Nanomaterials coupled with microRNAs for alleviating plant stress: a new opening towards sustainable agriculture

Abstract

Plant growth and development is influenced by their continuous interaction with the environment. Their cellular machinery is geared to make rapid changes for adjusting the morphology and physiology to withstand the stressful changes in their surroundings. The present scenario of climate change has however intensified the occurrence and duration of stress and this is getting reflected in terms of yield loss. A number of breeding and molecular strategies are being adopted to enhance the performance of plants under abiotic stress conditions. In this context, the use of nanomaterials is gaining momentum. Nanotechnology is a versatile field and its application has been demonstrated in almost all the existing fields of science. In the agriculture sector, the use of nanoparticles is still limited, even though it has been found to increase germination and growth, enhance physiological and biochemical activities and impact gene expression. In this review, we have summarized the use and role of nanomaterial and small non-coding RNAs in crop improvement while highlighting the potential of nanomaterial assisted eco-friendly delivery of small non-coding RNAs as an innovative strategy for mitigating the effect of abiotic stress.

Introduction

Global changes in the environment have resulted in unexpected fluctuations and extreme conditions in climate, which lead to significant losses in agricultural production. Environmental stresses adversely affect plant growth and development causing a 30–40% reduction in crop yields annually, around the globe (Stevanović et al. 2016) The stress induced losses present a serious limitation in attaining the anticipated food production goals. Therefore, to assuage the increased loss of crop productivity while appreciating the need for more production, there is an increased concern to mitigate the problem of plant stress (Calicioglu et al. 2019; Kerchev et al. 2020).

During the stress phase, several abnormalities develop in plant cells, for instance, the reactive oxygen species (ROS) increase which causes oxidative burst and results in multiple injuries such as membrane damage, reduction or loss in photosynthetic ability, etc. This disturbs the physio-morphological, biochemical and molecular pathways and is ultimately reflected as yield loss (Rajput et al. 2021). Plants have biological mechanisms to respond to or tolerate harsh environments such as enzymes for scavenging ROS (Gautam et al. 2017), molecular pathways for osmotic adjustments (Banti et al. 2013), etc.

One of the steps to realize the United Nations goal for “Zero Hunger” by 2030 is to employ efficient and effective agricultural practices for the generation of stress-tolerant and high-yielding crops (Calicioglu et al. 2019). The most important factor for improving agricultural productivity is to attain a balance between plant growth and response to stress (Van Nguyen et al. 2021). Scientists have discovered and employed different approaches to improve the performance of crop plants under stressful environments. The use of nanotechnology and small non-coding RNAs has been, individually, well described (Mitter et al. 2017; Jalil and Ansari 2019; Ashraf et al. 2021; Seleiman et al. 2021). In this article, we discuss, the applicability of nanomaterial assisted eco-friendly delivery of small non-coding RNAs as one of the novel strategies for improving plant’s tolerance to stress and increment in yield.

The role of nanomaterials

Nanomaterials or nanoparticles (NPs) are materials of 1 to 100 nm diameter, at least in one dimension that are bioactive in nature and can exhibit multiple quantum effects (Arya et al. 2019). Their small size and other physio-chemical properties allow them to traverse cellular membranes and exhibit their impact on the molecular machinery. Both metal and non-metal based NPs have been used in multifunctional roles in numerous disciplines (Torney et al. 2007; Nair et al. 2010; Siddiqui et al. 2015). NPs have also opened new avenues in various agricultural applications (Alejandro and Rubiales 2009; Murthy et al. 2020;). They can be used as pesticides (Alejandro and Rubiales 2009), herbicides (Siddiqui et al. 2015), growth and performance boosters (Palmqvist et al. 2017; Singh et al. 2021), fertilizers (Derosa et al. 2010; Zulfiqar et al. 2019), post-harvest disease management tools (Gonza´lez-Estrada et al. 2021), stress mitigators (Elsakhawy et al. 2018; Almutairi, 2019; Das and Das, 2019; Van Nguyen et al. 2021) and gene delivery agents (Makhotenko et al. 2018; Lee et al. 2019; Liu et al. 2020).

However, despite their importance, NPs have some drawbacks such as creating cell toxicity to metals, inducing oxidative stress (Begum and Fugetsu 2012; Slomberg and Schoenfisch 2012), causing a decline in germination and triggering loss of photosystem, yield, or nutritive value (Peralta-Videa et al. 2014; Barhoumi et al. 2015; Da Costa and Sharma 2016; Wang P. et al. 2016). The response of the plant to NPs has been shown to vary among species (Lin and Xing 2007), so the application of NPs needs to be well investigated and standardized before it can be functionally used.

Nanomaterials in abiotic stress tolerance

Plants have the ability to regulate their biochemical and genetic mechanisms for responding to different biotic and abiotic stresses and maintaining growth and development (Mishra et al. 2014; Elsakhawy et al. 2018; Alsaeedi et al. 2019; Ali et al. 2021). NPs can up regulate the genes that are involved in stress tolerance (Landa et al. 2012; Kaveh et al. 2013). There are numerous reports on the use of different NPs for alleviating abiotic and heavy metal stress in a variety of plants (Table 1). Under stressful conditions, they help the plants by triggering the defense systems, accumulating osmolytes, maintaining the ROS homeostasis, increasing nutrient uptake efficiency and stabilizing crop yields (Patra et al. 2016; Gautam et al. 2017; Morkunas et al. 2018). However, the procedures for their use and application need to be standardized as higher concentrations of NPs can cause oxidative stress in plants (Li et al. 2015; Khan et al. 2017). As an alternate strategy, NPs have been used to generate favorable growing conditions for the plants. The use of nano mixes of gypsum, calcium, or magnesium in managing and reclaiming salt-affected soils has been reported (Patra et al. 2016). Therefore, the different applications of NPs in plants have indicated that nanotechnology can contribute novel ideas for improving stress tolerance in plants.

Table 1.

The use of nanoparticles to increase abiotic stress tolerance in some major food and agricultural crops. [Some selected reports published since year 2010]

