Mechanisms of manganese toxicity and manganese tolerance in plants
锰对植物毒害及植物耐锰机理研究进展

The concentration of available Mn in soil is increasing with increased mining. Excess Mn2+ inhibits the uptake and activity of Ca2+, Fe2+ and Mg2+ and induces oxidative stress, which leads to decreased chlorophyll and rubisco contents, damaged chloroplast ultrastructure, reduced photosynthetic rate, and even death. However, a number of plant species have evolved on metalliferous soils and can tolerate high levels of Mn2+ in the soil and, more importantly, in the plant shoot. Thus, they are adapted to extreme soil metal environments. Investigating the mechanisms of Mn toxicity and detoxification can help develop plants suited for remediation of metal-contaminated soils via phytoremediation. Plants have developed various mechanisms, including compartmentalization, chelation, avoidance of uptake and exclusion, antioxidation, and ion interaction, to overcome Mn toxicity. The detoxification mechanisms in different organs are not the same. In roots, the exudation of organic acid mainly contributes to Mn detoxification (both internally and externally), uptake and transport. The storage of Mn in the root cell walls may keep the ion sequestered from the root cytoplasm. In leaves, the Mn preferentially accumulated in leaf epidermal cells may be associated with avoidance of damage to photosynthesis, because epidermal cells (except for guard cells) lack chloroplasts. At the cellular level, Mn ion or the Mn-chelate complex is predominantly sequestered in the vacuole of mesophyll cells and cell walls by compartmentalization. Furthermore, the trichomes of the epidermis may excrete Mn-chelating molecules. The conversion of Mn2+ to a metabolically inactive compound by organic acid or phenolic compounds, such as the Mn-oxalate complex, is one of the important detoxification mechanisms. The metal transporters involved in removing Mn from the cytosol or moving it to the vacuolar membrane, where Mn can be sequestered into a large and metabolically relatively inert intracellular compartment, play important roles in the Mn uptake, transportation and accumulation at whole plant level. Further detailed studies are needed to elucidate the molecular mechanism of Mn tolerance in plants.