Dissecting the apoplastic oxidative burst in Arabidopsis

每年全世界有20%以上的农作物因疾病而损失。此外，进入市场的受感染作物的质量、安全和营养也受到损害。因此，自农业诞生以来，作物保护一直是一个至关重要的因素。然而，许多自然植物防御的基本过程至今仍不为人所知。在鉴定抗虫害基因和防御过程方面已经取得了进展，但一些机制仍然不清楚。植物不能逃跑，并不断面临来自害虫和致病生物（病原体）的威胁。他们设计了一些防御措施，使他们能够保护自己免受这种挑战。它们产生许多化合物，这些化合物是每种植物的特征，可以通过对害虫的感官产生厌恶感（拒食剂）来抵御害虫，或者可以通过干扰微生物病原体的新陈代谢来杀死微生物病原体，从而使它们死亡（植物抗毒素）。植物没有免疫系统，在这种免疫系统中，专门的细胞识别、吞噬并杀死病原体，就像人体细胞（巨噬细胞）一样，把病原体暴露在“漂白剂”中。然而，它们可以产生类似的“漂白”化合物，如过氧化氢，这可能是致命的微生物。为了做到这一点，植物细胞在突然爆发中使用氧气，并在攻击部位将其转化为过氧化氢。这种“氧化爆发”可以在不同植物和微生物之间的各种相互作用中得到证明，是植物防御武器库的重要组成部分。然而，尽管它作为一种如此普遍的机制很重要，但我们尚不能确定它是如何发生的。植物进行这些反应的方式有很多种，有些是在细胞内部，有些是在细胞外部。可以肯定的是，显微技术可以在攻击部位对过氧化氢进行染色，并且总是可以看到它位于围绕所有植物细胞的墙壁中。因此，一些机制存在于此。有许多方法可以探索这一点。过氧化氢必须来自酶。一个大问题是哪一个？许多酶可以产生这种化合物。然而，有许多酶可以在细胞外完成这项工作，包括一种类型的过氧化物酶。如果过氧化物酶是负责的，它们必须与病原体被识别后立即发生的其他过程相关联。过氧化物酶通常使用过氧化氢，那么它们如何转变并产生过氧化氢呢？答案在于细胞壁周围环境的变化，而这些正是细胞对病原体做出反应时发生的事情！来自试图以植物细胞为食的病原体的分子被识别，并将信号发送到细胞中的各种成分。一个组成部分是带电分子的运动，使细胞内部变得更酸，外部变得更碱性。这允许过氧化物酶作为过氧化氢生产者。要做到这一点，它需要一些工作，我们知道这是传递到外面，但我们不知道它是什么。因此，我们需要证明缺乏这种特殊过氧化物酶的突变植物不能产生过氧化氢，因此在感染时会死亡。我们需要确认我们已经确定了实际的一个，因为在模式植物拟南芥中有73个。然后，我们需要确定底物，我们将通过找出它被转移到外面的确切时间来确定，然后我们将分析转移出来的东西，并找出在特殊过氧化物酶存在的情况下，哪一种可以产生过氧化氢。谁知道当我们证明了这一切，我们可能会使用这些信息来设计或培育更好的植物抗病性！

Over twenty per cent of the world's crops are lost to disease every year. In addition, quality, safety and nutrition are compromised in infected crops that make it to the market. Crop protection has therefore been a vital factor since the dawn of agriculture. However many basic processes in natural plant defence are still unknown to this day. Progress has been made in identifying genes for resistance to pests and the processes that bring defence about but some mechanisms are still not understood. Plants cannot run away and continually face the threat from pests and disease-causing organisms (pathogens) landing upon them. They have devised a number of defences to allow them to defend themselves against such challenges. They make a number of chemical compounds which are characteristic of each type of plant that can ward off pests by being obnoxious to their senses (antifeedants) or can kill microbial pathogens by interfering with their metabolism so that they die(phytoalexins). Plants do not have an immune system in which specialised cells recognise and engulf and kill pathogens by literally exposing them to 'bleach' as in human cells in the body (macrophages). However they can produce similar 'bleaching' compounds such as hydrogen peroxide which can potentially be lethal to microbes. In order to do this the plant cell uses oxygen in a sudden burst and converts it to hydrogen peroxide at the site of attack. This 'oxidative burst' can be demonstrated in all sorts of interactions between different plants and microbes and is an important part of the defence armory of the plants. However despite its importance as such a widespread mechanism we cannot yet be certain of how it occurs. A number of ways in which plants carry out these reactions have been proposed, some internal to the cells some external. What is certain is that microscopic techniques can stain the hydrogen peroxide at the site of the attack and invariably it can be seen located in the wall that surrounds all plant cells. Therefore some of the mechanism resides here. There are a number of ways to explore this. The hydrogen peroxide must come from an enzyme. A big question is which one? Many enzymes can produce this compound. However there are a number of enzymes that could do this job that are located outside the cell, including one type, peroxidase. If peroxidases are responsible they must be linked to other processes that occur immediately after the pathogen is recognised. Peroxidases normally use hydrogen peroxide so how can they turn around and produce it? The answer lies in changes in the environment surrounding them in the wall and these are precisely what happens when cells respond to pathogens! Molecules from the pathogen that is trying to feed on the plant cell are recognised and signals at sent to various components in the cell. One component is movement of charged molecules so that the inside of the cell becomes more acid and the outside more alkaline. This allows the peroxidase to function as a hydrogen peroxide producer. To do this it needs something to work on and we know this is passed outside but we don't know what it is yet. So we need to show that mutant plants that lack this special peroxidase cannot make hydrogen peroxide and will therefore die when infected. We need to confirm we have identified the actual one as there are seventy three in the model plant, Arabidopsis. Then we need to identify the substrate which we will do by finding out exactly when it is moved outside and then we will analyse what is tranferred out and find out which one can produce hydrogen peroxide in the presence of the special peroxidase. Who knows when we have proved all this we might use this information to engineer or breed better disease resistance in plants!