Nanoparticles and plant adaptations to abiotic stresses

Introduction

How to feed the projected population for the year 2050 of over 9.3 billion is always an urgent task for people working in agriculture. Improving the crop production efficiency is an obvious way to address the possible food shortage issue in 2050. However, abiotic stress including drought, salinity, and heavy metal stress always negatively affect crop production. Crop breeding to improve abiotic stress tolerance is a known strategy to address the above-mentioned food shortage issue. However, to date, the progress of crop breeding has not met expectations from the public, including farmers. Besides crop breeding, agronomy practices including foliar application of nanoparticles and seed nanopriming are other common approaches to improve plant abiotic stress tolerance. Combining the crop breeding and agronomy practices, we have a stronger confidence to face the possible food shortage in 2050.

Nowadays, nano-improved plant stress tolerance is widely reported. For example, the improvement of salinity stress tolerance by nanomaterials has been reported in many plant species including rice, wheat, wheat, maize, rapeseed, potato and cotton (Wu and Li 2022a). Compared with their counterpart bulk materials, nanomaterials have some advantages such as (1) a larger surface area, which means a higher surface area-to-volume ratio and probably more reaction sites, (2) they can be synthesised as nanozymes which can mimic enzyme activities such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and (3) they can do facile surface modifications which allow targeted delivery and controlled release (Wu and Li 2022a, 2022b). Moreover, nanomaterials can be used as sensors to detect stress signalling molecules or as delivery tools to enable transgenic events (Wu and Li 2022b). Overall, nanobiotechnology could be a potential approach to facilitate the development of sustainable agriculture.

In this special issue, we present advances in how nanobiotechnology can be used to improve plant performance under drought, salinity, and heavy metal stress etc.

Nanobiotechnology approach to improve plant drought tolerance

Drought is a global issue, causing severe loss in agricultural production. Many approaches have been employed to alleviate drought damage to crops, including irrigation, breeding programs of drought tolerant species, and applying chemicals etc. For example, Al-Huqail et al. (2023) showed that priming wheat seeds with a benzothiazine derivative (C₁₅H₁₁NO₄S) improved drought stress tolerance in wheat, by increasing photosynthetic pigments, and modifying stomatal behaviour, different gaseous exchange attributes, uptake fluxes of essential nutrients, and the activities of the antioxidant defence system. Further, nanomaterials also showed the ability to increase plant drought tolerance. Tran et al. (2023) found that compared with control, seeds treated with MSNs (Mesoporous silica nanoparticles) showed higher seed germination and greater primary root length in Arabidopsis. Another study by Sarwar et al. (2023) showed that silver nanoparticles (60 ppm) can protect tillering in drought-stressed wheat by improving leaf water relations and physiological functioning, showing significantly restored leaf physiological functioning and increased grains per plant (up to 22%).

Nanobiotechnology approach to improve plant salinity tolerance

Salinity stress is a chronic stress. It takes a long time to reset the soil salinity level back to normal. The increasing trend of land salinity across the globe threatens our food supply. To enhance food safety, besides reducing soil saline level, improving plant resistance to salinity is another approach. In this special issue, we showed that nanobiotechnology can help to improve plant salt tolerance. Javeed et al. (2023) showed that use of zinc oxide nanoparticles (ZnO NPs, hexagonal and spherical, 16–35 nm) improved salinity stress tolerance in Lagenaria siceraria L. by modulation of physiochemical attributes (increased total chlorophyll content, total soluble sugars and maintained the gas exchange parameters) and the antioxidant system (increased catalase, ascorbate peroxidase, and peroxidase activity). Another study (Kreslavski et al. 2023) showed that seed treatment with iron oxide nanoparticles (Fe₃O₄ NPs, 200 and 500 mg/L) improved salinity stress tolerance in wheat (Triticum aestivum L.), showing that NPs cannot only partially eliminate the negative effect of the salt on growth, PSII activity, and photosynthesis, but also decrease MDA content and show an increase in leaf ascorbate and total peroxidase activity. Furthermore, seed treatment with Fe₃O₄ NPs increased shoot Fe and manganese (Mn) content. These results showed that the designed nanomaterials cannot only improve plant salt tolerance, but also can enhance the nutrient level in plants.

Nanobiotechnology approach to alleviate heavy metal stress in plants

Besides drought and salinity, public concern about heavy metal stress in crop production has also increased rapidly. In this special issue, Jalil et al. (2023) summarised the use of nano zinc in the mitigation of heavy metal stress in plants. They further suggested that the first step of wider incorporation of nano zinc into agricultural practices should be a broad understanding of uptake, transport, signalling, and tolerance mechanisms of nano zinc in alleviating heavy metal stress in plants. Besides zinc based nanoparticles, different iron- and carbon-based nano compositions are known to enhance the removal of metals from contaminated sites and water by activating the functional groups that potentially target specific molecules of the metal pollutants to obtain efficient remediation (Javed et al. 2022). In this review paper, they further discussed different implementation barriers and the opportunities and research directions to facilitate the development of sustainable agriculture.

Concluding remarks and future outlooks

Obviously, the adoption of nanobiotechnology in agriculture is more and more a common measure. In this special issue, we show that nanomaterials can be used to improve plant stress tolerance. However, we do not explore in detail either the manner of uptake and distribution of nanoparticles in plants under stress conditions, nor the mechanisms through which they improve plant stress tolerance. Bhattacharya et al. (2023) discuss how nanoparticles regulate redox metabolism in plants during abiotic stress within hormetic boundaries. They argue that the action of NPs in modulating ROS generation and/or ROS detoxification is tightly coupled within the hormetic boundaries. They further suggest studying how nanoparticles control transcriptomic expression and modulate its crosstalk with signalling molecules. Moreover, besides the above-mentioned improvement of plant abiotic stress tolerance, nanobiotechnology can be also used to increase plant resistance to biotic stress. Ishaq et al. (2023) reported a facile one-step synthesis of gold (Au) NPs by using Viscum album. These Au NPs exhibit strong signals appearing at 9.7 and 2.3 keV measured by energy dispersive X-ray spectroscopy and are face-centred cubic in structure. Ishaq and colleagues screened out these Au NPs against gram-positive and gram-negative (Enterobacter, S. typhi, E. coli, and B. subtilis) bacterial strains, and found that Au NPs can damage bacterial cell membranes, resulting in the leakage of the cytoplasm and thus the death of the cell. Overall, we encourage more studies to be conducted to unveil the mechanisms behind nanomaterials’ improved plant stress tolerance and also the interactions between nanomaterials and plants, including their uptake and translocation in plants.