Plant Physiological Response to Toxic Metals = Plant Physiological Response to Toxic Metals

다국어 초록 (Multilingual Abstract)

Soil contamination with toxic heavy metals is one of the main abiotic stresses limiting crop growth and development. The heavy metals enter into the soil environment through natural process such as weathering of rocks and volcanic eruptions and anthropogenic activities such as industrialization, mining, pesticides and fertilization applications. The heavy metals mainly enters in the plant through the root system and other sources such as atmospheric depositions on the plant surface. Upon nutrient deficiency, plants release root exudates with chelator properties which could bind essential elements such as iron in the rhizosphere and increase its uptake by plants which also provides a gate for non-essential to enter in the plants. Heavy metal toxicity caused a reduction in root and shoot length as well as biomass. The photosynthetic pigments and gas exchange characteristics decreased in plant upon metal exposure. The heavy metal toxicity caused the overproduction of reactive oxygen species (ROS) in different parts of the plants which could led serious morphological, physiological and metabolic abnormalities in plants ranging from leaf chlorosis and to lipid peroxidation and protein degradation. Plants have evolved a variety of mechanisms to reduce the metal toxicity. The first defense strategy is the metal exclusion. Plants could reduce the metal uptake by root excretions and depositing on the root surface. When heavy metals enter the cytoplasm, plants release the cellular metal chelators such as nicotianamine, glutathione and organic acids like citrate, aconitate and malate which could bind metals and as a result free cellular metal ions decreased in plants. Upon metal stress, plants also release metallothioneins, sulfur-containing proteins, which could bind certain metals. Similarly, plants could sequester metal into distinct cellular compartments such as vacuoles. To scavenge ROS, plants have well-developed antioxidant defense systems comprising both enzymatic and non-enzymatic antioxidants. The activities of antioxidant enzymes increased in plants upon metal stress to keep the equilibrium between ROS generation and detoxification and thus, the strong antioxidant defense system is at least partially beneficial in heavy metal tolerance in plants. In addition, plants could enhance metal tolerance by triggering the production of signaling molecules such as abscisic acid, Jasmonic acid, proline or ethylene in different plant parts. In conclusion, heavy metals decreased growth, biomass, photosynthesis and yield of plants and plants could tolerate a certain level of metal stress by different mechanisms including root excretions, metal exclusion, phytochelation synthesis and compartmentalization.