Gene-for-gene coevolution between Albugo candida and Arabidopsis; mining non-host resistance genes for white rust control in Brassicaceae crops

植物病害对作物生产造成重大损失，并使农民付出巨大代价。例如，控制马铃薯枯萎病每年花费英国农民6000万英镑，亚洲大豆锈病每年花费巴西农民8亿美元。我们的目标是提供抗药基因，使疾病能够通过遗传而不是化学来控制。为了提供可靠的遗传解决方案，我们需要更好地了解植物及其病原体如何相互作用。白垩白病菌在十字花科植物中引起白锈病，包括芸苔类作物（如西兰花和油菜）和模式植物拟南芥。感染wr的植物极易感染其他疾病；我们希望了解发生这种情况的基本机制。病原体向宿主细胞传递一种叫做效应物的分子，这种分子会干扰宿主的免疫机制。植物抗性(R)基因识别这些效应物，然后激活免疫。为了克服R基因，病原体必须通过编码效应物的基因突变来逃避检测。我们的目标是确定WR效应库，而确定效应库的最佳方法是找到那些被R基因识别的效应库。植物育种家经常将野生亲缘种的R基因杂交到作物品种中。然而，单个R基因可以被抗性打破病原体种族迅速克服。我们的目标是从模式植物拟南芥中克隆多个WRR抗性基因，这些基因可以对抗感染芸苔或其他十字花科作物的WR菌株。通过转化具有多个独立作用的WRR基因的作物，降低了病原体中单个突变产生抗性破坏菌株的风险。在印度、澳大利亚和加拿大，芥菜（Brassica juncea）是一种重要的油籽作物，对贫穷的农民来说，杀菌剂价格昂贵，而且水资源资源量来源不足。拟南芥基因已被鉴定出抗芥菜双歧杆菌WR菌株；我们的目标是发现和部署更多这样的基因。油籽亚麻荠（Camelina sativa）经过基因改造，其籽油中25%的成分与鱼油中的成分相同，是“有益心脏健康”的多不饱和脂肪酸，但亚麻荠易受英国WR菌株的影响。我们将调查拟南芥的自然遗传变异，以鉴定和克隆针对这些菌株的额外WRR基因，并验证它们在芥蓝中的有效性，然后建立一个多基因堆来保护芥蓝免受已知英国菌株的WR。我们使用了一种遗传技巧来鉴定拟南芥中WRR基因的变异，这些基因对B juncea菌株的WR起作用。然而，这种方法并不能识别对甘蓝（西兰花、花椰菜、抱子甘蓝）菌株的耐药性变异。我们将测试一种不同的技巧，使我们能够突变所有可能赋予甘蓝菌株WRR的候选R基因，从而鉴定出针对这些菌株的新WRR基因。这类基因有可能成为抵抗大肠杆菌感染WR菌株的极好来源。为了鉴定被WRR基因识别的WR效应物，我们可以利用农杆菌将WRR基因与一组不同的效应物候选基因在烟叶中瞬间共表达，如果有识别，防御激活导致叶片浸润部分细胞死亡。因此，我们可以确定哪个WRR基因识别哪个效应物。这些知识对于确保不同的WRR基因确实识别不同的效应物至关重要，也是在未来实验中研究每种效应物如何抑制宿主免疫的前奏。这些研究将提供多基因堆叠，应该提供持久的抗性。利用拟南芥模型系统成功分离多个不同的WRR基因，将验证从小麦或马铃薯的野生近缘种克隆多个R基因的概念上类似的方法，以保护作物免受锈病或晚疫病的侵害。

Plant disease results in substantial losses in crop production, and imposes great costs on farmers. For example, control of potato blight costs UK farmers ~ £60M/year, and Asian soybean rust costs Brazilian farmers ~$800M/year. We aim to provide resistance genes that enable disease to be controlled by genetics rather than chemistry.To provide reliable genetic solutions, we need a better understanding of how plants and their pathogens interact. Albugo species cause white rust (WR) disease in crucifer plants, including Brassica crops (eg broccoli and oilseed rape), and the model plant Arabidopsis. WR-infected plants become extremely susceptible to other diseases; we wish to understand the basic mechanisms by which this happens. Pathogens deliver molecules called effectors to host cells that interfere with host immune mechanisms. Plant resistance (R) genes recognize such effectors and then activate immunity. To overcome R genes, pathogens must evade detection by mutations in genes that encode effectors. We aim to identify the WR effector repertoire, and the best way to identify effectors is to find those that are recognized by R genes.Plant breeders often use R genes from wild relatives by crossing them into crop varieties. However, single R genes can be rapidly overcome by resistance-breaking pathogen races. We aim to clone multiple WR resistance (WRR) genes from the model plant Arabidopsis that act against WR strains that infect Brassica or other crucifer crops. By transforming crops with multiple independently acting WRR genes, the risk is reduced that a single mutation in the pathogen will create a resistance-breaking strain. In India, Australia and Canada, Brassica juncea is an important oilseed crop, fungicides are expensive for poor farmers and there are insufficient sources of WRR. Arabidopsis genes have already been identified against B. juncea strains of WR; we aim to discover and deploy additional such genes. The oilseed Camelina sativa has been engineered to produce 25% of its seed oil as "heart-healthy" polyunsaturated fatty acids identical to those in fish oil, but C. sativa is susceptible to UK WR strains. We will survey Arabidopsis natural genetic variation to identify and clone additional WRR genes against these strains, and verify their efficacy in C. sativa, prior to building a multigene stack to protect C. sativa against known UK strains of WR.We used a genetic trick to identify variation in Arabidopsis for WRR genes that act against B juncea strain of WR. However, this trick did not work to identify variation for resistance to B. oleracea (broccoli, cauliflower, Brussels sprouts) strains of WR. We will test a different trick that enables us to mutate all candidate R genes that might confer WRR to the B. oleracea strains, and thus identify new WRR genes against these strains. Such genes have the potential to provide an excellent source of resistance against B. oleracea-infecting WR strains.To identify effectors from WR that are recognized by WRR genes, we can transiently co-express a WRR gene with a set of various effector candidate genes in tobacco leaves using Agrobacterium, and if there is recognition, activation of defence results in cell death in the infiltrated part of the leaf. We can thus identify which effector is recognized by which WRR gene. Such knowledge is essential to ensure that different WRR genes really do recognize different effectors, and also as a prelude to investigating how each effector suppresses host immunity in future experimentsThese studies will provide multigene stacks that should provide durable resistance. Success with this approach using the Arabidopsis model system to facilitate isolation of multiple distinct WRR genes, will validate conceptually similar approaches to cloning multiple R genes from wild relatives of wheat or potato, to protect the crop against rusts or late blight.