【摘要】 总结了近年来植物对一重要环境污染物镉的反应研究报道。探讨了镉对植物的各种毒性效应 ,并论述植物对镉胁迫采取的相应防御机制 ,如络合 ( PC、MT)、应激 (应激乙烯、应激蛋白 ) ,及额外防御机制 ,如细胞壁固定化、原生质膜排除、区域化、过氧化物酶等更多还原

【Abstract】 The paper summarizes progresses in the field of plants response to cadmium(Cd), an important and widespread heavy metal in enviroment.We analyse the toxic effects of Cd on plants and discuss the response mechanisms of plants to Cd stress including exclusion, immobilization and compartmentalization of the ions, synthesis of phytochelatins and metallothionein, inducements of stess protein and stess ethylene. The higher plants are able to uptake Cd depending on concentration and bioavailability of Cd in the soil. The processes of uptake were modulated by the presence of organic matter, pH, redox potential, temperature and concentrations of other elements. As soon as Cd enters the roots, it can reach the xylem through an apoplastic and/or a symplastic pathway, complexed by several ligands such as organic acids and/or, perhaps, phytochelations. Cd damages the roots by altering the synthesis of RNA and inhibits ribonuclease activity. Cd interacts with the water balance and damages the photosynthetic apparatus. Cd significantly reduces the normal H +/K + exchange and the stomatal opening, but how it dose so has yet to be established. Cd was found to produce oxidative stress by inhibit the activity of several antioxidative enzymes. Potential effects of Cd on host-pathogen interactions were very recently examined. In response to Cd stress, the plant can resort to a number of defense systems. Cd can be immobilized by means of the cell wall and extracellular carbohydrates. The importance of this mechanism may vary in accordance with the concentration of Cd supplied and the species involved, etc. But no differences in cell wall binding between normal and Cd-tolerant plants were observed. Preventing Cd ions from entering the cytosol through the action of the plasma membrane could theoretically represents the best defense mechanism. Cd stress gave increased levels of asparagine in root exudates. However, this response was probably due to disfunction of the plant membranes at Cd concentration above 1μm, rather than to a specific mechanism of amino acid induction aimed at directly chelating Cd ions. A deeper understanding of these events would be very useful in widening our knowledge of Cd exclusion in higer plants. Cd has a high affinity to metabolic processes of the sulphur metabolism, and its first effect is on ATP-sulfurylase, thus activate the synthesis of phytochelatins. Phytochelatins chelate Cd and form various complexs with Cd. As a result, it’s prevented from circulating as free Cd 2+ inside the cytosol. The production of phytochelatins is a widespread mechanism of Cd detoxification in higher plants. In animals, cyanebacteria and fungi, Cd can be complexed and detoxified by metallothioneins, a group of gene-encoded Cys-rich peptides, which generally lack aromatic acids. But there is no certain indication of the existence in higher plants of metallothioneins induced by Cd. Thus, their role in Cd detoxification seems to be at present of secondary importance as compared with phytochelatins and stress proteins. Vacuolar compartmentalization can prevent the free circulation of Cd ions in the cytosol and force them into a limited area, thus plays a very significant role in Cd detoxification and tolerance. The mechanism of compartmentalization was disscusd in this paper. Being subjected to various stresses, plants often start the synthesis of heat shock proteins, Ubiquitin and stress ethylene. But now it’s impossible to understand exactly the relationship between ethylene biosynthesis and Cd stress at a molecular/cellular level. Response to Cd in normal higher plants is a complex phenomenon. Cd can evoke a number of parallel and/or consecutive events at molecular, physiological and morphological levels. Indentification of individual biochemical pathway such as phytochelatin production and preteolysis, etc, is essential. Without integration into a cellular response, however such studies may lose direction and overemphasize the overall importance of a given pathway in achieving tolerance. Therefore, furthur efforts a更多还原