3. Nanoparticle–Plant Interaction: Transcriptomic and Proteomic Debate
3.1. Genomic Insight into Nanoparticle–Plant Interaction
Since nanotechnology has been recognized as the most cutting-edge, rapidly developing, and labor-saving technology by the scientific community as of late, it now has applications in almost every field of science. The toxicity caused by the constant deposition of NPs in the environment is not ignorable, especially in terms of plant performance. NPs interaction at genomic level is shown in
Figure 2. A bulk of the literature demonstrates the morphological and physiological phytotoxic effects of NPs, but very little demonstrates the gene-level toxicity of NPs. Researchers reported that CuO NPs were toxic to some agricultural crops like
Loliumrigidum,
Loliumperenne and
Raphanussativus,
Lolium rigidum,
Lolium perenne, and
Raphanus sativus, and after prolonged NP exposure, DNA was destroyed. The deposition of compounds that have been altered by oxidation led to the formation of mutagenic DNA lesions, which in turn caused rapid disruptions in plant growth
[66]. Similarly, ZnO NP toxicity in wheat seedlings was examined and it was discovered that nitric oxide (NO) mitigates this toxicity in wheat effectively
[67]. Additionally, figuring out the NPs’ genotoxic endpoints is a concern because they have been linked to metal’s genotoxic effects in plants
[68], but any generalizations regarding NP-induced phytogenotoxicity must be handled with care.
Figure 2. A brief illustration of genomic insight into plant–-NPs interaction.
3.2. Genes Regulation with Respect to NP-Induced Genotoxicity
At the molecular level, the basis of nanoparticle-induced phytotoxicity in higher plants is still poorly understood. However, transcriptomic analysis, upon NP exposure, has presented information about the up- and downregulation of different genes, which has provided some knowledge regarding NP toxicity at the genetic level in higher plants. For example, it was experimented that when maize root tissue was exposed to single-walled carbon nanotubes (SWCNTs), the genes for RTCS and SLR1 were upregulated and the genes for RTH
3 and RTH
1 were downregulated. Aquaporin also plays a significant role in nanoparticle acquisition. A variety of stress parameters control or regulate many stress-inducible genes in plants
[69]. In another study, it was found that an increase in aquaporin production in tomato seedlings upregulated the water channel and NtLRX1 genes
[70]. Some researchers treated
Eruca sativa with Ag NPs, recorded the proteomic profile of Ag-treated plants, and different gene and protein expressions. Ag NPs effected the number of proteins in the plant cell’s vacuole and endoplasmic reticulum (ER), indicating that these organelles are Ag NPs’ primary targets
[71]. When
Arabidopsis plants were treated with varying concentrations of gold (Au) nanoparticles, similar expression results for micro RNAs (miRs) were observed
[72]. When Au NP exposure was compared to a control, expression of some microRNAs, especially miR164, miR395, miR167, miR408, and miR398, were found to be downregulated. However, this study’s findings are in direct opposition to those of Burklew, Ashlock
[73] regarding tomato plant seedlings in significant ways. This further proves that NPs have a species-specific phytotoxicity. Moreover, NPs alter gene expression even at very low concentrations, suggesting that the impact of NP-caused toxicity at cellular level can be gauged using gene expression analysis
[74][75]. In this way, gene expression analysis assists in the identification and development of transgenic plants that are sensitive or tolerant to certain stresses
[76][77]. As a result, genomics analysis is profoundly useful for determining the mechanism of NP toxicity and relating any physiological or morphological changes at the genetic level in plants. The fundamental processes of electron transport chain signaling are disrupted by the NP-induced phytotoxicity, which ultimately affects the organism’s cell cycle.