Stress	Plants	Nanoparticle type (concentration)	Mode of action	Reference
Drought	Brassica napus	Yttrium doping-stabilized γ-Fe2O3	Reduction in levels of H2O2 and MDA accumulation, increase in leaf growth rate	Palmqvist et al. (2017)
	Glycine max	Metal-Based (Fe, Cu, Co, & Zn oxide) nanoparticles	Improves relative water content, drought tolerance index and biomass reduction rate, triggers drought-associated gene expression	Linh et al. (2020)
		Zinc oxide (0.5 and 1 g L− 1)	Increases seed germination, improves drought stress resistance	Sedghi et al. (2013)
		ZnO, B2O3, and CuO; ZnSO4·7H2O, H3BO3, and CuSO4·5H2O	Improver shoot growth and yield	Dimkpa et al. (2017)
	Hordeum vulgare	Chitosan NPs (30, 60 and 90 ppm)	Significantly increased the leaf area, the leaf color (SPAD), the number of grain per spike, the grain yield and the harvest index	Behboudi et al. (2018a)
	Lens culinaris	Silver (10, 20, 30 and 40 µg ml− 1)	Increase in germination percentage	Hojjat (2016)
	Linum usitatissimum	Nano-Titanium oxide (10 mg L− 1)	Enhanced chlorophyll and carotenoid content; improved growth and yield attributes; decreased H2O2 and MDA content	Aghdam et al. (2016)
	Oryza sativa	Iron oxide and hydrogel nanoparticles (0- 100 mg kg−1)	Ameliorated drought stress and cadmium toxicity	Ahmed et al. (2021b)
	Phaseolus vulgaris	Copper (0.03%)	Improved leaf water content, leaf water retention capacity, leaf transpiration rate and root weight at seedling stage	Van Trong et al. (2019)
	Saccharum officinarum	Silicon (50, 100, and 500 ppm)	Restored the water stress injuries integrated to facilitate the operation of antioxidant defense system	Verma et al. (2021)
	Solanum melongena	Zinc oxide (50 and 100 ppm)	Alleviating drought stress effects in saline soil	Semida et al. (2021)
	Sorghum bicolor	Cerium oxide (10 mg L− 1)	Increased leaf carbon assimilation rates, pollen germination and yield	Djanaguiraman et al. (2018)
		Zinc oxide (5 mg kg − 1)	Accelerated plant development, promoted yield, increased nutrient translocation acquisition	Dimkpa et al. (2019)
	Triticum aestivum	Copper (3 mg L− 1)	Satisfactory increase in nutrient uptake and water retention	Ahmed et al. (2021a)
		Iron (0-100mg kg − 1)	Improve germination rate, drought tolerance in Cd stress	Adrees et al. (2020)
		Selenium bio-fabricated (30 mg L− 1)	Remarkable increase in plant height, leaf area, number & length, shoot length, shoot fresh & dry weight, root length, root fresh & dry weight	Ikram et al. (2020)
		Silicon oxide (30 and 60 ppm)	Improved growth and yield under drought stress	Behboudi et al. (2018b)
		Silver (10 mg L− 1)	Satisfactory increase in nutrients uptake and water retention	Ahmed et al. (2021)
		Sodium silicate (1.0 mM)	Improved the growth of shoot (but not root); increased chlorophyll concentration; maintained leaf water potential; reduceds proline, H2O2 MDA accumulation and electrolyte leakage; increased glutathione concentration	Jalil and Ansari (2019)
		Titanium dioxide (0.02%)	Increase gluten and starch content	Jaberzadeh et al. (2013)
		Titanium dioxide (500, 1000 and 2000 mg L− 1) and sodium nitro-prusside (100 µM)	Increases percentage germination, germination energy, germination rate, root length, shoot length, root fresh weight, shoot fresh weight, vigor index and germination time	Faraji and Sepehri (2019)
	Zea mays	Nano-ZnO (100 mg L− 1)	Alleviate photosynthetic pigment degradation, maintain a higher net photosynthetic rate, enhance water use efficiency, and promoted drought tolerance	Sun et al. (2020)
		Potassium nano-silica (100–200 ppm)	Limited the negative effects of drought stress and improved drought tolerance	Aqaei et al. (2020)
Salinity	Brassica napus	Multiwalled carbon nanotubes (20 mg L− 1)	Enhance plant tolerance against salt stress by reestablishing redox balance and ion homeostasis	Zhao G. et al. (2019)
		Cerium (IV) oxide (200 and 1000 mg kg− 1; 500 mg kg − 1)	Induced high efficiency of the photosynthetic apparatus and reduced the effects of stress; shortened the root apoplastic barriers which allow more Na + transport to shoots and less accumulation of Na + in plant roots	Rossi et al. (2016); Rossi et al. (2017)
		Zinc oxide (10, 20 mgL− 1)	Reduced ion leakage and improved Hill reaction	Farouk and Al-Amri (2019); Hezaveh et al. (2019)
	Capsicum annuum	Manganese (0.1, 0.5, 1 mgL− 1)	Improved root growth	Ye et al. (2020)
	Cucurbita pepo	Nano-SiO2 (6 g L− 1)	Improved seed germination and growth characteristics by reducing malondialdehyde and hydrogen peroxide levels as well as electrolyte leakage	Siddiqui et al. (2014)
	Glycine max	Silicon (0, 0.5, 1, and 2 mM)	Increased K +  concentration, antioxidant activities and non-enzymatic compounds, but decreased Na +  concentration, MDA and ROS	Farhangi-Abriz and Torabian (2018)
	Helianthus annuus	FeSO4 (2 g L− 1)	Enhanced antioxidant activities	Torabian et al. (2018)
	Lycopersicum esculentum	Copper (250 mg L− 1)	Increased antioxidant activity, phenols, vitamin C, glutathione, and improved Na+/K +  ratio	Pérez-Labrada et al. (2019)
		Copper (10 mg)	Regulated the oxidative and ionic stress by promoting the expression of JA and SOD genes	Hernández-Hernández et al. (2018)
		Nano-Si (1, 2 mM)	Better adaptation under salinity stress, with improvements in root and shoot growth	Haghighi et al. (2012)
		SiO2 (250 and500 mg L− 1)	Maintained the concentration of chlorophylls, GSH, PAL activity and vitamin C	Pinedo-Guerrero et al. (2020)
	Oryza sativa	Nano Silicon (12.5 mg L− 1) and Nano Selenium (6.25 mg L− 1)	Positive influence on growth and yield, minimized the negative effects of salt stress	Badawy et al. (2021)
	Panicum virgatum	Graphene and multi-walled carbon nanotubes (200 mg L− 1)	Early germination, salt stress tolerance by reducing Na+ ions	Pandey et al. (2018)
	Pennisetum glaucum	Silver (0, 10, 20 and 30 mM)	Improved relative water content and proline contents; reduced the oxidative damage by improving antioxidant enzyme activities; regulat total phenolic and flavonoid contents	Khan et al. (2020)
	Solanum lycopersicum	Nano-silicon	Increased germination rate; up regulation of AREB, TAS14, NCED3 and CRK1; down regulation of RBOH1, APX2, MAPK2, ERF5, MAPK3, DDF2 and other salt responsive genes	Almutairi (2016)
	Solanum lycopersicum	Silicon & nano-silicon (1, and 2 mM)	Increased fresh and dry weights, root volume, chlorophyll content, photosynthetic rates and leaf water content; decreased sub stomatal CO2 and stomatal conductance	Haghighi and Pessarakli (2013)
	Solanum tuberosum	Zinc, Boron, Silicon, and Zeolite	Higher plant growth, and the lower concentration of leaf abscisic acid (ABA) and transpiration rate	Mahmoud et al. (2020)
	Sorghum bicolor	Graphene and multi-walled carbon nanotubes (200 mg L− 1)	Early germination; salt stress tolerance by reducing Na+ ions	Pandey et al. (2018)
	Triticum aestivum	Gold (300ppm)	Improved salt stress tolerance; maintained nitrogen metabolism, nitric oxide synthesis, and ions balance	Wahid et al. (2021)
		Polyhydroxy fullerenes (10, 40, 80, and 120 nM)	Lowered the levels of MDA and H2O2 and enhanced antioxidant activities of CAT, POD, and APX enzymes; improved chlorophyll, free amino acids, ascorbic acid and soluble sugars; enhanced K +  and P contents in root and P contents in shoot	Shafiq et al. (2019)
		Silver (0, 2, 5 and 10 mM)	Increased soluble sugars and proline contents; decreased CAT activity and increased POD activity	Mohamed et al. (2017)
		Silver (1mg L− 1)	Stimulated indole-3-butyric acid (IBA), 1-naphthalene acetic acid, 6-benzylaminopurine; increased pigment contents and chlorophyll stability index, auxins and cytokinins as well as ABA content	Abou-Zeid and Ismail (2018)
		Silicon	Improved salt stress tolerance, germination and chlorophyll contents	Mushtaq et al. (2019)
Heat	Lycopersicum esculentum	Selenium and nano-selenium	Improved plant growth parameters after a short-term pulse of high and/or low temperature stress	Haghighi et al. (2014)
	Zea mays	Cerium dioxide engineered	Increased H2O2 accumulation in phloem, xylem, bundle sheath cells and epidermal cells of shoots; up-regulation of the HSP70 in roots	Zhao et al. (2012)
UV-B Radiation	Triticum aestivum	Cadmium Telluride Quantum Dots	Programmed cell death and DNA laddering; inhibits root and shoot growth	Chen et al. (2014)
		Silicon (10 µM)	NO-mediated triggering of antioxidant defense system	Tripathi et al. (2017)
Flooding	Glycine max	Aluminum	Improved growth by regulation of energy metabolism and cell death,	Mustafa et al. (2015b)
		Silver	Increases the abundance of stress-related proteins; reduces generation of cytotoxic by-products of glycolysis and enhances seedling growth	Mustafa et al. (2015a)
Chilling or Cold	Cicer arietinum	Titanium oxide	Positive effect on cold tolerance	Amini et al. (2017)
			Alleviated membrane damage and ion leakage	Mohammadi et al. (2013)
			Decreased H2O2 and MDA content	Mohammadi et al. (2014)
			Increased expression of Rubisco and chlorophyll binding proteins	Hasanpour et al. (2015)
Oxidative	Hordeum vulgare	Silicon oxide	ROS accumulation; enhances barley's tolerance to nickel oxide nanomaterial	Soares et al. (2018)
	Oryza sativa	Silver (30, 60 mg L− 1)	Regulating proteomic changes such as accumulation of proteins, transcription and protein degradation, direct damage to cell wall, DNA, RNA, & protein, regulating cell division and apoptosis	Mirzajani et al. (2014)
Heavy metal: Gold	Vigna unguiculata	Gold	Withstand gold (Au3+) stress as the phenolics released by seed coat of germinating seeds reduce toxic Au3+ to form non/less toxic Au-nanoparticles	Shabnam et al. (2014)
Arsenics	Oryza sativa	Iron oxide	Improved iron uptake, oxidative stress tolerance and reduced arsenic accumulation	Bidi et al. (2021)
	Zea mays	Silicon	Ameliorated Arsenic toxicity	Tripathi et al. (2016)
Heavy metal: Cadmium	Brassica juncea	Hydroxyapatite	Reduces cadmium toxicity	Li and Huang (2014)
	Glycine max	Titanium oxide	Increases rate of photosynthesis and plant growth, reduce cadmium toxicity	Singh and Lee (2016)
	Oryza sativa	Silicon (19 nm, 48 nm and 202 nm size)	Gradual decrease of cadmium toxicity	Cui et al. (2017)
	Oryza sativa	Selenium and silicon combined (5, 10 and 20 mg L− 1)	Significantly reduced lead and cadmium accumulation	Hussain et al. (2020)
	Solanum lycopersicum	Chitosan nanoparticle (100 µgmL− 1)	Increased the shoot dry weight, net photosynthetic rate and SPAD index, while decreased the H2O2 and MDA levels	Faizan et al. (2021)
	Triticum aestivum	Titanium dioxide (2000 mg L− 1) and Sodium Nitro-prusside (100µM)	Improved seed germination and early seedling growth	Faraji and Sepehri (2018)
	Triticum aestivum	Silicon (300, 600, 900, 1200 mgL− 1)	Enhanced biomass and growth	Hussain et al. (2019)
	Zea mays	Silicon (0.25, 0.5, 1., 1.5 and 2 µmol L− 1)	Alleviated cadmium stress and increased dry matter	Silva et al. (2017)
Heavy metal-Chromium	Oryza sativa	Silicon	Alleviated aluminum phytotoxicity in acidic soils; enhanced the activities of antioxidant enzymes	Tripathi et al. (2021)
Heavy metal-Chromium	Pisum sativum	Silicon	Alleviated chromium (Cr6+) phytotoxicity	(Gangwar and Singh 2011; Tripathi et al. (2015))

