Mn3O4 NPs also possess excellent ROS-scavenging capacities, exerting multiple enzyme mimicking activities, for example, SOD- and CAT-like, as well as hydroxyl radical scavenging activities. A recent study reported that foliar application of Mn3O4 NPs at 1 mg per plant significantly alleviated salinity stress of cucumber plants. The authors found that Mn3O4 NPs increased endogenous low-molecular-weight antioxidants in the leaves, including resveratrol, chlorogenic acid, dihydroxycinnamic acid, benzenetriol, hydroxybenzoic acid, trihydroxybenzene, quinic acid and catechin. The multifunctional catalytic behaviour of Mn3O4 NPs arise from the coexistence of Mn and Mn oxidation states, and the switch between the II and III valence resembles the mechanism of redox enzymes, which is very similar to CeO2 NPs. In addition to directly acting as ROS-scavenger, NMs can act as carriers to deliver ROS-eliminating compounds to enhance plant stress tolerance. The authors of a recent study designed an ROS-responsive star polymer that successfully alleviated plant stress by simultaneous ROS-quenching and nutrient release. Specifically, RSP was foliar-applied to stressed tomato leaves. The RSP penetrated the leaf epidermis and entered into the chloroplasts where it efficiently eliminated H2O2, which subsequently triggered the release of the nutrient from the polymer. This study highlights the potential of using RSP as an ROS-responsive NM to manage short-term plant stress. Apart from foliar application, ROS-scavenging CeO2 NPs have been applied to roots. It has been reported that CeO2 NPs at 100 and 500 mg l−1 applied to hydroponic cultivated rice can significantly increase the nitrogen levels in roots and shoots by 6–12% and 22–30%, respectively, compared with controls. Similarly, ref. added graphene to the growth media of alfalfa ,macetas redondas and found that the amendment alleviated salinity and alkalinity stresses by modulating antioxidant defence systems and genes related to antioxidant defence and photosynthesis.

The treatment of graphene significantly increased dry biomass of alfalfa by 29.4% and 24.3%, respectively, in salinity and alkalinity conditions. In another study, root delivery of ROS-eliminating CeO2 NPs to hydroponically cultivated rice plants was shown to improve rice tolerance under salinity stress, increasing chlorophyll content and yield. ROS-scavenging NMs have also been reported as a seed treatment agent to improve stress resistance. For example, ref. found that cotton seeds primed with poly-coated CeO2 NPs at 500 mg l−1 for 24 h exhibited significantly increased root vitality by 114% under salt stress. Similarly, ref. found that salt-sensitive rapeseeds seeds primed with poly-coated CeO2 NPs exhibited enhanced salt tolerance by modulating ROS homeostasis. These studies suggest that using ROS-scavenging NMs to treat seeds can alleviate stress at germination stage , although the duration of this protective benefit is currently unknown. Figure 3 summarizes the current known or hypothesized mechanisms by which ROS-scavenging NMs alleviate plant stress.ROS are important signalling molecules that mediate redox signalling pathways and contribute to acclimatization against a range of stresses. One study demonstrated that an ROS wave is required to activate systemic acquired acclimation of plants to heat or high light stresses, highlighting the important biological function of this signalling molecule to the acclimation against abiotic stresses. In addition, an ROS-generating-associated gene respiratory burst oxidase homologue has been shown to be critical to plant stress responses. Given the known response of plants to select lower-level ROS that elicit redox signalling pathways, the concept of pretreating or priming plants with ROS-triggering NMs to stimulate defence systems and acquire systemic acquired acclimation may be an effective strategy to increase stress tolerance. In this strategy, plant stress resistance will be acquired via the initiated adaptive responses by ROS-triggering NMs. ROS-triggering NMs could be used to prime plants through a ‘stress memory’, which provides a mechanism for acclimation and adaptation, thereby improving the tolerance/avoidance abilities. Whereas ROS-scavenging NMs serve as a ‘curative’ strategy, ROS-triggering NMs are more like a ‘preventive’ strategy. Currently, only a limited number of studies have employed ROS-triggering nanozymes to increase plant stress resilience, and the researchers are primarily focusing on silver nanoparticles . AgNPs are known to catalyse ROS generation in cells.