3.3. Transcriptomic Insight into Nanoparticle–Plant Interaction
Numerous researchers have reported evidence of nanoparticle-induced alterations in plant gene expression. It is averred that exposure to ZnO and TiO
2 altered the gene expression found in
Arabidopsis thaliana roots. They concluded that ZnO NPs are the most toxic NPs among the three abovementioned NPs because they induced downregulation and upregulation of 826 and 660 genes for stress regulation, respectively, although the least toxic TiO
2 NPs only had an impact on 74 and 80 genes, respectively
[78]. Despite the researchers’ clear demonstration of up- and downregulation of genes caused by the phytotoxicity of nanoparticles, comprehensive details have not yet been portrayed. In a recently conducted study, they exposed
A. thaliana to various sizes and types of NPs under abiotic and biotic stresses to observe the early changes in gene expression. As a general response to nanoparticle-induced stress, they observed transcriptional repression and downregulation of gene expression
[79]. In another study, examination of proteomic, ionomic, and transcriptomic shifts have put
Arabidopsis into focus to determine how Au NPs affect growth and development. The expression of the MYB, BHLH, and WRKY gene families, which are involved in homeostasis of some essential metals and Fe, differed according to the transcriptome analysis of
Arabidopsis roots
[80]. It was inferred that Au impacted the normal homeostasis of essential elements like Mn, Fe, and Zn. By observing genetic and physiological responses of plants to Au, it has also been discovered that Au is uptaken by
Arabidopsis and then transported to the shoot. In addition, it has been observed to have an effect on the transcription of a specific group of genes, particularly cation transporters, with 869 genes being upregulated and 851 genes being downregulated. IRT1 and IRT2, iron-regulated transporters, have shown the lowest expression of genes. Downregulation of eleven aquaporin genes was also reported in this study. Metal uptake, particularly that of heavy metals, is thought to involve a number of these genes
[81]. Numerous studies have looked into the different kinetics that NMs use to acquire, translocate, and internalize NMs, as well as the cytotoxicity that is recorded at the level of gene expression. These studies also found that NMs affected plant growth and development, along with a number of other morphological and physiological effects. Although the genetic processes underlying these effects have not yet been discovered, it has been observed that these effects are species-specific. The transcripts of genes involved in vacuolar proton or cation exchange, hormonal responses, antioxidant systems, pathogen-responsive genes, decreases in photosynthesis genes’ transcript formation, aquaporin genes, as well as biogenesis and cell organization genes, were found to be upregulated in NP-mediated toxicity.
3.4. Proteomic Insight into NP–Plant Interaction
Over the past few decades, various multidisciplinary studies on the impacts of Ag NPs on plants have shown that they are toxic at many levels—cellular, morphological, physiological, etc. However, only a small number of studies have examined the detrimental proteomic effects of Ag NP stress on plants. In an effort to carry away the toxicity posed by Ag NPs, a gel-based proteomic study of
Oryza sativa seedlings was recently conducted
[82]. Ag NP-receptive proteins of were reported to be primarily associated with Ca
2+ signaling and regulation, the oxidative stress response pathway, transcription, protein degradation, cell wall synthesis, cell division, and apoptosis. Specific defense-related proteins, i.e., SOD, glutathione S-transferase (GST), and L-ascorbate peroxidase, were found to be overexpressed after exposure to Ag NP toxicity. Resultantly, this accelerated the generation of reactive oxygen species (ROS) under Ag NP treatment. Previous research has suggested that secreted ions or Ag NPs could disturb the cell’s metabolism by binding to Ca
2+/Na
+ATPases and Ca
2+ ion channels—the secondary messenger receptors. In a similar manner, a proteomic analysis of
Eruca sativa roots exposed to AgNO
3 and Ag NP stress revealed that both bulk metal and NPs can alter the structure of proteins associated with redox regulation and ultimately disrupt cellular homeostasis