Abbreviations used in Table: AREB- abscisic acid responsive element-binding protein; APX2-cytosolic ascorbate peroxidase 2; CAT- Catalase; CRK1- cysteine-rich receptor-like protein kinase 42-like; DDF- dwarf and delayed flowering; ERF-ethylene response factor; GSH-glutathione reductase; H₂O₂-Hydrogen peroxide; JA- Jasmonic acid; MAPK-mitogen-activated protein kinase; MDA- malondialdehyde; NCED3- 9-cis-epoxycarotenoid dioxygenase; PAL-phenylalanine ammonia lyase; POD-peroxidase; RBOH-respiratory burst oxidase; ROS- reactive oxygen species; SOD-superoxide dismutase; SPAD-soil plant analysis development; TAS14-abscisic acid and environmental stress-inducible protein

Nanomaterials in plant growth and development

NPs have been indicated as important effectors of plant agronomic traits and employed as technical intervention mechanisms to maximize crop productivity (Duhan et al. 2017; Verma et al. 2019; Ali et al. 2021). The use of NPs has been shown to enhance seed germination (Gopinath et al. 2014; Yin et al. 2018), shoot and root biomass, plant height and yield (Mahmood 2015), improve flower count, fruit production and metabolite content (Kole et al. 2013) and so on. There are several reports on the role of diverse NPs, which boost plant growth and development (Table 2). NPs have distinct physiochemical properties that inherently enhance plant growth and development (Zhao et al. 2020). Studies have also demonstrated the use of nano-fertilizers for nutrient delivery and nano-sensors for weather forecasts. These indirect applications can complement the potential of NPs to directly increase the agriculture productivity (Khan and Upadhyaya 2018; Ahmed et al. 2021a).

Table 2.

List of nanoparticles that support plant growth and development in some major food and agricultural crops. Some selected reports published since the year 2010

