Seed regeneration aided by nanomaterials in a climate change scenario: A comprehensive review

2.1 Seed regeneration

Changing climatic conditions due to global warming creates a drastic impact on the survival of many species, often causing them to approach extinction. Although this situation applies to plants, protecting some of the essential species is crucial due to rising uncertainties in the current climate which leads to extremely complex situations for human existence [14].

This is where seed regeneration (Figure 1) as a concept is necessary to understand for crop growth support during variable climate conditions. In simplest terms, seed regeneration means the ability to raise a plant from its seed stage under any adverse conditions, i.e., changing climate, lack of nutrients, existence of pests, and possible diseases that can affect the plant’s growth or desired output in any manner.

Figure 1

Overview of seed regeneration under climate change: Impacts on plant life cycle and population dynamics. Inspired by the study of Misra et al. [9].

Climate change is a vast topic, and even if we restrict the topic to the seed regeneration process alone, the number of factors that we need to consider is large (Figure 1). Still, it is an efficient way of discussing the problem, involving all kinds of challenges that we may face so that the selection process of nanomaterials can be narrowed down [15]. There are several factors causing climate change, but all can be pointed down to one primary reason: human activities. Initially, we will specifically discuss the lacking factors for a seed to grow if its original climatic condition or ecosystem is changed due to one or several reasons. The most common change would be the increase in temperature, UV exposure, and decrease in water resources, which are the immediate effects of global warming. Under these circumstances, plant growth has to encounter the mentioned factors, excess of which could hamper the same [16].

However, crops used for agriculture may also experience a lack of nutrients due to a decrease in soil quality or an improper supply of fertilizers. The usage of powder fertilizers that are sprinkled on plants may not be sufficient, and may require some added minerals. Seed regeneration has to take place irrespective of the external factors, where nanofertilizers serve the purpose for supplying nutrients to promote the plant growth [17,18].

During the selection of desired nanomaterials, there are high chances that there could be a cancellation of the effectiveness if the properties of nanomaterials being used may mutually inhibit their effects if not processed adequately. Hence, the growth stage becomes the most unreliable and unpredictable stage in the entire seed regeneration process, as there is no efficient way to determine whether the nanomaterials inhibit or exacerbate the process [19,20]. However, with further research, gold nanoparticles (NPs) have proven to be more efficient than both silver nanoparticles (AgNPs) and silicon NPs in preventing seed damage due to high heat exposure [21]. The most economical and feasible nanomaterials available have to be rigorously studied. As cost is a major factor, there has been an increased usage of AgNPs over gold nanoparticles (AuNPs) for research and utility purposes [22]. Based on such a topic alone, we can classify several nanomaterials best suited for the purpose while being economical and feasible. Such a common and essential discussion is seen in Figure 2.

Figure 2

Application of nanomaterials in agriculture. Adapted from the study of Paramo et al. [27].

2.2 Stages of seed regeneration

Seed regeneration involves several important steps that differ depending upon the type of seeds [23]. This crucial process serves the purpose of enhancing plant growth by creating new seeds from existing plant materials. The proper completion of seed regeneration will allow for the production of high-quality seeds with specific desirable traits. The seed regeneration process may vary depending on the type of seed that is being regenerated. However, the general steps required for the seed regeneration process begin at the selection of seed and ends at the growth of the plant, where the process will be as follows:

1. Selection of parent plants. This first stage involves analyzing characteristics to be achieved for the target population. In this step, the parent plants that have ideal features, like high yield, resistance to diseases, or other qualities that align with the ideal traits of the target population will be chosen.

2. Pollination. Once the parent plants have been carefully identified and chosen, further steps can be taken. The pollination process takes place through the transfer of pollen from the reproductive organs of the male to the reproductive organs of the female. In other words, pollen contained in the anther is passed to the stigma. This occurs to either the same or distinct species of flower. Aside from human action, pollination can also occur through naturally occurring factors, such as wind, insects, or birds. These naturally induced ways of pollinating are referred to as “cross-pollination,” where the insects and birds are referred to as the “pollinators.” A plant may gain the ability to fertilize itself, also known as “self-pollination.” [24].

