锰毒及植物耐性机理研究进展

综述了近些年国内外关于锰毒及植物耐锰机理的研究成果，并指出了存在的问题和发展前景。锰毒是酸性土壤上限制作物产量的重要因子，国内外针对锰毒及植物耐受机制进行了相关研究，但进展较为缓慢。锰对植物的毒害效应体现在不同的细胞组织及生理生化水平上，不同植物耐受锰的机理也存在差异性，但大都集中在有机酸的螯合解毒、内部积累、外部排斥及氧化等方面。某些锰胁迫所诱导的基因也被筛选出来，并且部分生物学功能得以鉴定。此外，锰与其他营养元素间的协同或拮抗作用也得以阐述，伴随锰超富积植物-商陆在中国的发现，对锰毒及植物耐性机理的深入研究和探讨，将会对植物修复技术的开展产生理论和实践意义。

Review of manganese toxicity & the mechanisms of plant tolerance

Heavy metals, persistent contaminants in the environment, have been increased due to human activities such as industry, agriculture and mining. Manganese (Mn), an essential trace element for plants in which it is involved in redox reactions as a cofactor for many enzymes, represents an important factor in environmental contamination. Excess Mn can lead to toxicity conditions in natural and agricultural sites. Manganese toxicity is one of the most severe growth limiting factors in acid soil which account for 21% of the total arable lands in China. The physiological mechanisms of Mn toxicity are still not fully understood. Excess Mn has various phytotoxic effects, including reduction of growth, photosynthesis and chlorophyll content, inhibition of enzyme activities and damage to chloroplasts. Moreover, Mn critical toxicity content varies widely with the plant species, age and soil nutrient balance. Generally, high Mn level has direct cytotoxic effects such as extensive cytoplasmic injures and plasma membrane ruptures in the outer root cap and meristematic cells. Many reports suggested that excessive Mn might cause the induction of oxidative stress. In this case, as a consequence of the uptake of toxic concentrations of Mn, several defense enzymes such as SOD, POD and CAT are induced in the chloroplasts and cytosol as protection against oxidative stress. The physiological, genetic and molecular mechanisms of Mn-tolerance in plants are not yet clear. The degree of tolerance varies widely in different species and environments. Some reports suggested that some low molecular weight organic acids may play an important role in accumulation of Mn in plants whereas some other studies have not found any clear relationship between them. It is generally accepted that a protective mechanism for Mn-tolerance in plants is probably constituted by a compartmentalization process which can segregate excess Mn from the key metabolic reactions. On the other hand, excess Mn seems to cause a more active extracellular POD covalently bound to the wall, which is considered to be involved in lignification processes. Some manganese-trafficking genes have been identified which are involved in sequestering free manganese ions in the cytoplasm. The more significant part of Mn-toxicity is its interactions with other mineral elements in particular with P, Ca and Fe. The application of P or Ca can be beneficial in the detoxification of manganese whereas Mn seems to interfere with Fe metabolism. Although many accomplishments have been achieved, many others remain unsolved. Various hypotheses have been proposed to explain the survival of plants in manganese contaminated environments, including the exclusion, avoidance and compartmentalization of metals, but no final conclusion was obtained. To date, more than 450 species of metal hyperaccumulator plants have been reported in the literature, of which more than 330 are nickel hyperaccumulators. The number of hyperaccumulators for manganese is low and their distribution is restricted in their distribution in some specific localities in the world. With the discovery of the plant Phytolacca acinosa Roxb, the first Mn hyperaccumulator in China, increasing research and development work on this plant should therefore be conducted in accordance with the situation that phytoremediation has become a topical research field in the last decade. Future emphasis should be laid on clarifying the patterns of absorption, migration, manganese-chelating, accumulation, compartmentalization, toxicity and detoxification, which can make phytoremediation practical.