Nanoparticle type (concentration)	Plant	Process	Mode of action	Reference
Carbon based
Carbon nanofibers (95 nm diameter)	Cicer arietinum		Increased germination rate; enhanced root and shoot length; increased protein and chlorophyll content	Ashfaq et al. (2017)
Water soluble carbon nano-oions (30 mg ml− 1)	Cicer arietinum	Biomass	Improved germination, growth, biomass and yield	Sonkar et al. (2012)
Carbon (50 mg L− 1)	Triticum aestivum	Growth	Enhanced growth of shoot and root lengths	Saxena et al. (2014)
Carbon nanodots (150 mg L− 1)	Triticum aestivum	Root growth	Increased root growth (ten times as that of the control)	Tripathi and Sarkar (2015)
Engineered Carbon (100–150 µM)	Vigna radiata	Growth and metabolism	Increased total chlorophyll content, protein content, plant biomass and antioxidant activity	Shekhawat et al. (2021)
Fullerol, carbon-based NPs	Momordica charantia	Biomass, fruit yield and phytomedicine content	Increased water uptake	Kole et al. (2013)
Powdered graphene (40 µg ml− 1)	Lycopersicum esculantum	Germination	Increased germination frequency	Zhang et al. (2015)
Graphene (5-200 mg L− 1)	Oryza sativa	Germination	Increased germination frequency at lower concentration but delayed germination frequency with increasing graphene concentration	Liu et al. (2015)
Graphene oxide nanosheets (5–50 mg L− 1)	Oryza sativa	Growth	Positive impact on plant growth, development, ABA, MDA and indole-3-acetic acid contents and enzymatic activities at low concentration	Shen et al. (2018)
Graphene and multi-walled carbon nanotubes	Panicum virgatum	Germination	Increased germination rate and biomass accumulation; salt stress tolerance	Pandey et al. (2018)
Graphene and multi-walled carbon nanotubes (200 mg L− 1)	Sorghum bicolor	Germination	Early germination; salt stress tolerance
Hydrated graphene ribbon	Triticum aestivum	Germination/ Growth	Enhanced seed germination, root differentiation and resistance to oxidative stress	Hu and Zhou (2014)
Graphene oxide (400 and 800 mg L− 1)	Vicia faba	Germination	Decreased levels of H2O2 and MDA; enhanced APX and CAT activity, increased proline content and seed-relative water content	Anjum et al. (2014)
Graphene oxide (100–1500 mg L− 1)	Zea mays	Germination/ Growth	Reduced effects of Cd2+ on seed germination and seedling growth	Yin et al. (2018)
Sulfonated graphene (50 mg L− 1)	Zea mays	Growth	Stimulates plant growth	Ren et al. (2016)
Interfacing carbon nanotubes (20 mg L− 1)	Zea mays	Biomass	Improved germination, mineral nutrient supply to the seedling through the action of the mutually opposing forces of inflow with water and retention	Tiwari et al. (2014)
Multi-walled carbon nanotubes (100 and 200 µg ml− 1)	Glycine max, Hordeum vulgare, Zea mays	Germination and seedling growth	Increased expression of genes encoding several types of water channel proteins	Lahiani et al. (2013)
Multi walled carbon nanotubes (40 mg ml-1)	Brassica juncea, Phaseolus Mungo, Cicer arietinum, Lycopersicum esculantum	Germination/ growth	Enhanced germination and water absorption	Ghodake et al. (2010); Tripathi et al. (2011); Khodakovskaya et al. (2011)
Multi walled Carbon nanotubes (40 and 80 µg ml− 1)	Lycopersicum esculantum	Germination	Enhanced seed germination and plant growth	Morla et al. (2011); Wang Q. et al. (2012)
Multi walled carbon nanotubes (50 and 200 µg mL− 1)	Lycopersicum esculantum	Growth	Increased number of flowers and fruits	Khodakovskaya et al. (2013); McGehee et al. (2017)
Helical multi-wall carbon nanotubes (MWCNTs), few-layered graphene, long MWCNTs and short MWCNTs (50 µg ml− 1)	Lycopersicum esculantum	Germination	Significant enhancement in the germination and growth of exposed tomato	Lahiani et al. (2016)
Multi-walled carbon nanotubes and impurities (75 wt %)	Medicago sativa, Triticum aestivum	Root growth	Internalization of roots	Miralles et al. (2012)
Multi-walled carbon nanotubes (20, 50 µg ml− 1)	Triticum aestivum Zea mays, Arachis hypogaea, Allium sativum	Growth	Significantly enhanced plant growth and biomass	Srivastava and Rao (2014)
Water soluble carbon nanotubes (6.0 mg ml− 1)	Cicer arietinum	Growth	Increased root and shoot length and branching; higher water absorption and retention	Tripathi et al. (2011)
Multi-walled carbon nanotubes (80 µg ml− 1)	Triticum aestivum	Root elongation	Faster root growth and higher vegetative biomass were observed	Wang X. et al. (2012)
Single walled carbon nanohorns ((25, 50 and 100 gml− 1)	Glycine max, Hordeum vulgare, Oryza sativa, Solanum lycopersicum, and Zea mays	Germination and growth	Early germination and increased shoot length	Lahiani et al. (2015)
Metal based
Alumina	Lemna minor	Biomass accumulation and root length	Increased root length, number of fronds per colony, efficiency of light reaction of photosynthesis	Juhel et al. (2011)
Cerium Oxide (125, 250 mg Kg− 1)	Hordeum vulgare	Biomass	Increased biomass with remarkable increase in P, K, Ca, Mg, S, Fe, Zn, Cu, and Al as well as enhanced methionine, aspartic acid, threonine, tyrosine, arginine, and linolenic acid contents in the grains	Rico et al. (2015b)
Cerium Oxide (0.1–10 mg L−1)	Solanum lycopersicum	Growth	Promoted plant growth and fruit maturity at low concentrations	Wang Q. et al. (2012)
Copper (30 ppm)	Triticum aestivum	Yield	Increased leaf area, chlorophyll content, fresh and dry weights and root dry weight	Mahmood (2015)
Copper (3 mg L− 1)	Triticum aestivum	Yield	Higher chlorophyll stability index, leaf succulence and leaf potassium content	Ahmed et al. (2021a)
Copper (69.4 µM)	Zea mays	Yield	Promoted growth, retained relative leaf water status, chlorophyll and carotenoid content; increased anthocyanin contents during drought	Van Nguyen et al. (2021)
Gold (10 ppm)	Brassica juncea	Growth and yield	Enhanced in growth and seed yield	Arora et al. (2012)
Gold (10, 100, 1000 mg L− 1)	Vigna radiata	Growth and photosynthesis	Increased root and shoot length and weight, photosynthesis enhancement	Das et al. (2017)
Gold (5–15 ppm)	Zea mays	Germination	Fast germination	Mahakham et al. (2016)
Iron Oxide (0.5 g L− 1)	Glycine max	Yield	Increased leaf and pod dry weight, increased grain yield	Sheykhbaglou et al. (2010)
Superparamagnetic iron oxide (0.2-2 mg ml− 1)	Glycine max	Germination /Growth	Increased chlorophyll content	Ghafariyan et al. (2013)
Mercuric chloride (0.1%)	Lycopersicum esculantum	Germination	Completely inhibits seed germination	Morla et al. (2011)
Nano-crystalline Cobalt (300 mg Ha − 1)	Glycine max, Vietnamese species DT-51	Germination, growth, and yield	Increases germination, chlorophyll index and yield	Ngo et al. (2014)
Nano-crystalline Copper (300 mg Ha − 1)
Nano-crystalline Iron (300 mg Ha − 1)
Iron oxide (20 mg L−1)	Triticum aestivum	Plant growth	Improved photosynthetic ability, biomass and nutrients	Rizwan et al. (2019)
Siliver (1; 2.5; 5 and 10 mg L− 1)	Hordeum vulgare	Root elongation	Increased root length	Gruyer et al. (2014)
Silver (30, 60 mg L− 1)	Oryza sativa	Root growth	Concentration-dependent increase or decrease in root growth	Mirzajani et al. (2013)
Silver (20–60 ppm)	Phaseolus vulgaris, Zea mays	Shoot and root elongation	Increased leaf surface area, chlorophyll, carbohydrate and protein contents	Zea and Salama (2012)
Silver (0.01-1.0 mg L− 1)	Triticum aestivum	Growth and development	Promotion of respiration intensity, seed vigor, and seed germination; increase in dry biomass of roots and aerial parts	Omelchenko et al. (2014)
Silver (50 and 75 mg L− 1)	Triticum aestivum	Growth	Improved growth and biomass under heat stress	Iqbal et al. (2017)
Nano-TiO2 (500, 2500 and 4000 mg L− 1)	Brassica napus	Growth and photosynthesis	Improvements in the morphological, physiological, antioxidant system	Li et al. (2015)
Titanium oxide (40 and 60 ppm)	Foeniculum vulgare	Germination	Enhanced yield indices such as germination value, vigour index and mean daily germination	Feizi et al. (2013)
Titanium dioxide (< 200 mg L− 1)	Lemna minor	Growth	Increased POD, SOD and catalase activity	Song et al. (2012)
Nano-TiO2 (0.05, 0.1. 0.2 g L− 1)	Lycopersicum esculentum	Photosynthesis	Improved photosynthetic efficiency	Qi et al. (2013)
Titanium oxide (1, 2, 10, 100, and 500 ppm)	Triticum aestivum	Germination	Faster mean germination time and increased shoot and seedling length	Feizi et al. (2012)
Titanium oxide (20, 40, 60 mg kg− 1)	Triticum aestivum	Growth	Increased root and shoot length and biomass	Rafique et al. (2014)
Titanium oxide (0.01%)	Vicia faba	Growth	Increased levels of chlorophyll b, soluble sugars and proline; enhanced activities of antioxidant enzymes; salinity tolerance	Abdel Latef et al. (2017)
Titanium oxide (0.01, 0.03%)	Zea mays	Yield	Significant increment in chlorophyll content, total chlorophyll (a + b), chlorophyll a/b, carotenoids and anthocyanins	Morteza et al. (2013)
Zinc Oxide (50 to 1600 mg L− 1)	Allium cepa	Germination and seedling growth	Stimulates seed germination and growth and development of seedlings	Tymoszuk and Wojnarowicz (2020)
Zinc oxide (1000 ppm)	Arachis hypogaea	Germination	Promotes germination and seedling vigour and early flowering; increased leaf chlorophyll content; increased stem and root growth and enhanced pod yield per plant	Prasad et al. (2012)
Zinc oxide (25 mg L− 1)	Brassica species	Oxidative signaling	Changes in nitrosative signalling	Molnár et al. (2020)
Zinc oxide (1.5 ppm)	Cicer arietinum	Shoot dry weight	Increased biomass accumulation, ROS levels and lowered MDA content	Burman et al. (2013)
Zinc oxide (10 ppm)	Solanum lycopersicum	Growth	Improved growth and photosynthetic attributes, enhanced activity of various antioxidant enzymes and higher accumulation of proline and protein content	Faizan et al. (2020)
Zinc Nanofertilizer (15 and 25 nm)	Pennisetum americanum	Photosynthesis	Increased photosynthetic pigments (chlorophyll and carotenoids)	Tarafdar et al. (2014)
Zinc oxide (0, 125, 250, and 500 mg kg -1)	Pisum sativum	Root elongation	Extended root growth	Mukherjee et al. (2014)
Zinc oxide (100 mg L−1)	Triticum aestivum	Plant growth	Improved photosynthetic ability, biomass and nutrients	Rizwan et al. (2019)
Zinc oxide (20 ppm) and Zn/Cu/Fe-oxide (50 ppm)	Vigna radiata	Growth and biomass	Improved absorption of essential elements	Dhoke et al. (2013)
ZnO (500, 1,000, 2,000 and 4,000 ppm)	Vigna radiata	Germination	Improved germination rate	Patra et al. (2013)
Others
SiO2 (100–2000 ppm)	Glycine max	Growth and development	Increased plant performance and reduced the uptake of mercury in the leaf epidermis and pericycle of roots and stems. Also showed enhanced photosynthetic content and antioxidant enzyme activities during mercury stress	Li et al. (2020b)
SiO2 (125–250 ppm)	Hordeum vulgare	Growth and development	Improved plant development, green pigments, photosynthetic activities, plant osmolyte and metabolite profiles	Ghorbanpour et al. (2020)
Silicon (0.2 and 0.4 mM)	Helianthus annuus	Germination	Increased seedling root and shoot length along with biomass and vigor index, and reduced mean germination time	Janmohammadi and Sabaghnia (2015)
SiO2 (1 mM)	Lens culinaris	Germination and growth	Improved the germination and early growth of plants under salinity stress	Sabaghnia and Janmohammadi (2015)
SiO2 (10–100 mg L− 1)	Oryza sativa	Germination	100% seed germination, increased length and dry weight of root and shoot	Adhikari et al. (2013)
Silicon (1 mM)	Oryza sativa	Growth and development	Enhanced gene expression and transportation of cadmium to vacuoles	Cui et al. (2017)
Silicon (5, 10 and 20 mg L− 1)	Oryza sativa	Growth and biomass	Promoted plant growth and photosynthesis	Hussain et al. (2020)
SiO2 (10 µM)	Pisum sativum	Growth and development	Protected seedlings and increased enzymatic activities	Tripathi et al. (2015)
Silicon and nano-silicon (2 mM)	Solanum lycopersicum	Growth	Increased plant fresh weight, water use efficiency, chlorophyll content, rate of photosynthesis and the leaf water content as well as decreased stomatal and sub stomatal CO2 conductance under salt stress	Haghighi and Pessarakli (2013)
Nano-SiO2 (8 g L− 1)	Lycopersicum esculentum	Germination	Increased seed germination time and rate, seed vigor index, seedling fresh weight and dry weight	Siddiqui and Al-Whaibi (2014)
Mesoporous Silica (200-2000ppm)	Triticum aestivum	Growth and development	Upregulated leaf gas exchange responses and growth development performance of plants	Sun et al. (2016)
Silicon (10 µM)	Triticum aestivum	Growth and development	Mitigated negative effects of UV radiation on plants	Tripathi et al. (2017)
SiO2 (50, 100 mg L− 1)	Triticum aestivum	Growth	Increased shoot and root fresh and dry weight	Karimi and Mohsenzadeh (2016)
Nano-Si (1.5, 3 mM)	Vicia faba	Flowering	Slightly improved flowering as compared with Si or the control	Roohizadeh et al. (2015)
Porous Silicon (15 kg ha− 1)	Zea mays	Germination	Improved nutrient availability to seeds	Suriyaprabha et al. (2012)
Silica Powder (100, 200, 300, and 400 mg kg− 1)	Zea Mays	Growth	Increased stem height, stem width, number of leaves and silica content in in-vivo condition, increased germination, water use efficiency and total chlorophyll content in in-vitro condition	Yuvakkumar et al. (2011)
Silicon (400, 2000 and 4000 mg L− 1)	Zea mays	Photosynthesis	Increased all photosynthetic pigments	Sharifi et al. (2016)
Sulfur; Nano-Crystalline (100–600 ppm)	Cucumis sativus	Germination and seedling growth	Significantly increased germination rate and seedling growth	Albanna et al. (2016)
Sulfur (500, 1,000, 2,000 and 4,000 ppm)	Vigna radiata	Germination	Improved germination rate	Patra et al. (2013)

