锰对植物毒害及植物耐锰机理研究进展

含锰矿渣的排放造成了严重的土壤锰污染。揭示锰毒害和植物的耐锰机制对于污染土壤治理具有重要意义。研究表明, 高浓度的Mn²⁺能够抑制根系Ca²⁺、Fe²⁺和Mg²⁺等元素的吸收及活性, 引起氧化性胁迫导致氧化损伤, 使叶绿素和Rubisco含量下降、叶绿体超微结构破坏和光合速率降低。而锰超累积植物则具有多种解毒或耐性机制, 如区域化、有机酸螯合、外排作用、抗氧化作用和离子交互作用等。根系主要通过有机酸的螯合作用促进植物对Mn²⁺的转运解毒, 同时能够将过量的Mn²⁺区域化在根细胞壁中; 叶片可通过酚类物质或有机酸螯合Mn²⁺, 并将其区域化在叶片表皮细胞和叶肉细胞的液泡中(或通过表皮毛将Mn²⁺排出体外)。其中, 金属转运蛋白在植物对Mn²⁺的吸收、转运、累积和解毒过程中发挥着重要作用。

Mechanisms of manganese toxicity and manganese tolerance in plants

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.