3. Seed development. Once pollination is successful, the seed development process may begin. In simple terms, after the pollen has been successfully transferred to the stigma, the fertilized ovule within the flower’s ovary turns into a seed. However, this process is a much more protracted procedure. This stage is a complex process and can be separated into two major phases, embryogenesis (cell division and morphogenesis), followed by maturation. Seed development begins with double fertilization, where one sperm will fertilize the egg to form the zygote. After multiple cycles of cell division, the zygote will form the embryo. The other sperm will merge with two polar nuclei forming a triploid cell, which will divide and form the endosperm (cellular or helobial). Layers of tissue will begin to stack in order to form the seed coat, which will serve the purpose of protecting both the embryo and endosperm from damage. Finally, maturation of the endosperm, embryo, and seed coat will begin, which involves the collection of carbohydrates, proteins, and lipids. Once maturation has finished, seeds are distributed from the parent plant, either through human-induced actions or natural occurrences such as wind, gravity, insects, birds, or water [25,26].

4. Seed harvesting. A successful completion of the seed development process will ensure that the seed is viable for germination. After the seeds have reached maturity and full development, the parent plants’ seeds are collected. There are different methods used for gathering the seeds, depending on the species, size, and structure of the mature seeds. These methods include handpicking, cutting, shaking, and beating.

5. Seed processing. Once the mature seeds have been collected, seed processing may begin. In this stage, the seeds must be cleaned to get rid of any chaff debris, or other contaminants. To maintain the quality of the seeds, they must be dried first. Later, they are stored in a cool, dry environment to prevent the absorption of moisture and pest infestation.

6. Seed testing. Once all of the seeds have been collected and stored properly, they must be tested prior to subsequent use. This will ensure that only viable seeds will continue the process. During testing, a seed is assessed through its viability, germination rate, genetic purity, and may also test the vigor and overall health of the seed. The germination test involves placing seeds in a setting with optimal environmental conditions, and the amount of seeds that germinate is recorded. Additionally, viability testing may involve X-rays to analyze the embryo and endosperm, floatation test to determine if the seed will sink (viable) or float (non-viable), and assess for electrical conductivity (those with lower conductivity are viable) [28].

7. Seed storage. When seed testing is complete, the viable seeds are placed in ideal environmental conditions to maintain the overall high quality of the seed. This optimal setting involves a cool, dry environment in order to block the entrance of moisture and degradation.

8. Seed treatment. As the seeds are stored, they may undergo seed treatment. However, this step is usually optional. Physical, biological, and chemical treatments can be applied to seeds to improve their vigor and resistance to stress. This is the step where nanomaterial-based intervention would be involved. These may include priming, coating, or sensors [29].

9. Seed distribution. Now, the seeds are ready to be distributed “for consumption to the users”, such as farmers.

10. Germination and plant growth. Finally, the seeds are planted in soil and must be taken care of properly.

Carefully completing these steps will help maintain a sustainable agricultural production by enhancing the resilience of plants. Properly managing seeds throughout this process will allow for strong, stress resistant, and long-term feasibility of seeds.

The seed growth life cycle as explained above, is complemented by understanding and implementation of seed regeneration process as explained in the below steps:

1. Selection of seeds. As mentioned earlier, this is a decisive step for selection of seeds that would require regeneration. Regeneration of wild seeds might prove quite tricky because recreating the entire ecosystem required for its growth is simply impossible. But the very factor depends upon the ecosystem that the seed grew in, and depending on these varying factors, results may also vary significantly. On the other hand, regeneration of domestic seeds might be a less tedious process, comparatively, since it can adapt to the certain idealized conditions, but not all seeds can be differentiated likewise and are highly dependent on the ecosystem they are subjected to [30,31].

2. Climatic conditions. The next step would be to select the climatic condition that we need the seed to grow in, it is almost impossible to regenerate a seed that can grow in all climatic conditions, so limiting the conditions may ease up the process that can be done by selecting specific climatic conditions. For instance, regenerating a seed in a condition of drought narrows down the recreation of its entire ecosystem for lack of water, high temperature, and lack of nutrients. On the other hand, if a flood condition has to be overcome, the case is opposite and requires a different approach. Being precise on the climatic conditions that we want a seed to grow in might narrow down the scope of the process [31,32].