Abbreviations used in Table: ABA- Abscisic Acid; APX-Ascorbate peroxidase; CAT- Catalase; H₂O₂- Hydrogen peroxide; MDA- Malondialdehyde; POD- Peroxidase; ROS- Reactive oxygen species; SOD- Superoxide dismutase

NPs have been tested as tools for rapid detection and precise quantification of plant metabolic flux, pesticide residues, as sensors for pathogens (bacterial, viral or fungal) and for various other agricultural applications of crop biotechnology (Duhan et al. 2017; Chaudhry et al. 2018). There are several reports on the use of different types of nanosensors based on plasmonics, fluorescence resonance energy transfer (FRET), carbon-based electrochemicals, nanowires and proteins (antibody). Single-walled carbon nanotubes (SWNTs) were used for near-infrared fluorescence monitoring of nitric oxide (Giraldo et al. 2014). Polystyrene NPs conjugated to FRET probes were designed to quantify and recognize the phytoalexins (Dumbrepatil et al. 2010). Gold-NPs have been proposed as DNA biochemical labels to detect Pseudomonas syringae on disposable screen-printed carbon electrodes (Lau et al., 2017).

Nano-fertilizers are synthesized or derived from traditional fertilizers, bulk materials, or plant extracts e.g. nano form of rock phosphate may increase the phosphorus availability for plants. They serve as economical and sustainable sources of plant nutrients (El-Saadony et al. 2021). This category also includes nano-scale additives in traditional fertilizers or traditional fertilizers coated or loaded on NPs such as a nano-membrane. The NPs have a higher surface area, which increases their retention capacity and helps to improve the solubility and dispersion of insoluble nutrients in the soil, thereby increasing nutrient bioavailability to the crops. They can serve to slow the release of nutrients or provide a network of channels that retain nutrient solubility (Rameshaiah et al. 2015; Selva and Balakrishnan 2017; Dapkekar et al. 2018; Khanm et al. 2018).

MicroRNAs as nano-regulators of plant stress response

Extensive studies have established the small (20–24 nucleotides long) non-coding RNAs (ncRNAs), as fine regulators of the morpho-genetic pathways (Goswami et al. 2019). They act by silencing genes or transcripts at the transcriptional and post-transcriptional levels, respectively (Ahmed et al. 2020b). The cellular machinery is intricately regulated by different classes of ncRNAs to bring about gradual changes in the phenotype or biochemical pathways by creating spatio-temporal gradients of proteins encoded by their target genes (Zhang et al. 2019).

