Biostimulants are materials that when applied in small amounts are capable of promoting plant growth. Nanoparticles (NPs) and nanomaterials (NMs) can be considered as biostimulants since, in specific ranges of concentration, generally in small levels, they increase plant growth. Pristine NPs and NMs have a high density of surface charges capable of unspecific interactions with the surface charges of the cell walls and membranes of plant cells. In the same way, functionalized NPs and NMs, and the NPs and NMs with a corona formed after the exposition to natural fluids such as water, soil solution, or the interior of organisms, present a high density of surface charges that interact with specific charged groups in cell surfaces. The magnitude of the interaction will depend on the materials adhered to the corona, but high-density charges located in a small volume cause an intense interaction capable of disturbing the density of surface charges of cell walls and membranes. The electrostatic disturbance can have an impact on the electrical potentials of the outer and inner surfaces, as well as on the transmembrane electrical potential, modifying the activity of the integral proteins of the membranes. The extension of the cellular response can range from biostimulation to cell death and will depend on the concentration, size, and the characteristics of the corona.

Selenium (Se) is an essential element in mammals; however, there is frequently an insufficient intake due to several factors. Different techniques have been used to deal with this problem, such as plant biofortification with Se in its ionic forms and, more recently, at the nanoscale. Additionally, despite the fact that Se is not considered an essential element in plants, it has been shown to stimulate (through still unknown mechanisms) plant metabolism, causing an increase in the synthesis of molecules with reducing power, including enzymes such as glutathione peroxidase, catalase and ascorbate peroxidase as well as non-enzymatic antioxidants such as phenolic compounds, glucosinolates, vitamins and chlorophylls. A positive correlation has also been shown with other essential elements, achieving an increase in tolerance to environmental adversities. This article describes the advances made in the field of the biofortification of horticultural crops with ionic Se and nanoselenium (nSe) from 2009 to 2019. The aspects covered include various concentrations used, the findings made regarding the impact these chemical forms have on plant metabolism, and indications of its participation in the synthesis of primary and secondary metabolites that increase stress tolerance.