Engineering wheat for take-all resistance

小麦是世界上人类和动物最重要的食物之一。在英国，小麦种植面积超过180万公顷，产值约为17亿英镑。虽然在过去的半个世纪里，产量有了显著的提高，但当前和未来对小麦和其他谷物的需求将要求加快提高生产力。当我们试图以可持续的方式为不断增长的全球人口（随着饮食偏好的变化）生产足够的食物时，这对社会来说是一个巨大的挑战。小麦全蚀病是由土壤真菌谷草芽孢杆菌（Gaeumannomyces graminis var. tritici, Ggt）引起的小麦根病，是世界范围内危害最大的小麦根病。英国植物育种行业和代表耕地农民的HGCA已经确定将抗全蚀性基因引入栽培小麦是一项首要任务（见英国植物育种家联盟和HGCA的随附信函）。至少有一半的英国小麦作物受到此病的影响，平均产量损失5-20%，在严重的全蚀条件下完全失败。保守估计，在英国，“全包”相关的产量损失成本每年在8500万英镑至3.4亿英镑之间。随着小麦连作，疾病严重程度增加，因此在同一块地里种植第二和第三种小麦可能在商业上不可行。随着粮食生产需求的增加和种植系统更加集约化，这一问题将进一步恶化。因此，全身性疾病对英国和世界粮食安全构成重大威胁，迫切需要制定简单、经济和可持续的疾病控制战略。目前的防治方法依赖于轮作、生物防治和杀菌剂，但没有一种方法能有效防止上述产量损失。实现简单、经济和可持续控制全身性疾病的最有效方法是通过遗传抗性。抵制“通吃”将代表小麦产量的逐步变化，从而确保粮食安全和提高行业竞争力。它还将扩大在连续种植季节种植小麦的能力，增加其地理分布，并减少化学和化肥投入。不幸的是，在栽培小麦品系中没有已知的主要抗全蚀病品种，因此迄今为止，这种疾病对育种者来说是难以对付的。相比之下，燕麦具有极强的抗全食性，并产生一种抗微生物的三萜糖苷（维那酸A-1），可以预防这种疾病。小麦和其他谷物不产生阿维那酸A-1，也不产生其他三萜苷。我们已经克隆并鉴定了大部分燕麦合成阿维那酸的基因。这些基因是目前对任何谷类或禾本科物种的全食性遗传抗性的唯一特征来源。燕麦离小麦太远，无法通过常规杂交或外源渗入引入合成阿文那酸的基因，因此遗传转化是将这些基因引入小麦的唯一可行选择。这项提议的目的是改造小麦，使其产生一套保护性的三萜，从而使小麦具有抗全蚀性疾病的能力。为了做到这一点，我们已经组装了一个定制的工具包，用于三萜代谢工程，使用来自燕麦和双科植物物种的特征基因和酶。

Wheat is one of the most important food items for humans and animals around the world. In the UK, wheat is sown on over 1.8 million hectares with a production value of ~£1.7 billion. Although significant increases in yield have been achieved during the last half century, the current and future demands for wheat and other cereals will require accelerated increases in productivity. This emerges as a grand challenge for society as we seek to produce enough food for a growing global population (with changing dietary preferences) in a sustainable manner. Take-all disease, caused by the soil fungus Gaeumannomyces graminis var. tritici (Ggt), is the most damaging root disease of wheat worldwide. The introduction of genes for take-all resistance into cultivated wheat has been identified as a top priority by the UK plant breeding industry and by HGCA on behalf of arable farmers (see accompanying letters from a consortium of UK plant breeders and from HGCA). At least half of UK wheat crops are affected by the disease, with average yield losses of 5-20% and complete failure under severe take-all conditions. Conservative estimates of the cost of take-all associated yield losses in the UK range from £85 m to £340 m per annum. Disease severity increases with successive wheat cropping, therefore growth of second and third wheat crops in the same fields can become commercially unviable. This problem will be exacerbated as the need for food production increases and cropping systems becomes even more intensive. Thus take-all disease represents a major threat to UK and world food security and there is an urgent need for simple, economic and sustainable strategies for disease control. Current control methods rely on crop rotation, biological control and fungicides, none of which are effective in preventing the yield losses indicated above. The most effective way to achieve simple, economic and sustainable control of take-all disease is through genetic resistance. Resistance to take-all would represent a step-change in wheat productivity, ensuring food security and enhanced industry competitiveness. It would also expand the ability to grow wheat in successive cropping seasons and increase its geographic distribution, and reduce chemical and fertiliser inputs. Unfortunately, there is no known major varietal resistance to take-all in cultivated wheat lines and hence the disease has so far proved to be intractable to breeders. In contrast, oats have extreme resistance to take-all and produce an antimicrobial triterpene glycoside (avenacin A-1) that provides protection against the disease. Wheat and other cereals do not make avenacin A-1 or appear to make other triterpene glycosides. We have cloned and characterised most of the genes for avenacin synthesis from oat. These genes are currently the only characterized source of genetic resistance to take-all from any cereal or grass species. Oat is too far removed from wheat to allow introduction of the genes for avenacin synthesis through conventional crossing or alien introgression, making genetic transformation the only viable option for introduction of these genes into wheat. The aim of this proposal is to engineer wheat to produce a suite of protective triterpenes that confer resistance to take-all disease. To do this we have assembled a customised toolkit for triterpene metabolic engineering using characterised genes and enzymes from oat and from dicot plant species.