The microRNA (miRNAs) constitute a highly conserved and a large class of the ncRNAs that influences all aspects of plant growth and development (Li and Zhang 2016; Djami-Tchatchou et al. 2017; Liu et al. 2018; Jatan and Lata 2019; Pagano et al. 2021). They also play a central role in governing the response of plants to biotic and abiotic stress conditions (Sunkar et al. 2012; Basso et al. 2019; Kushwaha et al. 2021). The miRNAs are transcribed as long primary miRNA (pri-miRNA) transcripts by RNA polymerase II (Pol II) from endogenous miRNA (MIR) genes (Lee et al. 2004). The pri-miRNA is cleaved by a protein complex containing DCL1 protein to produce the stem-loop structured precursor miRNA (pre-miRNA). The pre-miRNA is further processed to yield the miRNA duplex (miRNA/miRNA*). The miRNA duplex is stabilized by methylation at the 3’ ribose of the terminal nucleotide (Yu et al. 2005) and then exported to the cytoplasm (Park et al. 2005) where it gets associated with the RNA induced silencing complex (RISC) to form the functional silencer.

The expression of miRNAs is genetically determined and regulated by a variety of mechanisms including transcription factors (Boque-Sastre et al. 2015; Statello et al. 2021), hormones (Jha et al. 2020; Song et al. 2021) and target mimicry by long ncRNAs (Franco-Zorrilla et al. 2007; Wu et al. 2013; Wang J. et al. 2015). During stress, the levels of miRNAs are up or down regulated to differentially regulate the target genes expression and elicit a suitable response to stress (Sunkar et al. 2012; Hernandez et al. 2020; Janni et al. 2020). Microarray and next-generation sequencing analysis have revealed numerous stress-responsive miRNAs including miR156, miR159, miR167, miR168, miR171, miR319, miR396 and many others in different plant species like Arabidopsis, cotton, maize, rice, wheat, soybean, sweet potato, tomato and others. The role of miRNAs in response to different stresses and/or plants has been extensively reviewed (Lu and Huang 2008; Guleria et al. 2011; Sunkar et al. 2012; Shriram et al. 2016; Sharma et al. 2017; Kumar et al. 2018; Chaudhary et al. 2021). Some miRNAs respond to specific stresses and others are differentially regulated in response to various stresses (Ferdous et al. 2015; Goel et al. 2019; Ahmed et al. 2020a). A list of conserved stress-responsive miRNAs and their targets is presented in Fig. 1. The degree and nature of miRNA regulations also vary between crop species and/or cultivars. To a large extent, the miRNA variations are responsible for the differences in the response of plants to stress (Goswami et al. 2020).

Fig. 1.

Multi-stress responsive miRNAs and their targets in different plants. The miRNAs and their corresponding targets are shown in the inner rectangular boxes and are connected with arrows. The same color coding is used to indicate the miRNA target pair. The outermost rectangular boxes on both the upper and lower sides represent abiotic stresses regulating the respective miRNAs.

Abbreviations: Auxin F-box protein (AFB), APETALA2 (AP2), ATP Sulfurylase (ATPS), ARGONAUT protein (AGO2), Auxin response factors (ARF), Basic leucine zipper domain (Bzip), Cold stress (C), Copper proteins (Cu), Dicer Like1 (DCL1), dehydration-responsive element-binding protein (DREB), Drought stress (D), growth-regulating factor (GRAS, GRF), heavy metal stress (H), class III homeodomain leucine zipper (HD-ZIP), Laccases (Lac), Mitogen-activated protein kinase (MAPK), my elob lastosis (MYB), NAM, ATAF1/2 and CUC2 domain proteins (NAC), Nicotinamide adenine dinucleotide (NAD), Nuclear factor Y subunit A (NF-YA), Nucleotide-binding site leucine-rich repeat (NBS-LRR) proteins (NBS-LRR), PHOSPHATE2 (PHO2), pentatricopeptide repeat proteins (PPR), Peroxidase (POD), Polyphenol oxidase (PPO), Salt stress (S), scarecrow-like (SCL), Superoxide dismutase (SOD), Squamosa promoter-binding protein/like (SBP/SPL), Pi homeostasis responsive protein- SPX, high temperature/heat stress (T), TEOSINTE BRANCHED/CYCLOIDEA (TCP), transport inhibitor response1 (TIR1)

The direct evidence for the involvement of miRNAs in mitigating stress has come from studies involving the generation of miRNA based transgenic plants. These studies suggested the role of miR398, miR156 and miR159 in improving plant thermotolerance and generating heat stress memory. Arabidopsis plants overexpressing miR156 exhibited enhanced tolerance to heat stress (Stief et al. 2014). Likewise, plants overexpressing miR159 showed more sensitivity to heat stress (Wang Y. et al. 2012). Recently it was shown that miR398 which targets the Cu/Zn superoxide dismutase (CSD) also participates in plant thermotolerance response and its expression is regulated by the long ncRNAs, NAT398b and NAT398c (Li et al. 2020a). It was earlier reported that overexpression of miR398-resistant form of cytosolic CSD1 and chloroplastic CSD2 in Arabidopsis accumulated more CSD transcripts and showed greater heavy metal stress tolerance (Sunkar et al. 2006).

In Oryza sativa, overexpression of miR166 improved cadmium tolerance, by reducing the metal induced oxidative stress in plants. In another study, it was shown that knockdown of miR166, using the Short Tandem Target Mimic (STTM) system, resulted in morphological changes that conferred drought resistance in rice (Zhang et al. 2018). Overexpression of miR160 in Arabidopsis improved seed germination and seedling survival under heat stress (Lin et al. 2018; Ding et al. 2020). Recently, it was shown that STTM knockdown double mutants of miR160 and miR165/66 in A. thaliana exhibited moderate drought tolerance but the leaf phenotypes were compromised. The comparative analysis of single and double STTM lines suggested that miR160-directed regulation of auxin response factors (ARFs) contributed to leaf development via the auxin pathway, whereas miR165/166 mediated HD-ZIP IIIs regulation conferred drought tolerance through ABA (Abscisic acid) pathway (Yang et al. 2019).

Studies on tomato plants constitutively overexpressing miR169 revealed its role in enhancing plant tolerance to drought stress by reducing water loss, by lowering the stomatal aperture index and stomatal conductance by > 30% (Zhang et al. 2011). Arabidopsis plants overexpressing miR169 were hypersensitive to nitrogen starvation (Zhao et al. 2011). This suggested that miR169 is capable of improving plant tolerance to drought stress and nitrogen deficiency. Similarly, under drought conditions, transgenic Arabidopsis plants that overexpressed miR394 exhibited restricted water loss during leaf transpiration, which ultimately increased tolerance to drought stress (Ni et al. 2012).

Transgenic studies elucidated a role for miR396, miR172, miR402 and miR397 in improving plant performance under salinity, dehydration and cold stress conditions. Rice plants that constitutively overexpressed miR396 exhibited stunted root growth and decreased plant development during salinity and alkalinity stress treatments (Gao et al. 2010). This suggested that miR396 acts as a negative regulator of plant response to stress. Also, overexpression of miR172c was shown to confer salt stress tolerance to soybean plants but it increased sensitivity to ABA in transgenic Arabidopsis plants (Li et al. 2016). Overexpression of miR402 promoted Arabidopsis seed germination under stress conditions (Kim et al. 2010a). Overexpression of miR395 affected plant tolerance to salinity, drought and heavy metal (cadmium) stress, as evidenced by a decrease in seed germination and seedling growth (Kim et al. 2010b; Zhang et al. 2013). Transgenic Brassica napus overexpressing miR395d showed higher tolerance to cadmium by triggering the accumulation of sulphur-containing compounds for chelating cadmium (Zhang et al. 2013). This was evidenced by a lower degree of cadmium-induced oxidative stress and higher levels of chlorophyll, glutathione, and non-protein thiols in the transgenic plants compared with their wild-type plants (Zhang et al. 2013). Similarly, miR397 overexpression resulted in a significant increase in cold stress tolerance by up regulating cold-regulated C-repeat binding factors (Dong and Pei 2014).