3. Methods of seed regeneration. After seed selection, climatic conditions are set to desired variables. The next step is to find the most efficient way for seed regeneration. It is certainly impossible to ideally meet all the conditions by using simple physical methods, but utilizing chemical methods to provide nutrients for it, or simply the presence of nanomaterials can enhance the properties required to facilitate seed regeneration, considering that it depends on the conditions that we choose the most desirable nanomaterials that can accommodate the needs. In contrast, SiO₂ nanomaterials can be used for drought resistance, AgNPs are used for agricultural purposes, TiO₂ NPs can enhance fennel seed germination [33]; thus, depending on the utility, a wide range of choices of nanomaterials are available [31,34].

4. Growth of plants from seed. It is always uncertain that the imitation of the ecosystem required for the seed to grow into a plant will necessarily function. There are chances that these factors may inhibit the very growth it was intended for. There are no methods to evaluate explicitly to see how the nanomaterials function, and would require an observation up to several days [35]. There is always the risk of failure and lack of data on nanomaterial’s functioning, but the necessity of seed regeneration far exceeds these factors.

Being a very tedious process, despite current research in the field, there are certain aspects of the process that could be further regulated using nanomaterials and emerging technologies that could be incorporated to make the process comparatively simpler and manageable [17,31,35].

3.1 Synthesis of nanomaterials for seed regeneration

The factors that affect the characterization and synthesis of NPs are extract concentration, size, temperature, pH of the solution, the concentration of the raw materials used, and synthesis methods [14]. Generally, nanomaterials can be synthesized in two different ways, which include top-down and bottom-up approaches. Comparatively, bottom-up is the most preferred because of its cost-efficiency. Due to their antimicrobial properties, the most widely used nanomaterials are metal NPs such as copper, silver, and zinc; polymers are also used, where carbon-based nanomaterials are used as antimicrobial agents. In general, nanomaterials applied in trace amounts act as bio-stimulants that help induce plant innate immunity, increasing tolerance to diseases to suppress biotic stresses [42]. Considering the bio-stimulation potential of nanomaterials in host plants, the application of NPs as foliar and seed application can be a practical and preventive approach before the onset of pathogen infection by boosting the plant’s innate immunity.

However, the particular characteristics regarding the range of specific concentration associated with biostimulation should be further examined [43]. There are two sources of NPs: biotic and abiotic. AgNPs can be synthesized in various ways. However, the biological route is considered as non-toxic, non-harmful, biocompatible, and economical. Here AgNPs are synthesized both chemically and biologically (Figure 3).

Figure 3

Synthesis of AgNPs using green synthesis. Adapted from the study of Roy et al. [47].

The biotic form of NPs is biodegradable, eco-friendly, and prepared from organic sources. On the other hand, the abiotic form of NPs can be retrieved from inorganic sources like salts that are unsafe, mostly non-biodegradable [44]. Due to their size (1–100 nm), shape, and structure, NPs display improved properties over other materials. Compared to other metallic NPs, AgNPs exhibit unique physicochemical properties imparting antioxidant and antimicrobial activities. Various methods have been used to produce NPs, but synthesis using plant extracts is popular due to its advantages, including rapid development, cost-effective protocol, single-step method, non-pathogenicity, and environmentally friendly [45,46].

The synthesized AgNPs are generally characterized using several characterization techniques to distinguish synthesized NPs [48]. For example, researchers have used UV–Vis spectrometer, Energy dispersive X-ray spectroscopy, selected area electron diffraction (SAED), and high-resolution transmission electron microscopy to characterize synthesized AgNPs by using Ananas comosus, spherical NPs of an average diameter of 12 nm were described in the transmission electron microscopy (TEM) micrographs [49]. The magnesium hydroxide NPs (Mg(OH)₂NPs) were synthesized by using the modified co-precipitation method [50]. The Zea mays L. seeds were sterilized, treated, and various concentrations of Mg(OH)₂NPs were observed on the growth of Zea mays L. seeds compared to the untreated Zea mays L. seeds [51].

