Functions of a novel chitinase-like effector family unique to aphids

蚜虫本质上是迷人的昆虫，但它们也是全球农业中的破坏性害虫，通过吸食植物汁液和传播许多导致植物疾病的病毒造成损失。蚜虫种群可以非常迅速地增加，这是由于它们的克隆繁殖和它们不寻常的“俄罗斯娃娃”缩短世代。虽然化学杀虫剂被广泛使用，但由于蚜虫产生抗性而不再能够控制，因此经常出现问题。此外，某些农药可能存在环境或健康问题，导致一些农药被撤回或限制。有几种作物，如番茄、甜瓜、大豆和水稻，已知对一些蚜虫和相关物种具有遗传抗性。相关的基因被称为R基因（抗性），通常编码识别害虫入侵的受体蛋白。然而，昆虫反过来可以进化出新的方法来克服植物的R基因防御。总的来说，迫切需要找到新的强大和可持续的方法来管理农业害虫。在这种情况下，我们比较了蚜虫之间的分子差异，这些蚜虫的繁荣或死亡取决于它们攻击的植物类型，最近在蚜虫唾液中发现了一类新的蛋白质，它们与几丁质酶有很大的相似性。几丁质酶存在于所有昆虫中，对于维持几丁质聚合物在它们的外骨骼和口器中是必需的。然而，这些唾液蛋白似乎无法充当几丁质酶，因此我们需要找到另一种功能来解释它们如何使蚜虫受益。有趣的是，这种蛋白质组是蚜虫所独有的，甚至在其他密切相关的吸汁昆虫中也不存在。由于蚜虫是在3亿年前进化成一个独特的物种，我们认为类几丁质酶蛋白质有着古老的起源，这可能是这些生物取得巨大成功的基础。我们最初的想法是，类几丁质酶蛋白质可能仍然具有粘附在几丁质上的能力，实际上是在清除这些分子的碎片。这将阻止宿主植物检测到几丁质的存在，使蚜虫在植物免疫系统的雷达下活动。其他关于真菌的研究表明，植物的细胞表面携带有检测几丁质的受体蛋白，没有理由认为这些受体不能检测蚜虫的几丁质。我们的第二个想法来自于发现一种特殊的蚜虫几丁质酶样蛋白，它与完全相反的过程密切相关：通过R基因触发植物免疫。在这里，我们将直接测试这两个概念。为了揭示几丁质酶样蛋白的工作原理，我们将采取广泛的研究方法。我们想为整个家族制定出共同的规则，同时也想发现导致植物免疫和蚜虫死亡的一个成员的具体特性。首先，我们将制造纯化形式的蛋白质，以使实验室规模的实验成为可能。特别是，我们将测试这些蛋白质是否真的与几丁质结合，或者它们是否具有其他一些意想不到的功能。我们还将寻找蛋白质或植物细胞的其他成分，这些蛋白质或植物细胞的其他成分与进入的几丁质酶样唾液蛋白结合。众所周知，免疫抑制和免疫激活模式都是由蛋白质与蛋白质之间的相互作用引起的，以应对真菌和细菌疾病，因此我们将在我们的蚜虫系统中寻找相似之处。我们将与法国的同事合作，应用一种名为CRISPR的基因组编辑技术，特异性地敲除编码免疫激活唾液蛋白的基因。这将给我们新的蚜虫，看看他们是否比仍然有这种基因的蚜虫做得更好或更差。最后，我们发现植物免疫力在压力环境下受到抑制，包括由于气候而日益普遍的干旱，因此我们将探索免疫鲁棒性的机制。

Aphids are intrinsically fascinating insects, but they are also damaging pests in global agriculture, causing losses through feeding on plant sap and through spreading many viruses that cause plant diseases. Aphid populations can increase extremely fast due to their clonal reproduction and their unusual "Russian Doll" telescoped generations. Although chemical pesticides are widely used, problems often arise due to the aphids evolving resistance and no longer being able to be controlled. In addition, there may be environmental or health issues associated with certain pesticides that have led to some being withdrawn or restricted. There are a few crops such as tomato, melon, soybean and rice, with known genetic resistance to some aphids and related species. The genes involved are called R-genes (for resistance) and commonly code for receptor proteins that recognise invasion by the pest insect. However, insects in turn can evolve new means to overcome the plant's R-gene defences. Overall there is a pressing need to find new robust and sustainable ways to manage pests in agriculture.In this context, we compared molecular differences between aphids that thrived or died depending on which plant type they were attacking, and recently discovered a new class of proteins in aphid saliva that distantly resemble chitinase enzymes. Chitinases are present in all insects, being essential for maintaining the chitin polymer in their exoskeleton and mouthparts. However, these salivary proteins appear unable to act as chitinase enzymes, so we need to find another function to explain how they benefit the aphids. Intriguingly, this protein group is unique to aphids, being absent even from other closely related species of sap-sucking insects. Because aphids evolved as a distinct group of species around 300 million years ago, we think that the chitinase-like proteins have ancient origins that may underpin the enormous success of these organisms.Our starting idea is that chitinase-like proteins may still have the ability to stick to chitin, in effect mopping up fragments of these molecules. This would prevent the host plant from detecting the presence of chitin, allowing the aphids to go under the radar of the plant's immune system. Other work on fungi shows that plants carry chitin-detecting receptor proteins on their cell surface, and there is no reason to think that these receptors couldn't also detect aphid chitin. Our second idea comes from finding one particular aphid chitinase-like protein that is strongly associated with exactly the opposite process: triggering plant immunity through an R-gene. Here we will directly test both these concepts.To uncover how chitinase-like proteins work, we will take a wide range of research approaches. We want to work out the common rules for the whole family, and also to discover the specific properties of the one member that leads to plant immunity and aphid death. First, we will make purified forms of the proteins to enable laboratory scale experiments. In particular, we will test whether the proteins really bind to chitin, or whether they have some other unexpected functions. We will also hunt for proteins or other components of plant cells that bind to the incoming chitinase-like salivary proteins. Both immune-suppressing and immune-activating modes are commonly known to result from protein-to-protein interactions in responses to fungal and bacterial diseases, so we will look for parallels in our aphid system. Working with colleagues in France, we will apply a genome editing technology called CRISPR to specifically knock-out the gene that codes for the immune-activating salivary protein. This will give us new aphids to see if they do better or worse than aphids that still have this gene. Finally, we discovered that plant immunity is suppressed under stressful environments, including drought that are increasingly prevalent due to climate, so we will explore mechanisms of immune robustness.