miR393 has been implicated in response to multiple stresses and it is known to target the transcript for transport inhibitor response protein 1 (TIR1). Rice seedlings overexpressing miR393 were more sensitive to salinity and alkalinity treatment (Gao et al. 2011), while plants overexpressing a miR393-resistant TIR1 exhibited increased salt stress tolerance. The plants also showed increased water-use efficiency, delayed senescence, stabilized chlorophyll content and higher seed germination rate (Chen et al. 2015). In another study, characterization of transgenic Agrostis stolonifera (creeping bent grass) plants overexpressing pri-osa-miR393a revealed that plants exhibited enhanced tolerance to drought, heat and salt stress (Zhao J. et al. 2019). The transgenic plants had fewer but longer tillers bearing leaves with reduced stomatal density and denser cuticles increased uptake of potassium and enhanced expression of the small heat-shock protein. This indicates that miR393 has the potential for improving plant tolerance to multiple stresses.

miR319 is usually upregulated during multiple stress conditions (Sunkar and Zhu 2004; Zhou et al. 2010). In creeping bentgrass, constitutive expression of miR319 increased tolerance to salt and drought stress (Zhou et al. 2013). These transgenic plants exhibited increased water retention and cell membrane integrity and accumulated less Na⁺ under salt stress conditions (Pieczynski et al. 2013). Osa-miR319 overexpression resulted in cold stress tolerance in rice by reducing OsPCF5 and OsPCF8 expression levels (Yang et al. 2013). Taken together, it can be interpreted that miR319 overexpression can enhance tolerance of plants to multiple environmental stresses.

Overexpression of miR408 in the model plant Arabidopsis enhanced tolerance to salinity, cold and oxidative stress, but increased sensitivity to drought and osmotic stress (Ma et al. 2015). The heterologous expression of Salvia miltiorrhiza miR408 in Nicotiana benthamiana also showed a similar response. The plants were more tolerant to salt stress and showed reduced ROS accumulation (Guo et al. 2018). Moreover, overexpression of osa-miR408 in perennial ryegrass (Hang et al. 2021) and chickpea (Hajyzadeh et al. 2015) plants led to improved drought tolerance. The transgenic plants showed morphological changes, such as curled leaves and sunken stomata, which could be related to decreased leaf water loss. This suggests that miR408 plays a vital role in improving plant tolerance to abiotic factors that cause osmotic stress.

There are also reports on the use of artificial miRNAs (amiRNAs), to target stress-responsive genes for manipulating plant responses. For instance, amiRNAs designed to knock down nuclear cap-binding protein 80 (CBP80) or Abscisic Acid Hypersensitive 1 (ABH1) resulted in higher tolerance to drought stress in potato plants (Pieczynski et al. 2013). These results indicate that miRNAs could be promising candidates for the development of crop cultivars with enhanced multiple stress tolerance, thus contributing to agricultural productivity.

Nanomaterial assisted delivery

The efficient application of nanomaterials in the medical science sector has triggered translation interests in the agriculture sector as well (Singh et al. 2021). Several studies have reported using NPs for the targeted delivery of bioactive molecules for various applications. The NPs have an extremely small size and bind at multiple sites, so they can easily penetrate the cell and enable the efficient delivery of their cargo of bioactive molecules (Sako et al. 2021).

The NPs can be delivered in plants by application through soil, spraying aerial surfaces, incubation, irrigation, injection, hydroponic treatment, biolistic gene guns and so on (Wang et al. 2019; Roychoudhury 2020). Different reports describe the incubation of seeds, roots, pollen, isolated cells and protoplasts with the NPs (López-Moreno et al. 2010; Martin-Ortigosa et al. 2012; Arif et al. 2018; Wang et al. 2021). When supplied to roots they can be directly absorbed but when delivered through aerial or foliar applications they can enter through stomata, wounds, trichomes, stigma, hydathodes, cuticles, or lenticels (Eichert and Goldbach 2008; Dietz and Herth 2011). In plants, their uptake is size dependent and NPs can traverse the cell wall by endocytosis. Their entry within the cells can also be facilitated by carrier proteins, aquaporins, and other transporters. Within the plants, their transport can be through apoplastic or symplastic routes. The movement of NPs within different parts of the plant is both active as well as passive (Wang P. et al. 2016).

NPs have been employed for important and accurate application in the genetic transformation of animals and plants (Wang et al. 2019; Lv et al. 2020). Most studies reported in animals and medical research utilized magnetic NPs (Nasiri et al. 2020) and limited information is also available from plants (Barhoumi et al. 2015; Wang et al. 2017; Zhao et al. 2017). One such report describes the successful generation of transgenic cotton seeds employing NPs using a tissue culture independent approach (Zhao et al. 2017). The plasmid DNA was packaged with magnetic NPs and forced into the pollen via a magnetic field. The transformed pollens carrying the NPs loaded with plasmid DNA were used for pollination. The resulting seeds showed stable genetic inheritance of the transgene.

A large number of experiments conducted using the NPs have shown that the method of delivery and size of the target material determines the success of the experiment. The nanotechnology-assisted delivery methods include a variety of nanomaterials like chitosan-based (Behboudi et al. 2018a; Faizan et al. 2021), liposomes-based (Karny et al. 2018), protein and peptide-based (Ng et al. 2016; Chuah and Numata 2018; Miyamoto et al. 2020), aptamers (Karimi et al. 2018), inorganic NPs (Loh et al. 2016), quantum dots (Al-Salim et al. 2011; Modlitbová et al. 2018), and polymers such as polyethyleneimine (Sadeghpour et al. 2018), poly lactic-co-glycolic acids (Fukamachi et al. 2019), and dendrimers (Pasupathy et al. 2008; Chaudhary et al. 2018).

Silica (Si) NPs have been shown to play a crucial role in plant stress tolerance (Ahmad and Akhtar 2019), by increasing photosynthesis, enhancing chlorophyll concentration and decreasing the rate of transpiration (Ahmad and Akhtar 2019). Mesoporous silica NPs have been used as efficient platforms for drug delivery. They have well-developed surface chemistry and are biocompatible, easily degradable and safe (Kim et al. 2008; Slowing et al. 2008; Finnie et al. 2009). A study reported the use of surface-functionalized mesoporous silica NPs as effective carriers of bioactive nucleic acids. The modification of NPs involved attaching polyethyleneimine polymers to the surface by non-covalent bonding. This provided a cationic property to the surface, which enabled an increase in the attachment of DNA and small interfering RNAs (siRNAs) constructs. These NPs also exhibited higher cellular uptake and successful cargo delivery (Xia et al. 2009). It was shown that NPs coated with 10 kDa polyethyleneimine polymer were effective in transducing HEPA-1 containing siRNA construct against green fluorescence protein expression (GFP). The construct was able to knock down GFP and exhibited reduced cytotoxicity for the NPs.

It was further proved that NPs are not only facilitating the transport of molecules but also aid the delivery of functional genetic material without the requirement for DNA integration (Demirer et al. 2019). There are many reports on efficient delivery using functionalized high aspect ratio nanomaterials via diffusion. For instance, chitosan-complexed single-walled carbon nanotubes were successfully used to deliver chloroplast-selective genes by using the lipid exchange envelope penetration method (Kwak et al. 2019). The chloroplast-selective gene was efficiently delivered to different plants including A. thaliana (mesophyll protoplasts), Eruca sativa, Nasturtium officinale, Nicotiana tabacum and Spinacia oleracea. Transient protein expression profile of the insert proved the delivery efficiency (Kwak et al. 2019), while gene expression analysis in Eruca sativa, Gossypium hirsutum, Nicotiana benthamiana, and Triticum aestivum indicated no transgene integration (Demirer et al. 2019).

Carbon nanotubes application in plants is very limited because of the cell wall rigidity. To ensure cell penetration multiwalled carbon nanotubes have been used in Catharanthus roseus protoplast transformation. The results indicated that nanotubes move via the protoplast membrane and target the nuclei, plastids, and vacuoles (Serag et al. 2010). Later on, a combination of cup-stacked carbon nanotubes with cellulase was reported for traversing the cell wall (Serag et al. 2012).

