Recent advances in metabolomics for studying heavy metal stress in plants
Introduction
Heavy metal pollution in ecosystems has emerged as a global concern. Heavy metals (HMs) apply to groups of metals with atomic densities 5-fold higher than that of water (or greater than 4 g·cm−3) [1]. Due to intensified human activities, geochemical rock weathering and other environmental factors (volcanic eruptions, acid rain, and continental dust), the contents of HMs including cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), mercury (Hg), aluminum (Al), nickel (Ni), and zinc (Zn) in the soil have risen sharply [2,3]. Heavy metals are difficult to eliminate after they enter the environment in excessive amounts, especially after entering the soil, and they have a tendency to become organic in organisms.
Plants are generally highly sensitive to toxic metals, and a small amount of toxic metal stress can induce different physiological symptoms in plants. Excessive HMs can inhibit seed germination and seedling growth, damage the antioxidant enzyme and membrane systems, induce chromosomal aberrations, and cause plant death in severe cases (Fig. 1). Among HMs, some are essential trace elements necessary for the growth and development of plants (such as Cu, Zn and Fe). However, when the concentrations of these HMs in the ground exceed a certain level, they become more toxic to the cells and have a negative impact on the physiology, morphology and biochemistry of plants (Fig. 2) [[4], [5], [6]]. Some Heavy metals are nonessential for the growth of plants, such as Cr, Hg, Cd and Pb. Even lower concentrations will decrease product safety and quality, and higher concentrations have toxic effects on plants [7].
As plants are primary producers in the food chain, excessive HMs accumulate in their leaves, stems, roots and fruits, which not only seriously interferes with plant growth and development, but also endangers animals and humans via the food chain [8,9]. Therefore, research on the mechanism of crop heavy metal damage and resistance is very important. There are more than 200,000 kinds of metabolites in plants. Metabolic pathways such as organic acid metabolism, amino acid metabolism and glycolytic metabolism were altered under heavy metal stress [10], which is presented in Table 1, Table 2. Plant metabolomics is the science and technology for qualitatively and quantitatively analyzing the metabolic components of plants before and after encountering external disturbances or stimuli and studying plant metabolic networks and related gene functions [11,12]. Metabolomics can sensitively reflect subtle changes in the metabolism of organisms under adverse stress. Research on heavy metal injury and resistance mechanisms in plants is useful for breeding highly resistant plants and advancing phytoremediation technologies. This review discusses recent studies on the metabolomics analysis of HMs in crops, including their uptake, accumulation in specific regions, detoxification and phytoremediation.
Section snippets
Technologies applied in metabolomics studies on heavy metal stress
Metabolomics can sensitively reflect subtle metabolic changes in organisms under adverse stress and is used to study the abnormalities of all metabolic pathways in the organism by observing the changes in metabolites upon stress or disturbance. Metabolomics can more truly reflect the physiological and pathological conditions of the organism's system than other techniques. The production and metabolism of small molecules are the final results of a series of events. They can more sensitively and
Metabolomics for plant heavy metal stress response (nonessential elements for plants)
Heavy metals damage the membrane system and antioxidant system in plants through 4 pathways (Fig. 1): (1) by altering the plasma membrane permeability, (2) by binding proteins and altering their activity, (3) by binding to ADP/ATP reactive and phosphate groups, and (4) by interfering with ion balance to disrupt the membrane and antioxidant systems in plants [7,23]. These phenomena lead to plant stunting, leaf chlorosis, loss of yield and quality, and in severe cases, death (Fig. 2). Plant
Metabolomics for plant heavy metal stress response (essential elements for plants)
Essential heavy metal elements (Cu, Zn, Fe) in plants play biochemical and physiological roles. Their two main functions are involved in redox reactions and the synthesis of several enzymes.
Heavy metal stress detoxification
The main effect of hazardous metals on plants is their interactions with the functional groups of intracellular molecules, especially polynucleotides and proteins [85]. Various plant genes and metabolites are regulated upon subjection to heavy metal stress, including their detoxification mechanisms, stomatal behavior antioxidant systems, pathogen defense mechanisms and programmed cell death mechanisms [86]. As shown in Fig. 1, the first to be affected by heavy metal contamination in the soil
Phytoremediation
With the development of industrialization, environmental pollution is becoming an increasingly serious problem, and heavy metal accumulation in the soil is increasing, thereby causing a global biological problem of heavy metal poisoning. Global methods for combatting heavy metal pollution must be considered in the 21st century. Both the physical (soil replacement and thermal desorption processes) and chemical (immobilization techniques and soil washing) methods currently used to reduce soil
Conclusions and future perspective
Metabolomics techniques have been extensively used in the area of plant science research. Since small changes in gene and protein expression are amplified at the metabolic level and the number of metabolites is much smaller than that of genes and proteins, metabolomics can provide a more intuitive understanding of the mechanisms by which plants cope with stress. Heavy metal stress is abiotic stress in plants that causes stunted growth, affects crop yield and quality, and can result in plant
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the support of this program by the National Key Research and Development Program of China (2017YFC1600805), the Zhejiang Province Welfare Technology Applied Research Project (LGN20C130006) and China agriculture research system program of China (CARS-30-4-01).
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