Nanoforms of essential metals: from hormetic phytoeffects to agricultural potential

Abstract

Vital plant functions require at least six metals (copper, iron, molybdenum, manganese, zinc, and nickel), which function as enzyme cofactors or inducers. In recent decades, rapidly evolving nanotechnology has created nanoforms of essential metals and their compounds (e.g. nZnO, nFe2O3) with a number of favourable properties over the bulk materials. The effects of nanometals on plants are concentration-dependent (hormesis) but also depend on the properties of the nanometals, the plant species, and the treatment conditions. Here, we review studies examining plant responses to essential nanometal treatments using a (multi)omics approach and emphasize the importance of gaining a holistic view of the diverse effects. Furthermore, we discuss the beneficial effects of essential nanometals on plants, which provide the basis for their application in crop production as, for example, nanopriming or nanostimulator agents, or nanofertilizers. As lower environmental impact and increased yield can be achieved by the application of essential nanometals, they support sustainable agriculture. Recent studies have actively examined the utilization of green-synthesized metal nanoparticles, which perfectly fit into the environmentally friendly trend of future agriculture. Further knowledge is required before essential nanometals can be safely applied in agriculture, but it is a promising direction that is timely to investigate.

Keywords: Hormesis; nanofertilization; nanometals; nanopriming; nutrient deficiency; omics.

Fig. 1.

Unique properties of nanometals compared with the bulk form, and the phytoeffects and application of nanometals in plant cultivation. The nanoforms of essential metals possess greater surface area and hardness, specific optical, magnetic, and electrical properties, and show antimicrobial effects. Essential nanometals promote growth/yield and induce the synthesis of phytochemicals and protection against abiotic and biotic stressors. The positive effects of essential nanometals on the physiology of plants can be utilized during agricultural approaches such as nanopriming, nanostimulation, nanofertilization, and nanopesticide application. For further details refer to the text.

Fig. 2.

Factors determining plant responses to nanometals and the different levels at which plant responses can be examined. Among the characteristics of nanometals, their type, dosage, size, agglomeration, crystal structure, and surface charge are the most important determining factors. The response to essential nanometals also depends on the species, metal tolerance capacity, and developmental stage of the plant. Additionally, the method of application (e.g. foliar spray, irrigation, hydroponics) and the period of exposure to the nanometal also influence its effect. Plants show multi-level responses to nanometal exposure. Genomics, transcriptomics, proteomics, metabolomics, and phenomics studies provide a holistic view about the complex effects of essential nanometals on plants.

Fig. 3.

Hormetic effect of increasing doses of essential nanometals on plants. At lower concentrations, nanometals promote plant biomass production and physiological processes, whereas higher nanometal concentrations exert inhibitory effects.