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

陆地植物能够适应不断变化的环境条件，如光、温度、水供应、气体和营养物质，这是它们不朽生命的先决条件。近年来研究表明，各种环境胁迫可诱导植物的氧化损伤。事实上，体内电子自旋共振光谱（ESR）显示，在干旱、紫外线、强光、除草剂和其他环境胁迫的早期阶段，叶子细胞的氧化水平会升高（Heber et al. 1996）。在这项研究中，旨在描述环境胁迫细胞的命运是如何以及哪些因素决定的，研究了植物早期水分胁迫的生化事件。结果表明：(1)生菜根部高渗透水分胁迫诱导叶片中MDA自由基的产生，甚至在叶片发生任何可见变化之前。(2)弱渗透胁迫对叶片叶绿体（Cp） CuZn-superoxide dismu . More tase （SOD）具有特异性失活作用，但不引起叶片萎蔫。(3)较强的渗透胁迫导致叶片不可逆萎蔫，导致过氧化氢酶和Cp-SOD失活，而抗坏血酸过氧化物酶（APX）和光系统II反应中心保持完整。(4)氧化应激诱导剂methylviologen能快速特异地灭活菠菜中Cp-APX。叶绿体清除h2o2能力的丧失导致细胞中其他对h2o2敏感的酶失活。因此，MV对叶片氧化损伤的发生可归因于Cp-APX的失活。这些结果揭示了抗氧化酶是环境胁迫的敏感靶点，以及其主要防御作用的重要性。靶酶对特定类型的环境胁迫具有特异性。这一发现不仅将为抗逆性作物的分子育种策略设计提供指导，而且对如何调控防御酶的特异性失活以及为何调控提出了新的问题。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