The chitosan-based nanomaterials can be prepared through chemical methods like interfacial polymerization, in situ polymerization, and interfacial polycondensation. Furthermore, physicochemical methods are also used to enhance specific traits, such as complex coacervation, ionic gelation, and spray cooling. There are physical methods that may involve the use of centrifugal extrusion – spheronization, spray-drying, and fluid bed coating. Among these, ionic gelation is considered the standard method to prepare copper and zinc-based chitosan NPs. To characterize the various physicochemical properties of advanced nanomaterials, methods such as scanning electron microscopy (SEM), dynamic light scattering (DLS), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) were used [52,53].

Chitosan, a biopolymer, is well known because of its broad-spectrum antimicrobial activity. Chitosan can be synthesized biologically in the Czapek-Dox broth medium in which Penicillium oxalicum, an endophytic fungus, was grown for 3 days. The extracellular proteins produced by the fungus were precipitated with ammonium sulfate saturation of 80% (W/V), and then the precipitated proteins of 5 mg were made to pass through the presaturated carboxymethyl-cellulose column. The unbounded proteins removed from the column were checked and collected for the required protein content. It will be utilized to prepare chitosan NPs [54].

The chitosan was made to dissolve at 0.5% (w/v) with acetic acid of 1% (v/v) and the corresponding pH of 4.8. The chitosan solution of 15 mL was added to the anionic proteins of (180 µg/mL) removed from the void volume of the (6 mL) column stirred for 30 min under magnetic stirring and kept overnight at room temperature [55]. After incubation, the colloidal suspension was centrifuged so that the precipitate was made to wash twice to remove the unreacted substances and then freeze-dried. Hence, the highly stable NPs are delivered by biological preparation with high zeta potential and low polydispersity Index compared to other methods [56].

To produce stable and uniform ZnO NPs, various methods have been developed, where such methods are employed to produce ZnO NPs are the reaction of metabolic zinc with alcohol, hydrothermal technique, precipitation method, vapor transport synthesis, and green synthesis. The uptake, accumulation, and translocation of NPs depend on the charge, size, chemical configuration, and stability of the NPs and plant species used [57].

Biosynthesis of gold NPs was conducted at room temperature with Cassia auriculata L. leaf extract. Characterization techniques such as UV-Vis spectroscopy, X-Ray diffraction, and TEM have been used to characterize the extracted bio-nanogold particles. Pennisetum glaucum (L.) R.Br. is considered an essential plant for biofuel and food production. The effect of the application of bio-nanogold on the plant Pennisetum glaucum (L.) R.Br. was studied [58]. There was another study which focused on green synthesis of magnesium hydroxide NPs, along with its efficacy in germination seeds. Both in vitro and in vivo promotion of plant growth were observed in Zea Mays L. at various concentrations and the improved seed germination of 100% when the concentration of NPs is 50 ppm [59].

3.2 Prerequisite nanomaterial properties

The small size of nanomaterials enables direct nutrient and pesticide delivery to the seed, ultimately augmenting the efficiency, minimizing negative environmental impact, and enhancing plant growth. This targeted delivery to the plant allows for quicker and more precise intake of nutrients, water, and develops a resistance to diseases. In becoming more sustainable through various climatic changes, nanomaterials present such qualities vital for a plant’s survival [60,61]. In any case, the reaction of metal NPs on plants focuses on the phase of development (Table 1).

3.3 Nanomaterial types

The benefits of supplementing NPs in agriculture can be analyzed by the penetration and transportation of NPs in plants. NPs, due to their unique properties, help in the growth of plants. They are more biologically active than their bulk counterparts due to the extraordinary surface region per mass unit of NPs [63,64]. However, the impact of NPs relies upon the properties of both plants and NPs. Such nutrients may either be classified biotic or abiotic. The biotic NPs are prepared naturally and are biodegradable, whereas the abiotic NPs are prepared from inorganic ones, which are not safe [65]. Nanomaterials have been found to have beneficial outcomes in sustainable farming yield production, and numerous investigations revealed their positive effect on different crops (Table 2).

