing their efficacy, stability, and targeted distribution, ensuring both plant health and minimal environmental damage. Even though there is strong evidence that metal oxide nanoparticles (MONPs) can activate plant defence mechanisms against bacterial diseases, there are still a number of important research gaps and obstacles that need to be overcome before these discoveries can be implemented into widely used and sustainable agricultural practices. The long-term buildup and environmental destiny of MONPs in soil–plant systems are key concerns. The persistence, transformation, and mobility of MONPs in complex soil matrices are still unclear, despite laboratory studies showing their effectiveness. Their possible environmental buildup in agricultural areas over several cropping cycles and their ensuing effects on non-target organisms, such as the beneficial soil microbiota essential for nutrient cycling and plant health, are also poorly understood. There is still much to learn about the long-term ecological effects of broad MONP application, especially with regard to biodiversity and ecosystem services. Furthermore, it is currently difficult to develop standardised procedures for evaluating phytotoxicity and efficacy across a variety of plant species and environmental conditions, even if dose optimisation is acknowledged as being crucial.

Practical challenges also include the cost-effectiveness and scalability of manufacturing MONPs for extensive agricultural usage, particularly complicated formulations intended for targeted distribution or controlled release. To ensure the safe and responsible integration of nanotechnology into plant disease management, future research efforts should prioritise the development of biodegradable or more environmentally benign nano-formulations, thorough ecotoxicological studies in pertinent agricultural ecosystems, thorough lifecycle assessments of MONP products, and the establishment of strong regulatory frameworks. To fully utilise MONPs as long-term instruments for boosting plant resistance to bacterial pathogens, these deficiencies must be filled.

Limitation of the work

“Metal oxide nanoparticles as promising agents for triggering defence mechanisms in plants against bacterial diseases” is a review paper that has a number of intrinsic limitations. First off, because the subject is still in its infancy and is developing quickly, there are currently few thorough and extended investigations on the safety and effectiveness of different metal oxide nanoparticles in a range of plant systems. Because of this, it may be difficult to reach firm findings or create generally accepted standards. Due to a lack of adequate data, a review that just looks at plant defence may unintentionally ignore or downplay these important environmental factors. Lastly, there is still much to learn about the economic feasibility and practicality of large-scale agricultural use of metal oxide nanoparticles.

Author contributions

My KBW(Lecturer) is the corresponding author who reviwed the data and analysis the review article. GYL (Associate professor) supervise the work.

Funding

There is no fnancial fund obtained to do this work.

Data availability

All the data analyzed during this study are included in this article.

Declarations

Ethics approval and consent to participate

The authors state that this publication does not contain any information concerning human experiments or the use of human tissue samples.

Consent for publication

Not applicable: this manuscript has no personal data from the authors.

Competing interests

The authors declare no competing interests.

Clinical trial number

Not applicable.