Mechanism of the oxidative stress -induction at early stages of water stress in plants

陆地植物对不断变化的环境条件的适应能力。G.光、温度、水分、气体和营养物质是它们不动生命的先决条件。近年来的研究表明，各种环境胁迫都会引起植物的氧化损伤。实际上，体内电子自旋共振光谱法（ESR）显示，干旱、紫外线、强光、除草剂和其他环境胁迫在这些胁迫的早期阶段提高了叶细胞的氧化水平（Heber等人，1996年）。本研究旨在阐明环境胁迫下细胞命运的决定因素，并对植物在水分胁迫早期的生化反应进行了研究，得到以下结果：（1）高浓度拟南芥对莴苣根系造成的水分胁迫诱导了叶片中单脱氢抗坏血酸（MDA）自由基的产生，甚至在叶片发生任何可见变化之前。(2)叶绿体（Cp）CuZn-超氧化物歧化酶 ...更多信息 弱渗透胁迫不诱导叶片萎蔫，使叶片中的SOD酶（SOD）特异性失活。(3)较强的渗透胁迫导致不可逆的叶片萎蔫导致过氧化氢酶失活，除了Cp-SOD，而抗坏血酸过氧化物酶（APX）和光系统II的反应中心保持完整。(4)氧化应激诱导剂甲基紫精失活的Cp-APX在菠菜非常迅速和特异性。叶绿体清除H_2O_2能力的丧失导致细胞内其它H_2O_2敏感酶的失活。因此，MV的叶片氧化损伤的发展归因于Cp-APX的失活。这些结果揭示了抗氧化酶作为环境胁迫的敏感靶标，以及它们的主要防御作用的重要性。目标酶对特定类型的环境胁迫具有特异性。这一发现不仅将导致耐逆作物分子育种的战略设计，而且还提出了新的问题，如何以及为什麽这种特定的防御酶的失活是受调控的。少

Versatile capacity of land plants to tolerate ever-fluctuating environmental conditions, e. g. light, temperature, water supply, gases and nutrients, is the prerequisite for the immotile life of them. It has been shown recently that various environmental stresses induce oxidative damage in plants. Indeed, in vivo electron spin resonance spectrometry (ESR) revealed that the oxidation level of leaf cells is raised by drought, UV, strong light, herbicides and other environmental stresses at early stages of these stresses (Heber et al. 1996). In this study, aiming at describing how and what factors determine the fate of environmentally stressed cells, biochemical events at early stages of water stress in plants were investigated.The following results were obtained: (1) Water-stress made by a high osmoticum on lettuce roots induced higher production of monodehydroascorbate (MDA) radicals in the leaves, even prior to any visible changes of leaves. (2) Chloroplastic (Cp) CuZn-superoxide dismu … More tase (SOD) in leaves was inactivated specifically by a weak osmotic stress that did not induce leaf wilting. (3) A stronger osmotic stress leading to irreversible leaf wilting resulted in the inactivation of catalase in addition to Cp-SOD, while ascorbate peroxidase (APX) and the reaction center of photosystem II were kept intact. (4) The oxidative stress-inducing agent methylviologen inactivated Cp-APX in spinach very quickly and specifically. Resulting loss of H2O2-scavenging capacity of chloroplasts lead to inactivation other H_2O_2-sensitive enzymes in cells. Thus, development of oxidative damages of leaves by MV is ascribed to the inactivation of Cp-APX.These results revealed the antioxidant enzymes as sensitive targets of environmental stresses, as well as the importance of their primarily defensive roles. The target enzymes were specific to specific types of environmental stress. This finding will not only lead to strategic designing of molecular breeding of stress-tolerant crops, but also pose new questions how and for what such specific inactivation of the defense enzymes is regulated. Less