4.1 Nanopriming

Nanopriming is a technique commonly used to enhance germination and growth process by priming the seeds with nanopriming solutions. These solutions are made of NPs, often silver, copper, silicon, or zinc, and are applied to the seed surface prior to planting. This solution interacts with the seed by penetrating through the seed coat, enabling many cellular processes and stimulating biochemical and physiological changes in the seed, enabling seedling vigor and better germination rates. The NPs that pass through the seed coat interact with the seed can facilitate delivery of nutrients and growth hormones and enhance antimicrobial properties or also enhance. This will allow the seed to continue to grow under harsh environments, including drought, salinity, or temperature stress. Furthermore, this direct delivery ensures the seed receives essential materials for its growth and development, as well as protecting the seeds from infections and improving the crop yield [75].

4.2 Nano-fertilizers

Nano-fertilizers are combinations of various fertilizers containing the most suitable nanomaterials with desired properties to improve the plant’s ability to store and absorb nutrients, thus enhancing both the plant’s growth and yield. This method allows for a direct transport of nutrients to the seed. Nano-fertilizers often are made of nutrient-rich particles that have a high surface area and enhanced chemical reactivity, promoting efficient absorption and storage of nutrients within the plant. In storing such minerals, as the seed germinates and grows, the NPs will slowly release these nutrients to enhance it. Nano-fertilizers are efficient in direct transport of nutrients to the seed and roots, which maximizes nutrient usage and reduces nutrient runoff or over-fertilization [85].

4.3 Nano-pesticides

Nano-pesticides are made of active chemicals like herbicides, fungicides, or insecticides that are embedded in NPs to protect the plant from pests and diseases, as well as ensuring that these active chemicals are released at a controlled rate. Nano-pesticides penetrate through the seed coat and interact with the seed during germination, to protect it from various diseases. Nano-pesticides are effective through their controlled release of pesticides delivered to the plant. This allows for a targeted delivery and a minimized resistivity to pests. Nano-pesticides can be applied in different ways [86,87]:

1. Foliar application: Applied directly on the plants surface where they penetrate plant tissues.

2. Soil application: Applied to the soil, allowing absorption through the plant’s roots.

3. Seed treatment: Applied as a coating to the seed, for early-stage protection of the plant.

4.4 Nano-coating

Nano-coating covers the seed and coats them with a single thin layer of specified nanomaterials in order to protect them from harsh conditions presented by the external environment, ultimately improving the plant’s resilience. Similar to nano-priming, nano-coating involves the use of a solution made of nanomaterials that is applied onto the seed. However, in nano-coating, common nanoparticle solutions contain polymers, metals, metal oxides, or carbon-based materials. Through a variety of coating techniques, such as dipping, spraying, or vacuum-assisted methods, the thin coating is placed on the seed and enhances the plant’s overall health. This coating allows for improved water absorption, efficient nutrient delivery, enhanced resistance to diseases, and an increased tolerance to stresses [88].

4.5 Nano-sensors

Nano-sensors are implemented into agricultural practices to observe and analyze environmental conditions immediately. Nano-sensors are used to effectively monitor a plant’s health and observe specific qualities, while providing real-time information. This method allows us to track the plants’ amount of moisture, temperature, nutrient levels, pH levels, detection of pathogens, and environmental stressors. This allows for data collection for farmers and agronomists in order to properly assess the quality and growth of the plant. This will help improve the crop yield. Nano-sensors are made of materials typically ranging from 1 to 100 nm, such as nanowires, NPs, and nanoplates [89]. This detection method can involve different mechanisms including:

1. Optical detection: Observed changes in visual properties (absorption and scattering).

2. Electrochemical detection: Chemical reactions causing current or voltage.

3. Mechanical detection: Observed changes in mechanical properties (mass and deformation) [71].

4.6 Nanoparticle-mediated gene delivery

Nanoparticle-mediated gene delivery is an advanced method used to create plants with desirable traits. During this process, NPs are chosen and used to supply desired genes or genetic materials to the target seeds. These ideal characteristics may include improved yield, enhanced resistance to diseases and pests, or improved stress tolerance and nutrient content [90]. This method works by utilizing the chosen NPs as carriers delivering specific DNA into the seeds’ cells. NP-mediated gene delivery offers several advantages, including precision of genes, efficiency of delivery, and versatility of choosing the NP transporters. This mechanism involves a variety of steps, which includes