A previous study reported that seed priming with AgNPs enhanced the tolerance of pearl millet to salinity stress by activating the antioxidant enzymes. AgNP seed priming significantly increased the fresh and dry weights of plants by 58% and 34%, respectively, compared with plants grown in 150 mM salt. The underlying mechanisms may be that AgNPs activated defence pathways during seed priming, forming the ‘stress memory’ and subsequently enhancing resilience to stress. The mechanisms for AgNPs generating ROS have been reported in recent studies. By using electron spin resonance, ref. demonstrated that AgNPs can directly produce • OH in the presence of H2O2, and Ag is generated during this process; importantly, Ag ions did not catalyse the production of • OH. Similar results were obtained by another study. A more recent study demonstrated that AgNPs possess peroxidase-mimicking activities, which catalyse oxidation of substrate TMB in the presence of H2O2. The formation of • OH by AgNPs is similar to a Fenton reaction in which AgNPs act as a Fenton-like reagent. Under a changing climate, the frequency of seed exposure to abiotic stresses will increase, which could result in reduced germination and loss of vigour, threatening crop yield. As such, accelerating the germination speed and enhancing the seed vigour are critical. One study reported that AgNP seed priming accelerated the germination speed and yield of Chinese cabbage . Another study showed that AgNP priming promoted the germination, growth and yield of watermelons . Nanoscale zero valent iron , also known to be a Fenton-like reagent, can catalyse the generation of ROS. Another study used nZVI as an ROS-modulator to pretreat rice seeds. This study found that priming generated an optimum level of endogenous ROS via Fenton’s reaction, resulting in higher seed germination rate and greater seed vigour. Unfortunately, the study did not further evaluate whether nZVI priming can increase the stress tolerance of seeds or seedlings, but given the observed hormone biosynthesis upregulation and increased antioxidant enzyme activity, nZVI seed priming should be explored as a potential strategy to promote the stress tolerance of rice and other plant species. Collectively, ROS-modulating NM-based seed treatments may be a promising strategy to mitigate climate-change-associated stress. Under stress conditions, although ROS over-accumulation is common, ROS can still act as signalling molecules, which interplay with other signalling molecules such salicylic acid to activate defence-related genes.

These metabolic processes help plants to establish systemic acquired acclimation or systemic acquired resistance, enhancing the resistance to abiotic or biotic stresses. Therefore, NMs that can trigger the upregulation of defence-related hormones/ signalling molecules or genes may also enhance stress resistance. One study reported that silica nanoparticles at 100 and 1,000 mg l−1 enhanced disease resistance of Arabidopsis plants by up-regulating the production of salicylic acid, a defence hormone and an important signalling molecule. Similarly, another study demonstrated that copper-based NMs successfully alleviated damage of soybeans to sudden death syndrome by triggering the upregulation of a broad array of defence-related genes . One study reported that foliar spray of commercial Cu2 nanoparticles significantly increased antioxidant defence-related genes, for example, SOD, GPX, MDAR and WRKY transcription factor, in cucumber plants, although the potential benefits to stress tolerance were not evaluated. Taken together, these studies demonstrate that NMs that can trigger ROS production or stimulate defence pathways can activate systemic acquired resistance of plants, enhancing the protection against disease or stresses through a classic adaptation response .For both ROS-scavenging NMs and ROS-triggering NMs, efficiently penetrating barriers and entering into plant cells is critical for modulating ROS levels and participating in metabolic activities. As such, an in-depth understanding of the uptake pathways of NMs is fundamental for the efficient application of nanobiotechnology in agriculture. Here, we briefly discuss the possible uptake pathways for NMs into plants, including foliar,maceta de 10 litros root and seed application . Foliar application has been the most extensively used delivery pathway. There are several pathways for the entry of nanoscale particles into leaves, including the cuticle and stomata. One study reported that foliar-applied gold nanoparticles in wheat can reach the mesophyll by either the cuticle or stomata, and move through the plant vasculature. Recent studies have noted that the stomatal uptake pathway is more efficient than the cuticle for Cu-based NPs. In addition, nanoparticle properties and leaf biointerface are important factors influencing the uptake and translocation of NPs. More mechanisms for foliar-uptake pathways can be referenced to excellent reviews. Compared with foliar application, delivering ROS-modulating NMs to plants via the roots has been less investigated. It has been reported that the application of ROS-modulating NMs to plant roots is more effective under hydroponic cultivation than under soil-grown conditions. In the soil system, NMs will undergo aggregation, adsorption and dissolution, confounding interaction with plant roots. A recent study compared the leaf and root application of ROS-scavenging CeO2 NPs on alleviating salt stress of cucumber, and found that foliar-sprayed CeO2 NPs enabled better cucumber salt tolerance than root application. The pathways for NMs entering into the root include apoplastic and symplastic pathways. Using ROS-modulating NMs to prime seeds could be a more cost-effective and environmentally friendly strategy than foliar and root application. This type of approach would not only reduce the release of engineered NMs in the environment but would also result in decreased worker exposure to these materials. Using ROS-modulating NMs to treat seeds might be a promising strategy to increase seed quality, promote growth and increase the yield. Additionally, nano-enabled seed treatment is an efficient way to load mineral nutrients into seeds.

