# Nanotechnology in Plant Metabolite Improvement and in Animal Welfare
## 4 Possible Mechanism of AgNPs in Plant Cells  
Literature addresses the possible mechanisms of growth improvement and biomass accumulation due to AgNP administration. The significant increase in some macronutrient contents and the accumulation of larger amounts of N, Mg, and Fe in the shoots of sugarcane exposed to AgNPs may contribute to the increased shoot number and length [29; 43]. Nitrogen is a constituent of chlorophyll, proteins, nucleic acid, and hormones. The photosynthetic activity leads to the formation of biomass in plants therefore, it is possible that this mechanism helps improve growth in plants exposed to AgNPs. However, information on the mode of AgNPs action on nutrient absorption is lagging behind; but, in any case, it may be speculated that these particles may enhance the plant uptake inflicting damage to the cell wall. Thus, cell wall malfunction may lead to higher uptake of nutrients and water; incidently, the higher penetration of drugs consequent to cell membrane permeabilization has been observed in both plant and animal cells [8]. Another possible mechanism of growth stimulation mediated by AgNPs, can be ascribed to PGR-responsive gene expressions. As a matter of fact, investigations demonstrated that these nanoparticles decreased the expression of ethylene- and ABA-responsive genes, resulting in callus regeneration rate [47]. Under closed container culture conditions, plant development will be highly influenced by self-produced gases, such as ethylene, which possibly regulate senescence. The introduction of AgNPs in olive cells showed positive impacts on cell growth, reducing necrosis and improving growth vigor [69]. The rapid growth effects on plant cultures might be due to the inhibitory effect of silver ions on ethylene production. Silver ions from AgNPs act as ethylene perception inhibitors, by replacing the copper cofactor, leading to the downregulation of genes involved in ethylene production [47; 70]. It has been also reported that the application of AgNPs decreased the _ACS_ transcript level on the AgNPs-elicited culture of _T. undulata_[59]. The expression of _ACS_ had been widely studied to understand growth and stress response, as it catalyzes the conversion of S-adenosylmethionine to 1-aminocyclopropane-1-carboxylic acid, the precursor of ethylene. This low expression of _ACS_ may explain how AgNP-treated culture showed a decreased concentration of ethylene. It was suggested that the action of AgNPs consisted of replacing ethylene in the receptors located at the level of the endoplasmic reticulum thus leading to the saturation of ethylene production. A significant decrease of ethylene content in AgNPs-elicited culture was also reported and another study showed that the particles affect root growth improvement through the upregulation of auxin [60]. Dynamic biosynthesis and signaling of PGRs are also influenced by the application of nanoparticles. Some of them include auxin, abscisic acid (ABA), and ethylene [71]. However, another study also demonstrated that nanoparticles are involved in the determination of plant growth and development [72]. [PERSON] et al. [72] showed that the hormonal pool is affected by the application of nanoparticles. It has been suggested that different plant growth and physiological responses in various plant species might result from different pools of PGRs. A high concentration of nanoparticles suppressed the biosynthesis of cytokinesis and auxins in the shoot apical meristem (SAM). Similar to that, jasmonic acid was also decreased during nanoparticle treatment. Meanwhile, accumulation of abscisic acid, _cis_-zeatin, and salicylic acid was observed in plant roots upon nanoparticle exposure. This indicates that nanoparticle serves as a source of plant stressors [72].