Nano-Priming against Abiotic Stress: Comparison
Please note this is a comparison between Version 1 by Abdul Salam and Version 3 by Dean Liu.

Agriculture is directly linked to human life, providing food for survival and health. It is threatened by a number of challenges, such as climate change, resource depletion, and abiotic stresses, including heavy metals (HMs), salinity, drought, etc. Various strategies have been employed to palliate the phytotoxic effects of these stressors from the soil–plant system. Nanotechnological approaches have emerged as a promising tool for increasing crop productivity and promoting sustainable agriculture. 

  • seed priming
  • nanoparticles
  • abiotic stress
  • germination

1. Seed Nano-Priming

Seed nano-priming is one of the effective methods that alter the metabolism of seeds along with their signaling pathways, influencing germination, establishment, and plant lifecycle. Several studies have demonstrated that seed nano-priming has a variety of advantages, including enhanced plant growth and development, and higher nutritional quality. Nano-priming can regulate biochemical processes while maintaining the balance among growth hormones of plants and reactive oxygen molecules [1][36]. It is used to boost plant growth and metabolism, regulate physiology under abiotic stress, and improve germination synchronization. It also increases crop resistance to biotic or abiotic stress environments, which assists in minimizing the use of pesticides and fertilizers [2][37]. According to the latest research, seed nano-priming can stimulate various genes during germination, particularly those attributed to plant stress tolerance [3][4][5][38,39,40]. The application of nanoparticles via seed priming is an innovative area of study, and the preliminary results have been promising [6][7][8][41,42,43]. It can also be utilized for seed protection since many NPs have antimicrobial properties via antimicrobial compounds [9][44]. Furthermore, nano-priming can potentially target the bio-fortification of seeds to promote food production and quality [10][45]. After priming, nanoparticles find their way to the seed tissues and remain there. Images of confocal microscopic observation show the localization of NPs in seed tissues after 24 h priming.

2. Priming-Induced Molecular Responses against Abiotic Stresses

Nanoparticles can interact with plant cells and settle in different cell compartments. The seed nano-priming alters biochemical pathways and gene expression profiles during or after seed germination [11][125]. The first stage of seed germination is the imbibition phase, in which the seed takes up water. Seed priming with NPs at this stage can cause the activation of seed-located water channel genes, i.e., aquaporin (AQP) genes that enhance the water uptake capacity of the seeds [4][12][39,126]. When seeds of different crops such as soybean, corn, and barley are sprayed with MWCNTs, the expression of AQP genes is induced [12][126]. When rice seeds (Oryza sativa L. cv. KDML 105) were primed with Ag NPs, overexpression of AQP genes was observed [4][39]. AQP genes enable water transport across biological membranes and aid in transferring ROS (such as H2O2), nutrients, and gases (NH3 and CO2), eventually resulting in enhanced germination rates and subsequent growth. An et al. [3][38] analyzed the transcriptome profile of CeO2 NPs-primed cotton seeds coated with polyacrylic acid under saline and non-saline conditions. Under no salinity stress, 7799 variable genes were expressed in the seeds primed with NPs compared to the control. While under salt stress, ten ion homeostasis regulating genes and 13 ROS pathway genes were expressed in seeds, resulting in salt stress tolerance and improved growth. Ye et al. [5][40] observed that MnSOD (Mn superoxide dismutase) was upregulated in primed seeds which enhanced the SOD enzyme levels to avoid the phytotoxic effects caused by the ROS. Primed seeds were also better resistant to biotic stresses compared to controlled seeds. The expression of genes for polyphenol oxidase, peroxidase, and phenylalanine ammonia-lyase was upregulated in seed primed with chitosan NPs which impart them resistance against oomycete Schlerospora graminicola i.e., cause of downy mildew disease [13][127]. Plaksenkova et al. [14][128] demonstrated the over-expression of miRNA-156 and miRNA-159 in the barley seeds primed with ZnO NPs. These microRNAs impart resistance to plants against abiotic stress factors.
Several studies have been performed, but still, extensive research is required to better understand the molecular mechanisms taking place after seed priming with NPs and the expression of resistance genes. Variable NPs with variable coatings are used for priming purposes, and each produces a different response at the molecular level. Moreover, NPs can serve as co-factors or signals that can improve the regulation of gene transcription related to abiotic stress responses [15][94].

