The role of an AS1 co-repressor in plant defence

重要的是要了解植物如何抵抗疾病，尤其是因为超过10%的作物产量因疾病而损失，近10亿人营养不良。我们的认识主要来自对油菜近缘植物拟南芥的研究。拟南芥已经表明，植物对病原体的反应不同，比如灰霉菌-被称为坏死菌，因为它杀死植物细胞并以其内容物为食-以及在活的植物组织中繁殖的生物营养微生物和病毒。当植物感觉到有侵袭的生物时，它会产生水杨酸(SA)作为荷尔蒙信号，在受攻击的地方和植物的其他地方诱导抗性。相比之下，当植物感觉到坏死性病原体或被草食动物破坏时，它会产生茉莉酸(JA)，有时还会产生乙烯(ET)。SA启动一组基因的表达，其中一些基因编码抗微生物蛋白，而JA和ET启动不同的防御基因。最近，我们发现AS1蛋白的丢失通过允许JA诱导防御基因达到比正常水平更高的水平来增加对坏死性病原体的抗性。这令人惊讶，因为10多年来，我们一直认为AS1的唯一作用是控制叶片的发育。(AS代表不对称的叶片，描述了缺乏AS1蛋白的突变体。)当AS1与一种被称为AS2的伙伴蛋白结合时，它控制着叶片的发育，因为它的突变体类似于AS1突变体。有趣的是，缺乏AS2的突变体具有正常的抗病能力，这意味着AS1不需要它的发育伙伴AS2来调节防御基因。最近，我们发现了AS1的另一个伙伴，我们称之为PIP，并发现AS1可能只有在与PIP结合时才调控防御基因。重要的是，虽然缺乏PIP的植物对坏死菌更具抵抗力，但它们似乎发育正常，对细菌病原体表现出正常的抵抗力。在这里，我们的目标是通过识别PIP调节的过程和基因来更全面地了解PIP的作用。在这样做的时候，我们将检验这样一个假设，即降低PIP活性的唯一效果是提高对坏死性病原体和可能的食草动物的抵抗力，从而提供在不减少产量或诉诸基因操作的情况下培育抗病能力增强的植物的潜力。我们将首先确认PIP和AS1在植物细胞的细胞核中相互结合，并检查它们本身是否受到病原体或食草动物攻击的调节。然后，我们将测试缺乏PIP的植物是否对广泛的坏死性病原体和食草动物更具抵抗力，同时保持对一系列生物营养物的正常抵抗力。我们还将测试对蚜虫的抗性，建议与SA信号有关，因此不应受到影响。同时，我们将通过依次对缺少这两种蛋白的植物中表达的所有基因进行测序，确定受AS1和PIP调控的基因，而不是AS2。由于AS1通过与其DNA结合来控制其靶基因的表达，我们还将通过PIP对AS1结合的基因进行测序。根据DNA序列预测这些基因的功能将表明，PIP是否只调控与防御有关的基因，或者它是否可能具有其他作用。我们将通过检查它们在缺乏PIP的植物中是否发生改变来研究任何潜在的新角色。我们知道，没有PIP的植物在温室里生长正常，对灰霉病的抗性更强。我们还将测试在室外自然生长的植物是否也是如此，因此降低PIP活性的育种是否可以在不影响产量的情况下提高抗病能力。

It is important to understand how plants resist disease, not least because over 10% of crop yields are lost to disease and nearly 1 billion people are malnourished. Our understanding has come mostly from research on the brassica relative, Arabidopsis. Arabidopsis has shown that plants responds differently to pathogens such as the grey mold Botrytis - termed a necrotroph because it kills plant cells and feeds on their contents - and to biotrophic microorganisms and viruses which multiply in living plant tissue. When a plant senses an attacking biotroph it produces salicylic acid (SA) as a hormonal signal to induce resistance at the site of attack and elsewhere in the plant. In contrast, a plant produces jasmonic acid (JA), and sometimes ethylene (ET), when it senses a necrotrophic pathogen or is damaged by a herbivore. SA turns on the expression of a set of genes, some of which encode antimicrobial proteins, while JA and ET turn on different defence genes. Recently, we found that loss of the AS1 protein increases resistance to necrotrophic pathogens by allowing JA to induce defence genes to higher levels than normal. This came as a surprise because for over 10 years we had thought that the only role of AS1 was to control leaf development. (AS stands for Asymmetric Leaves and describes mutants lacking AS1 protein.) AS1 controls leaf development when it binds to a partner protein - called AS2 because its mutants are like AS1 mutants. Intriguingly, mutants lacking AS2 have normal disease resistance, implying that AS1 does not need its develpmental partner AS2 to regulate defence genes. More recently, we identified an additional partner of AS1, which we called PIP, and found that AS1 probably regulates defence genes only when bound to PIP. Importantly, while plants that lack PIP are more resistant to necrotrophs, they appear to develop normally and to show normal resistance to bacterial pathogens. Here we aim to understand more fully the role of PIP by identifying the processes and the genes that it regulates. In doing so, we will test the hypothesis that the sole effect of reducing PIP activity is to increase resistance to necrotrophic pathogens, and possible herbivores, providing the potential to breed plants with increased disease resistance without reducing yield or resorting to genetic manipulation. We will first confirm that PIP and AS1 bind to each other in the nuclei of plant cells and examine whether they are themselves regulated by pathogen or herbivore attack. We will then test whether plants lacking PIP are more resistant to a broad range of necrotrophic pathogens and to herbivores, while maintaining normal resistance to a range of biotrophs. We will also test resistance to aphids, which is suggested to involve SA signalling and so should be unaffected. In parallel, we will identify the genes that are regulated by AS1 and PIP, but not AS2, by sequencing all the genes that are expressed in plants lacking each of the proteins in turn. Because AS1 controls the expression of its target genes by binding to their DNA, we will also sequence the genes to which AS1 binds together with PIP. Predicting the functions of these genes from their DNA sequences will suggest whether PIP regulates only genes that are involved in defence, or whether it might have other roles. We will investigate any potentially new roles by examining whether they are altered in plants lacking PIP. We know that plants without PIP grow normally and are more resistant to Botrytis in the greenhouse. We will also test whether this is also true for plants growing naturally outside, and therefore whether breeding for reduced PIP activity might increase disease resistance without compromising yield.