“Heavy metals” is a collective term widely applied for the group of metals and metalloids
with an atomic density above 4 g/cm3 [1]. Non-essential toxic plant heavy metals include
arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), mercury (Hg), nickel
(Ni), and vanadium (V); whereas others are essential, such as copper (Cu), iron (Fe),
manganese (Mn), and zinc (Zn). Heavy metals cause harmful effects in plants, animals,
and humans as a result of long-term or acute exposure. Toxicity from heavy metals
is increasing due to the extensive release from industrial, agricultural, chemical,
domestic, and technological sources, which in turn contaminate the water, soil, and
air. Natural phenomena, such as volcanic eruptions and sea movements, also contribute
to the natural cyclization of metals on the earth, and human activities often alter
the rate of release and transport by increasing emissions by a few orders of magnitude.
Heavy metals penetrate the human body through water, food, and air. Inside an organism,
they bind to cellular structures, thereby damaging the performance of essential biological
functions. Metals, for example, easily bind to the sulfhydryl groups of several enzymes
that control the speed of metabolic reactions: the “new” metal-enzyme complex leads
to the loss of the catalytic activity of the enzyme. The level of toxicity from heavy
metals depends on several factors, including time of exposure, dose, and the health
status of the people exposed.
The European Environment Agency (EEA) reported that of the 1000 industrial plants
that released heavy metals into the air in 2016, eighteen accounted for more than
half of the total pollution, suggesting a great responsibility on the part of a few
large companies (Figure 1) [2].
An additional issue is the biomagnification (or bioaccumulation) caused by the very
slow rate of elimination of heavy metals from an organism. Bioaccumulation, in ecology
and biology, is the process whereby the accumulation of toxic substances in living
beings increases in concentration following a rise in the trophic level: the higher
the trophic level, the stronger the concentration of heavy metals. Biomagnification
is also expressed as the concentration increase of a pollutant in a biological organism
over time.
To limit the risks for humans and the environment, many countries have legislated
limits for each heavy metal. Specific limits have been defined in drinking, waste,
and surface waters (lakes, rivers, seas). There are also limits in foods and animal
feed, because heavy metals can easily enter the food chain through plants (or algae)
and are subsequently bioaccumulated into the higher trophic levels. The risk for human
health is due to directly eating edible plant tissues, or indirectly through eating
animals that have in turn fed on herbivores or directly on edible plant tissues. Understanding
the mechanisms for regulating the storage and distribution of heavy metals in plants
is the basis for improving the safety of the food chain.
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.