3. Effects on Photosynthesis under Abiotic Stress

Photosynthesis is the process of conversion of sunlight energy into organic compounds taking place in higher plants, some microorganisms, and algae [16][129]. Nano-priming of the seeds has found its way into the plant’s photosynthesis mechanisms. Seed priming with ZnO NPs has improved photosystem and gas-exchange attributes resulting in increased photosynthetic activity [17][46]. The improved photosynthetic efficiency is attributed to the protective role of ZnO NPs against oxidative damage induced by abiotic stress. Lower concentrations (0.01%) of aluminum oxide (Al2O3) NPs on Hibiscus sabdarifa L. cultivars significantly enhanced the biochemical functions (proline, proteins, soluble sugars, chlorophyll a and b, and carotenoid levels), physiological properties, growth characteristics (fresh and dry mass, root and shoot length, and dry mass), and the activity of various antioxidant enzymes (ascorbate peroxidase (APX), CAT, peroxidase (POD), and SOD) [18][130]. The increase in photosynthetic rate could be attributed to the stimulation of water splitting in the electron transport system, as Pradhan et al. [19][131] observed in the case of Mn NPs. Rubisco (Ribulose-1, 5-bisphosphate carboxylase/oxygenase) is the key enzyme in photosynthesis that integrates CO2 into biological compounds. Rubisco activity is induced by TiO2-primed chickpeas [20][132]. In Mentha piperita, TiO2 (200 mg L−1) enhanced the carotenoid and chlorophyll a and b content [21][133]. Similarly, Yang et al. [22][134] observed that TiO2 priming raised Rubisco activity and enhanced photosynthesis by water splitting and oxygen evolution, normalizing the provision of light energy from photosystems (PS Ⅱ and PS Ⅰ) and light absorption in the chloroplast.
Iron plays its part in the electron transport system of photosynthesis and respiration, helping photosynthesis, reproduction, and initial seed germination [23][135]. Rui et al. [24][136] reported that the need for natural sources of iron in Arachis hypogaea could be replaced by iron oxide (Fe3O4) priming. This enhanced the Fe level, chlorophyll content, root and shoot length, and activity of phytohormones (abscisic acid and gibberellic acid) and antioxidant enzymes. The growth and photosynthetic rate of rice seedlings could be enhanced by priming with low zero-valent iron NPs (nZVI). When physiological and biochemical changes in nZVI-primed Oryza sativa were observed, there was a significant increase in expression of OsGAMYB and OsGA3Ox2 (to mediate mobilization of seed storage food reserves efficiently and control the activity of hydrolases) [25][137]. The photosynthetic pigments content was enhanced by applying Al2O3 and TiO2 NPs in wheat leaves [26][138]. Moreover, in soybean, Rubisco activity [27][139], stomatal conductance, and transpiration rate were promoted by CeO2 NPs [28][140]. Gold NPs stimulated oxygen evolution and the electron transport chain in mung bean leaves [29][141].
It was reported by Abdel-Latef et al. [30][142] that when ZnO NPs were used as a priming agent for lupin seeds, they elevated photosynthetic pigments and growth parameters (fresh and dry weight, root and shoot length) by alleviating salt-stress-induced changes. Hussain et al. [7][42] reported that under cadmium (Cd) stress, SiO2 NPs–primed seeds increased the chlorophyll a and b content, carotenoid content, photosynthetic rate, plant biomass, and reduced Cd uptake, antioxidant enzyme activity, and ROS production. Seed germination and plant growth were reported to be elevated when sorghum seeds were primed with Fe3O2 NPs [31][143]. Biomass and photosynthetic pigments were increased when treated with 500 mg L−1 concentration of Fe3O2 NPs, while leaf water content was enhanced by 100 and 500 mg L−1 concentrations. The enhanced photosynthetic parameters improved biomass production and maintained biochemical balance, alleviating the drought stress from the wheat seedlings (cultivar HI 1544) when they were primed with 15 mg L−1 of SiO2 NPs [32][83]. Similarly, Abou-Zeid et al. [33][86] observed an increase in root and shoot growth and improved photosynthesis and ultra-structure of leaves when wheat cultivars were primed with ZnO NPs. Silicon dioxide NPs have also been reported to enhance the photosynthetic rate of plants by inducing the synthesis of photosynthetic pigments [34][144]. However, further investigation is needed to explore the molecular mechanisms underlying NPs’ improved photosynthesis and related attributes.
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