植物对重金属耐性的分子生态机理

植物适应重金属元素胁迫的机制包括阻止和控制重金属的吸收、体内螯合解毒、体内区室化分隔以及代谢平衡等。近年来,随着分子生物学技术在生态学研究中的深入应用,控制这些过程的分子生态机理逐渐被揭示出来。菌根、根系分泌物以及细胞膜是控制重金属进入植物根系细胞的主要生理单元。外生菌根能显著提高寄主植物的重金属耐性,根系分泌物通过改变根际pH、改变金属物质的氧化还原状态和形成络合物等机理减少植物对重金属的吸收。目前,控制菌根和根系分泌物重金属抗性的分子生态机理还不清楚。但细胞膜跨膜转运器已得到深入研究,相关金属离子转运器被鉴定和分离,一些控制基因如铁锌控制运转相关蛋白(ZIP)类、自然抵抗相关巨噬细胞蛋白(Nramp)类、P_1B-type ATPase类基因已被发现和克隆。金属硫蛋白(MTs)、植物螯合素(PCs)、有机酸及氨基酸等是植物体内主要的螯合物质,它们通过螯合作用固定金属离子,降低其生物毒性或改变其移动性。与MTs合成相关的MT-like基因已经被克隆,PCs合成必需的植物螯合素合酶(PCS), 即γ-Glu-Cys二肽转肽酶(γ-ECS) 的编码基因已经被克隆,控制麦根酸合成的氨基酸尼克烟酰胺(NA)在重金属耐性中的作用和分子机理也被揭示出来。ATP 结合转运器(ABC)和阳离子扩散促进器(CDF) 是植物体内两种主要膜转运器,通过它们和其它跨膜方式,重金属被分隔贮藏于液泡内。控制这些蛋白转运器合成的基因也已经被克隆,在植物中的表达证实其与重金属的体内运输和平衡有关。热休克蛋白(HSP)等蛋白类物质的产生是一种重要的体内平衡机制,其分子机理有待进一步研究。重金属耐性植物在这些环节产生了相关响应基因或功能蛋白质,分子克隆和转基因技术又使它们在污染治理上得到了初步的应用。

MOLECULAR MECHANISMS OF PLANT TOLERANCE TO HEAVY METALS

Plants have evolved many adaptive mechanisms to cope with heavy metal stress, including governing uptake of heavy metal ions, detoxification by chelation, intracellular sequestration and cellular homeostasis to minimize the damage from exposure to nonessential metal ions. Mycorrhizae, root exudates and cellular membranes are three key factors that regulate heavy metal uptake. Ectomycorrhizae can significantly enhance the heavy metal tolerance of the host plant, and root exudates reduce the absorption of heavy metal ions by changing the pH and redox state of rhizsphere as well as chelation. However, the molecular mechanisms of these processes are not yet clear. The trans_membrane transporters have been fully researched and have been either identified or isolated. Many genes are involved, such as the ZIP (ZRT IRT related proteins) family, natural resistance associated macrophage proteins (Nramp) and P1B_type ATPase family were discovered and cloned in recent years. The primary chelators of heavy metals in plants are metallothioneins (MTs), phytochelatins (PCs), organic acids and amino acids. They alleviate the toxicity of metal ions in plants by chelation. MT_like genes encoding MTs and genes encodingγ-glutamyl_cysteine synthetase have been cloned. Gene expression in tested plants has demonstrated the role of genes in tolerance to heavy metals. Genes of phytochelatins synthetase (PCs) helping synthesis of PCs, i.e.γ-Glu_Cys, also have been cloned. The nicotianamine gene, an amino acid involved in biosynthesis of muginetic acids, has been shown to contribute to tolerance and hyperacumulation to heavy metals. ABC_type (ATP_binding cassette) transporters and CDF (cation diffusion facilitators) as well as other trans_membranes transporters help to sequester the heavy metals in vacuoles. The genes governing these transporters have been cloned and expressed in plants, which show to contribute to heavy metal adjustment. Plants with heavy metal tolerance have d eveloped pertinent genes or functional proteins in these aspects. In recent years, we have increased our understanding of the molecular mechanisms of plant heavy metal tolerance and have used plants in the remediation of heavy metal contamination. Understanding molecular mechanisms of plant heavy metal tolerance might be a key step for heavy metal remediation.