Editorial for Special Issue “Heavy Metals Accumulation, Toxicity, and Detoxification in Plants”

"Heavy metals" is a collective term widely applied for the group of metals and metalloids with an atomic density above 4 g/cm3 [...].

. Environmental pressures of heavy metal releases to air, 2016 [2]. An eco-toxicity approach (USEtox model, https://usetox.org/model) was applied to illustrate spatially the combined environmental pressures on Europe's environment caused by releases of the selected pollutants. This gives information about the location of source of heavy metals and the low or high levels in air as indicated in the upper left corner of the figure.
This special issue, entitled "Heavy Metals Accumulation, Toxicity, and Detoxification in Plants", explores three main issues concerning heavy metals: (a) the accumulation and partitioning of heavy metals in crops and wild plants; (b) the toxicity and molecular behaviors of cells, tissues, and their effects on physiology and plant growth; and (c) detoxification strategies, plant tolerance, and phytoremediation.

Authors
Title Heavy

Metals Type
Małkowski et al. [7] Hormesis  This special issue, entitled "Heavy Metals Accumulation, Toxicity, and Detoxification in Plants", explores three main issues concerning heavy metals: (a) the accumulation and partitioning of heavy metals in crops and wild plants; (b) the toxicity and molecular behaviors of cells, tissues, and their effects on physiology and plant growth; and (c) detoxification strategies, plant tolerance, and phytoremediation.
The issue contains a total of 19 articles (Table 1). There are four reviews covering the following topics: phytoremediation [3], manganese phytotoxicity in plants [4], cadmium effect on plant development [5], the genetic characteristics of Cd accumulation and the research status of genes and quantitative trait loci (QTLs) in rice [6], and fifteen original research articles, mainly regarding the impact of cadmium on plants [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Cadmium is therefore the predominant topic of this special issue, thus confirming the focus of the research community on the negative impacts determined by cadmium or cadmium associated with other heavy metals. Interestingly, we did not receive any manuscripts on other heavy metals such as arsenic, chromium and mercury despite their danger for human health.
The cadmium research articles come from China, Poland, Italy, Canada, Pakistan, and the United States. These studies investigate different molecular mechanisms or approaches, using model plants such as Arabidopsis and tobacco [17,18,20] or hyperaccumulator plant species [9,16,19,21] to unravel their molecular strategies in heavy metal accumulation. Other articles focus on how to prevent cadmium from entering the food chain by investigating edible plants such as Zea mays [7], durum and bread wheat [12,13], or animal feeding plants such as Lolium multiflorum.
The studies reveal some common strategies in terms of the molecular mechanisms involved. Some plants activate the production of small proteins such as glutathione S-transferase (GST) and small heat shock protein (sHSP) [9,11,21] or antioxidants [16]. In order to alleviate heavy metal toxicity, other plants respond by activating a complex metabolism-like auxin pathway [7,8,17]. Plants also produce specific metallothionines and phytosiderophores [10,12] to chelate heavy metals or to activate heavy metals transporters such as heavy metal ATPase (e.g., HMA2 and HMA4) and ATP-binding cassette (ABC) transporters [12,13,18,19,21].
The studies in this special issue highlight considerable genetic variability, suggesting different possibilities for accumulation, translocation, and reducing or controlling heavy metals toxicity in plants.
Heavy metal pollution is still one of the world's great challenges. In the future, the main research objective should be to identify and characterize the genes controlling the uptake and translocation of heavy metals in a plant's above-ground organs in order to produce (i) phytoremediation plants that efficiently move heavy metals in the stem and leaves or (ii) plants dedicated to human nutrition that transport heavy metals only in trace amounts to seeds or fruits.

Conflicts of Interest:
The authors declare no conflict of interest.

QTLs
Quantitative trait loci sHSP small heat shock protein GST glutathione s-transferase HMA heavy metal ATPase ABC ATP-binding cassette