By using transmission electron microscopy, one study observed that FeNPs were absorbed on the watermelon seed coat during the priming process and slowly translocated into the seed endosperm. By using transmission electron microscopy with energy-dispersive X-ray spectroscopy, another study also demonstrated the presence of FeNPs inside the seeds after the priming. These results demonstrate that NPs can effectively penetrate the seed coat and enter into seeds, although the mechanisms of action remain unclear and must be evaluated.Using NMs to modulate ROS homeostasis is a promising strategy to enhance stress tolerance of crop species. It is a rapidly developing area of research, with the vast majority of publications dating from only 2017 to 2022. We propose that additional mechanistic studies are needed to explore the potential of this approach. First, compared with the ROS-scavenging NMs strategy, the ROS-triggering NMs approach remains largely unexplored. Using ROS-triggering NMs to stimulate defence pathways at early growth stage, for example, seed or seedling, could enhance the immunity or resistance of plants to abiotic and biotic stresses. This ‘preventive’ strategy combined with an ROS-scavenging NM-based ‘recovery’ strategy may provide a versatile and effective solution for stress tolerance. Notably, ROS-scavenging nanozymes can sometimes also induce early ROS stimulation to enhance plant stress tolerance. As such, ROS-scavenging nanozymes could be applied as a ‘preventive’ strategy as well. In addition, ROS-triggering NMs may temporarily shift energy and resources to defence pathways, potentially impacting carbon and nitrogen metabolism in ways that could compromise biomass accumulation. As such, the regimen of application will be highly important. For example, one could apply ROS-triggering NMs simultaneously with nutrients in order to more broadly support the plant’s defence system. Abiotic stresses often occur in combination or in succession, and as such, future studies need to evaluate the performance of ROS-modulating NMs under multiple stressor scenarios. Furthermore, a comprehensive understanding of the mechanisms for modulating ROS via NMs to enhance plant stress tolerance is needed prior to deployment of these strategies in the field. The relevant mechanisms include the following: the pathway for NM entry into leaves,roots and especially seeds; the catalytic mechanisms of nanozymes, and how the size, surface charge, pH and environmental conditions impact the catalytic activities of NMs; the intracellular kinetics of ROS-scavenging or ROS-triggering mechanisms of NMs; the precise metabolic pathways by which ROS-triggering NMs enhance plant stress resistance ; and the cellular, biochemical and molecular level response of plants to NMs under different treatment regimens, especially transcriptome and metabolic reprogramming induced by ROS-triggering NMs. The orthogonal approach of transcriptomics, proteomics, metabolomics and epigenetics will be a powerful tool to address these questions. Nano-enabled stress tolerance strategies represent a rapidly developing interdisciplinary field of research. We need to pay attention to new findings from both plant and NMs fields, for instance, new knowledge regarding the response mechanism of plants to abiotic and biotic stresses, and state-of-the-art ROS-regulating nanozymes, which will push forward the application of nanozymes in crop stress tolerance. For example, a study recently reported that Huanglongbing, a devastating disease of citrus, is an immune-mediated disease that stimulates the production of ROS as well as the upregulation of genes encoding ROS-producing NADPH oxidases.