[71]. However, the fact that the exposure of Ag NPs is sufficient to alter the membrane proteins of vacuoles and endoplasmic reticulum suggests that these NPs primarily target both of these specific organelles. According to these findings, Ag NP phytotoxicity is caused by its unique chemical and physical properties rather than its continuous release to the atmosphere
[71]. The agricultural sector is an important field in terms of the application of gold nanoparticles. After exposing
Arabidopsis thaliana to Au NPs, a proteome analysis revealed the induction of glutathione S-transferases in the shoot. Additionally, researchers have also suggested that auxin, the plant hormone, plays a significant role in shaping Au-induced root system architecture
[80]. In a similar way, Ag NPs at three distinct particle sizes (2, 15, and 50–80 nm) and concentrations (0.2, 2, and 20 ppm) were used in a series of experiments on soybean seedlings
[83]. They demonstrated that, under flood stress, Ag NPs of 15 nm particle size promoted soybean seedling growth rate and germination potential in comparison to larger and smaller Ag NPs. The proteins altered by the exposure of Ag NPs were mostly linked to stress signaling, the detoxification pathway, and cellular metabolism. The concentration of these proteins increased under flood stress, but the proteins rapidly decreased when exposed to Ag NPs. Additionally, treatment with Ag NPs demonstrated a shift in metabolism from fermentative to cellular processes. According to the findings, soybean seedlings subjected to Ag NP treatment under flooding stress experience less oxygen loss as a result of treatment with Ag NPs, which have a particle size of 15 nm and 2 ppm. Additionally, soybean seedlings were found to be damaged at elevated Ag NP concentrations with particle sizes of 15 nm
[83]. A comparative study with Ag, Al
2O
3, and ZnO NPs on two-day-old soybean seedlings subjected to flooding stress was recently conducted
[84]. Surprisingly, treatment with 50 ppm Al
2O
3 NPs resulted in increased vigor and growth in soybeans. The primary functions of these Al
2O
3NP-responsive proteins were lipid metabolism, glycolysis, protein degradation, and protein synthesis. Under treatment with Al
2O
3 NPs, a fivefold increase in the negative NmrA-like transcriptional regulatory protein family was also observed. The proteomic results suggest that the growth of flood-stressed soybean seedlings may be attributed to regulation of energy metabolism and reduced proliferation of root cells. Similar to Ag and Au NPs, various cell systems have been used to investigate other NP toxicities. For instance, it has been demonstrated that CeO
2 NPs inhibit maize growth by hampering stress-responsive protein expressions like CAT, heat shock protein, and APX
[85]. Mediation of NP toxicity has been reported by the decline and destabilization in cell membrane potential, lowering ATP levels in cells, elevating stress-responsive and quorum-sensing proteins, expression of oxidative stress-tolerant proteins, Ca
2+ release and its signaling, transcriptional proteins, as well as downregulating essential proteins related to growth-regulating mechanisms like cell metabolism, photosystem-II functioning, and sulfur assimilation. A brief description of NPs interaction at transcriptomic and proteomic level is shown in
Figure 3.
Figure 3. An illustration of transcriptomic and proteomic analysis of the plant–-NPs interaction. ABI: ABA-insensitive; AOX: alternate oxidase; ADH: alcohol dehydrogenase; AREB: ABA-responsive element-binding proteins; APX: ascorbate peroxidase; ARGOS: auxin-regulated gene involved in organ size; CycB: cyclin B; BiP: binding immunoglobulin proteins; Cys: cysteine synthase; GR: glutathione reductase; ETC: electron transport chain; GlyII: glyoxalase II; HKT1: high-affinity K+ transporter 1; GPX: glutathione peroxidase; HSP: heat shock protein; miR: microRNA; MDHAR: monodehydroascorbate reductase; MMR: mismatch repair gene; OEE2: oxygen-evolving complex 2; MT: metallothionein; PCNA: proliferating cell nuclear antigen; PIN: peptidyl–prolyl cis–trans isomerase NIMA-interacting; PCS1: phytochelatin synthase 1; PIP1: plasma membrane intrinsic protein 1; RbcL: large subunit of Rubisco; PR: pathogenesis-related; RD29A: responsive to desiccation 29A; SAR: systemic acquired resistance; ROS: reactive oxygen species; SCR: scarecrow; SOS1: salt overly sensitive 1; TF: transcription factors; SOD: superoxide dismutase; SHR: short root.