A recent study indicated that organelle-targeting peptide NPs could be used to deliver DNA into intact plants (Ng et al. 2016). NPs loaded with a chloroplast-targeting peptide fused with DNA were used to deliver the peptide-DNA complex to plastids via cellular membrane (Thagun et al. 2019). This bypassed the need for stable genetic transformation and facilitated the transformation of plastids for regulating transient gene expression. Even though much progress has been made in this direction, there are many mechanistic issues that remain to be understood (Chuah and Numata 2018).

Delivery of non-coding RNAs

In humans and animals, nano-carrier-guided miRNA delivery has been successively done to treat various diseases (Chaudhary et al. 2018; Fu et al. 2019; Lee et al. 2019; Dasgupta and Chatterjee 2021), but in plants the application is still limited and poorly studied. In plants, miRNAs and siRNAs have been widely reported in mitigating plant stress and enhancing productivity (Khraiwesh et al. 2012; Betti et al. 2020; Abdellatef et al. 2021). Thus, the use of NP mediated delivery of ncRNAs can open exciting avenues with a range of innovative applications.

It was shown that primarily the presence of metallic NPs was shown to affect the expression or accumulation of miRNAs (Fig. 2) involved in controlling heavy metal homeostasis, regulation of SOD gene expression and stress-related gene expression (Dugas and Bartel 2008). For instance, it was shown that exposure of Arabidopsis seedlings to gold NPs down regulated the expression of six miRNAs (miR164, miR167, miR395, miR398, miR408, and miR414). The effect was more visible at 80 µgml^− 1 concentrations of gold NP as compared to 10 µgml^− 1 (Kumar et al. 2013). The miR397 was up regulated at both concentrations of gold NPs while expression of miR399 was not affected. The differential expression of miRNAs was also reported in switchgrass, treated with nano-Titanium (IV) oxide (TiO₂) and this affected germination rate, plant growth and development (Boykov et al. 2019). At increased concentrations of TiO₂ the biomass, root number and branching were reduced.

Fig. 2.

List of selected heavy metal stress responsive miRNAs in plants. The miRNAs listed in the larger circle are responsive to the heavy metal mentioned above in the respective circle. The miRNAs listed in the smaller (overlapping) circle are responsive to the heavy metals listed above the neighboring circles. miR398 responds to Copper, Zinc and Iron in addition to Manganese, Mercury, Aluminum and Cadmium but not to Arsenic

Several heavy metal responsive miRNAs have been investigated in many plants in response to arsenic (Liu and Zhang 2012; Srivastava et al. 2013), aluminum (Chen et al. 2012; Wu et al. 2018), cadmium (Ding et al. 2011; Zhou et al. 2012; Zhou et al. 2017; Gao et al. 2019; Pegler et al. 2021), manganese (Valdes-Lopez et al. 2010), mercury (Chen et al. 2012), lead (Wang Y.L. et al. 2015) etc. The heavy metal regulated miRNAs act by controlling uptake and transportation of metals, folding and assembly of proteins, hormone signaling, ROS scavenging and so on (Gielen et al. 2012; Ding et al. 2020).

The bottleneck in the use of metallic NPs could be troubleshot by employing alternative nanomaterials. In one of the first reports, it was shown that topical application of dsRNA loaded with double hydroxide clay nanosheets increased plant resistance to viral disease for at least 20 days (Mitter et al. 2017). This innovative approach is environmentally friendly and designed to improve plants without the request for plant gene transfer. In another recent study, it was shown that a combination of clay nanosheets and artificial miRNA constructs efficiently prevented Tomato yellow leaf curl virus (TYLCV) infection in tomato plants (Liu et al. 2020). The recombinant plasmid DNA expressing pre-amiRNAs were loaded on to degradable clay nanosheets and sprayed on tomato plants. This treatment reduced the TYLCV viral load in plants when evaluated 35 days after spray and showed increased H₂O₂ levels. These reports suggest that clay nano-sheet surface coatedartificial microRNAs can be promising control tools for begomovirus and other stresses (Fig. 3). However detailed studies are still required to unravel the mechanism and understand how the external RNA is incorporated to plants. The protocols also require to be standardized for RNAi conditions like optimum construct length, concentration, etc.

Fig. 3.

A hypothetical model to represent the application of nanomaterials to augment plant stress tolerance. A variety of stresses act on plants to influence their physiological, biochemical and molecular pathways. The response of plants to stress a cumulative action of these changes. The use of small non-coding RNAs (ncRNAs) and nanoparticles separately has shown to improve the stress tolerance of plants. The application of nanoparticles coated with ncRNAs is emerging as an attractive new strategy to improve plant response to stress

Conclusion and Perspectives

The ever increasing demand for food, limited availability of agricultural land and continuously changing climate have imposed serious challenges for sustainable agriculture. Great progress has been made in identifying a variety of gene products in controlling the plant responses to abiotic stresses. The small ncRNAs occupy a special place in this context as they have helped to understand the mechanisms responsible for maintaining genetic homeostasis by regulating gene expression and silencing. The miRNAs, an important class of the small ncRNAs, constitute the central hub of gene networks that regulate plant biology. Functional studies on miRNAs have highlighted their association in controlling plant response to abiotic stress. These studies are still in their infancy and it is a matter of time before key miRNAs will be identified and selected as real targets for improving crop performance under environmental stress.

Scientists around the globe are also searching for novel approaches to improve agricultural productivity and nanotechnology is emerging as an attractive and innovative option. The exclusive physiochemical attributes of NPs have facilitated numerous applications in agricultural and biomedical sectors. This technology holds the potential for delivery and controlled release of biomolecules like agrochemicals, genetic materials or other molecules to the target site to improve disease resistance, nutrient use efficiency, growth, crop yields and stress tolerance (Nair et al. 2010; Ditta and Arshad 2016). The technology, however, is still in its infancy and most of the studies on NPs are limited to understanding their impact on plant growth. It was shown that metallic NPs might prove fatal or detrimental to plants so greater efforts will also be required to gain mechanistic insights on the role of NPs in eliciting the desired plant responses. Additionally, lot more optimizations will be required in the crop specific selection of a NP and its mode of application, followed by testing the reproducibility of the response in a wide range of plant species before advancing to field-level applications.

Recently the focus has shifted on their direct or indirect utilization for crop improvement and abiotic stress management. Preliminary studies have shown the applicability of NPs to overcome the barrier of environmental stress and have direct agricultural relevance in times of global climate change. Therefore, nanotechnology has the potential to complement traditional breeding and genetic engineering approaches. NPs can easily enter plant cells through a size-dependent mechanism and can accumulate in different tissues including newly developed seeds. The NPs have been shown to improve the antioxidant potential in plants by radical scavenging to tackle the stress induced ROS. Detailed insights into the physiological, biochemical and molecular mechanisms of NPs in plants will encourage greater application to improve plant growth and yields under stress conditions. The interest has shifted to identifying and synthesizing different types of NPs using plant extracts or microbes.

The advancement in approaches for employing NPs to deliver, detect and influence miRNAs has unfolded the potential use of nanomaterials for revolutionizing the agronomic sector. The relatively recent reports on the use of clay NPs loaded miRNAs as new effective tools for crop improvement have triggered great excitement. The NP assisted delivery of ncRNAs or constructs expressing ncRNAs appears as an effective mechanism for regulating plant stress tolerance. These studies have also reflected on the molecular crosstalk between the NPs and the cellular machinery providing new directions for detailed investigations. Detailed studies in this direction may lead to the development of novel technologies for crop improvement.

Authors’ contributions

TAG: conceptualization, writing- original draft and preparing the tables and figures; NSM: conceptualization, supervision, editing, and administration. Both authors have read and approved the final manuscript.

Data Availability

Not applicable.

Declarations

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethics approval

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Informed Consent

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