Micronutrients: the Borderline Between their Beneficial Role and Toxicity in Plants

Management of crop nutrition by applying macro and micro-nutrients is important for obtaining high yields and ensuring the quality of consumed products. Traditional approaches to cropping have typically focused on macronutrients or increasing fertilizer doses to increase crop yield and/or improve quality, while ignoring the use of micronutrients. Thus, excessive fertilizer rates are applied, which results in toxicity effects on plants, soils, and microorganisms, as well as in mineral leaching to the ground water, causing severe environmental (eutrophication, nitrates pollution etc.) and health problems. Despite the low dosage requirements, microelements are essential for the growth and development of plants, serving various key functions in metabolism. Recently, intensive research has been conducted on micronutrient use, revealing diverse and complex mechanisms of micronutrient uptake, translocation, physiological use, deficiency and toxicity effects. In addition, these studies have defined strategies to optimize the use of micronutrients in crop production.

Micronutrients (Fe, Zn, Cu, Se, Mn, I, Si, B, Mo) are equally important for plant growth and development as macronutrients and their absence or deficiency may have severe effects on plant growth and crop quality. Micronutrients are key elements in plant tolerance against biotic and abiotic stress, as cofactors of many of the enzymes controlling and ameliorating stress responses, as well as many of those synthesizing and sensing plant hormones. Moreover, they are not only required to optimize beneficial plant-microbe interactions, but they also play an important role in plant innate immunity, either as cofactors of enzymes, or directly as part of redox or nutritional immunity responses to plant pathogens.

Conversely, with excess concentrations of micronutrients in soil, accumulations can reach toxic levels, affecting soil health and performance. Research on soil restoration and phytoremediation from mineral toxicity with resistant plant species is currently of great interest.

Besides their role in plant and soil health, micronutrients are essential in human diets, supplied mostly through dietary sources. Deficiencies of essential micronutrients (mostly Fe, Zn and Cu) is widespread in many areas of the world, including developed countries. Conventional strategies to solve mineral deficiencies in human diet, also known as hidden hunger, are by direct supplementation or through food fortification. Biofortification aims to increase the available concentrations of an element in food crops by means of fertilization and plant breeding.

This research topic aims to gather current research that focuses on the beneficial role of micronutrients, as well as the extent to which their accumulation may be excessive and cause negative effects on plant metabolism or soil health. Contributions to this Research Topic should focus on:

• The beneficial role of micronutrients in plant metabolism, growth and yield;
• The role of micronutrient application in crop quality (nutritional value, chemical composition, bioactive properties);
• The role of micronutrients in secondary metabolism;
• The methods of micronutrient applications (foliar, root dipping, fertigation etc.) as biofortification agents;
• The role of micronutrients as stress alleviating factors;
• Micronutrient and microorganism interactions, and their potential role as protective agents against pathogen infections;
• Soil restoration and phytoremediation from mineral toxicity with resistant plant species.