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于对芸苔属植物拟南芥的研究。拟南芥已经表明，植物对灰霉菌灰霉病（被称为坏死菌，因为它杀死植物细胞并以其内容物为食）和在植物活体组织中繁殖的生物营养微生物和病毒等病原体的反应不同。当植物感知到生物营养物的攻击时，它会产生水杨酸（SA）作为一种激素信号，在受到攻击的部位和植物的其他部位诱导抗性。相反，当植物感知到坏死性病原体或被食草动物破坏时，会产生茉莉酸（JA），有时还会产生乙烯（ET）。SA开启一组基因的表达，其中一些基因编码抗菌蛋白，而JA和ET开启不同的防御基因。最近，我们发现AS1蛋白的缺失通过允许JA诱导防御基因达到比正常水平更高的水平而增加了对坏死性病原体的抵抗力。这是一个惊喜，因为10多年来我们一直认为AS1的唯一作用是控制叶片发育。（AS代表不对称叶片，描述缺乏AS1蛋白的突变体。）AS1通过与伴侣蛋白结合来控制叶片发育，这种蛋白被称为AS2，因为它的突变体与AS1突变体相似。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不需要其发育伙伴AS2来调节防御基因。最近，我们确定了AS1的另一个伙伴，我们称之为PIP，并发现AS1可能只有在与PIP结合时才能调节防御基因。重要的是，虽然缺乏PIP的植物对坏死性营养更有抵抗力，但它们似乎发育正常，对细菌病原体也表现出正常的抵抗力。在这里，我们的目标是通过识别过程和它所调节的基因来更全面地了解PIP的作用。在此过程中，我们将验证这样一个假设，即降低PIP活性的唯一效果是增加对坏死病原体和可能的食草动物的抗性，从而提供了在不降低产量或求助于基因操作的情况下培育具有更高抗病性的植物的潜力。我们将首先确认PIP和AS1在植物细胞核中相互结合，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具有更强的抗性，同时保持对一系列生物营养物的正常抗性。我们还将测试对蚜虫的抗性，这被认为涉及SA信号，因此应该不受影响。与此同时，我们将通过依次测序在缺乏AS1和PIP的植物中表达的所有基因来确定受AS1和PIP调节的基因，而不是AS2。由于AS1通过结合靶基因的DNA来控制靶基因的表达，我们也将对AS1与PIP结合的基因进行测序。从这些基因的DNA序列预测它们的功能将表明PIP是否只调节与防御有关的基因，还是它可能有其他作用。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作用。我们知道，没有PIP的植物在温室中生长正常，对葡萄孢菌的抗性更强。我们还将测试这是否也适用于自然生长在室外的植物，从而测试降低PIP活性的育种是否可以在不影响产量的情况下提高抗病性。

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.