The present review discusses the role of silver (AgNPs) and selenium (SeNPs) nanoparticles at different concentrations in the regulation of plant defence responses to the biotic stressors. Study of the role of the above nanoparticles (NPs) has generated considerable interest because these caused significant changes in the framework of plant growth and their metabolism and play an important role in responses to biotic stress factors. Numerous results of metabolomics studies provide evidence that NPs change the profile of metabolites and their concentrations. NPs were applied as potential tools to improve the growth of plants, plant tolerance to abiotic stresses and food production, but research on the environmental safety of their use in agriculture is still necessary. The response of plants to the application of NPs depends on their concentration, plant species, exposure time and stage of development.

The main goal of this study was to investigate the effect of lead (Pb) at various concentrations, as an abiotic factor, and the cross-talk between Pb and pea aphid (Acyrthosiphon pisum (Harris)) (Hemiptera: Aphididae), as a biotic factor, on the defence responses of pea seedlings (Pisum sativum L. cv. Cysterski). The analysis of growth parameters for pea seedlings demonstrated that Pb at a low concentration, i.e., 0.025–0.0625 mM Pb(NO₃)₂, caused a hormesis effect, i.e., stimulation of seedling growth, whereas Pb at higher concentrations, i.e., 0.01–0.325 mM Pb(NO₃)₂, inhibited growth, which manifested as the inhibition of length and fresh biomass. The differences in the level of the main defence-related phytohormones, such as abscisic acid (ABA), jasmonic acid (JA) and salicylic acid (SA), and indole-3-acetic acid (IAA)—an auxin stimulating plant cell growth—depended on the dose of Pb, aphid infestation and direct contact of the stress factor with the organ. A high accumulation of soluble sugars in the organs of pea seedlings both at sublethal doses and hormetic doses at early experimental time points was observed. At 0 h and 24 h of the experiment, the hormetic doses of Pb significantly stimulated invertase activities, especially in the roots. Moreover, an increase was observed in the pisatin concentration in pea seedlings growing in the presence of different concentrations of Pb and in the case of cross-talk between Pb and A. pisum in relation to the control. Additionally, a significant induction of the expressions of isoflavone synthase (IFS) and 6α-hydroxymaackiain 3-O-methyltransferase (HMM) genes, which participate in the regulation of the pisatin biosynthesis pathway, in pea seedlings growing under the influence of sublethal 0.5 mM Pb(NO₃)₂ and hormetic 0.075 mM Pb(NO₃)₂ doses of Pb was noted. The obtained results showed that the response of P. sativum seedlings depends on the Pb dose applied, direct contact of the stress factor with the organ and the duration of contact.

This study attempted to elucidate the alterations in ultrastructure of root cap cells and some metabolic reactions in yellow lupine (Lupinus luteus L.) root cells induced by silver nanoparticles (AgNPs) during the seed germination stage and seedling development. Also, the role of AgNPs in defense mechanisms against hemibiotrofic patogen Fusarium oxysporum f.sp. lupini was studied. Therefore, ultrastructural analysis of root cap cells of L. luteus L. cvs. Diament and Mister were performed by transmission electron microscopy (TEM). Additionally, intracellular reactive oxygen species (ROS), particularly hydrogen peroxide (H₂O₂) was detected in root cells using confocal microscopy. The obtained research results showed distinct alterations in ultrastructure of root cap cells after pretreated with AgNPs or during cross-talk between AgNPs and F. oxysporum f.sp. lupini. It was surprising that the application of AgNPs caused local and clearly visible thickenings of the cell wall (CW) with a wide electron-dense apoplast space (AS) in the root cap cells. Also, an interesting observation was an increased presence of dictyosomes (D), characterized by a highly developed cisternae system in cells of the AgNPs and AgNPs + F. oxysporum variants in comparison to the control. An increase in the processes of endocytosis and exocytosis was noticed, which is associated with the transport of material enclosed or inserted into membrane vesicles. Besides, F. oxysporum infection induced strong autophagic activity and the occurrence of multivesicular bodies (MVBs) and multilamellar bodies (MLBs). Furthermore, in response to AgNPs or F. oxysporum alone, or in combination with AgNPs and F. oxysporum, increase in green fluorescence emission associated with ROS, especially H₂O₂ in relation to the control.