1. NP synthesis: Creation of nanoparticles suitable for gene delivery.

2. DNA encapsulation: Encasing genetic material within the nanoparticles.

3. Seed treatment: Applying the nanoparticles to seeds.

4. Cellular uptake: Nanoparticles delivering the DNA into seed cells.

5. Expression of the transgene: Activation and expression of the introduced gene.

4.7 Nanoclay-based soil amendments

Nanoclay-based soil amendments are not directly applied to seed itself but aim to improve the seed’s environment to create optimal conditions for the plant to grow. This method uses nanoclay particles that are placed in soil to improve the seed’s surroundings. The chosen nanoclay is often taken from natural materials such as montmorillonite, kaolinite, or bentonite, as they carry specific physical and chemical properties. Nanoclays have high surface area, are rich in nutrients, and have antimicrobial properties. All these properties improve water retention, nutrient availability, enhanced soil structure, and prevention against diseases [91].

4.8 Nano-enabled delivery systems

Nano-enabled delivery systems are advanced methods that provide an efficient delivery mechanism using nanomaterials to improve the plant’s growth process, similar to the discussions in 4.6 but in a generalized utilization. These substances vary from delivering specific nutrients, pesticides, growth regulators, or certain genetic material. These carriers enhance germination, stress resistance, and the overall performance of the plant. Nano-enabled delivery may include nutrient delivery, pesticide delivery, gene delivery, and growth regulatory delivery. All of these methods allow for a precise delivery of chosen materials to the seed in order to promote the health and growth of the plant [83].

4.9 Electrospinning

Electrospinning is a versatile and effective process for producing continuous polymer strands with diameters ranging from a few nanometers to several micrometers. This method is utilized to make polymeric nano-microfibers and nano-microparticles separately for various applications [92,93]. Electrospinning offers several advantages, such as improved bioavailability of pesticides, enhanced specificity, improved controlled release and ease and safety in handling [92]. Roughly 90% of the seed harvests are for human and animal food consumption, which makes seed health a crucial factor. The inoculums present in the seeds can cause outbtreaks in the field and spread to the unaffected areas, introducing new microbes. Studies have shown that seeds treated with fungicides using electrospinning exhibit higher germination rates compared to untreated seeds. The fungicide effectively inhibits fungal growth on seeds, reducing fungal activity and protecting seed quality from harmful parasites [92,94].

8.1 Additively manufactured seed coatings

Implementing nanomaterials into additively manufactured seed coating will enhance the overall productivity of agriculture due to enhanced seed germination rates and early plant growth. This method will also allow for a controlled distribution of nutrients throughout all weather conditions. Further research of additively manufactured seed coatings will lead to optimization of the coatings for greater efficiency in the delivery of required nutrients. This will maximize the crop yield while simultaneously minimizing the amount of materials used [134,135].

8.2 Nano-sensors for soil property detections

Using nanosensors in agriculture allows for great precision in agriculture by providing an accurate real-time monitoring of the soil properties, including moisture levels, content of nutrients, and pH levels. Implementing such devices will let researchers or users analyze the data and properly adjust levels to optimal ones. Future research works can focus on improving the sensitivity and reliability of nano-sensor devices in order to provide necessary data. Furthermore, such devices can be used in a variety of farming industries in order to streamline operations related to agriculture and encourage sustainable practices [71].

8.3 Anti-bacterial protection and prospective nanomaterials

Through further research, nanomaterials can provide strong anti-bacterial protection for plants. There is a scope for reducing reliable on standard pesticides, and focusing on performance of NPs as a multipurpose coating while also being sustainable itself. Currently, researchers are focused on NPs with innate antimicrobial properties, such as AgNPs [136]. In the future, it would be beneficial to continue conducting research on a variety of nanomaterials, like nanocarbons (carbon nanotubes, graphene).

8.4 Controlled delivery of nutrients

An important aspect of enhancing plant strength and resilience is finding an efficient and environmentally safe method to control the delivery of nutrients to the plant. It is crucial that the delivery mechanisms being studied or implemented is precise and exact, allowing for accurate and proper growth of the plant. Additionally, alongside targeted delivery, a more meticulous study of specific nanomaterials is needed to evaluate their impact on nutrient uptake and reduction of nutrient wastage. Delivery mechanism may be influenced due to the material combinations used in NPs and their respective properties. This necessitates study of various material combinations and their delivery mechanisms specifically in the context of seed regeneration.