HEI10: a master switch for recombination in plants

为了满足不断增长的全球人口的需求，社会正面临着提高作物产量和质量的重大挑战。解决这一挑战的方法之一是培育和进一步改良作物种质。例如，野生种群的抗病和抗逆性特征有助于将其引入优良品种。在这种情况下，必须在许多世代中导入有益的性状，以获得改良的品系。作物育种的这一过程在很大程度上依赖于后代在繁殖过程中自然发生的性状重新分类。具体地说，多样性是在称为减数分裂的特殊细胞分裂过程中产生的，减数分裂导致配子(性细胞)的产生。在减数分裂过程中创造遗传多样性的一种机制被称为交叉。在这里，同源染色体在物理上配对和交换互换区域，这产生了新的遗传变异组合。然而，在许多最重要的作物物种中，染色体上的交叉模式高度倾斜，包括小麦、玉米、大麦和番茄。因此，尽管含有影响重要农艺性状的重要基因，但基因组的某些区域无法进行育种。当在植物、动物和真菌之间进行比较时，交叉水平通常较低，大多数物种在每个减数分裂过程中每条染色体都表现出1-2个交叉。然而，物种内部和物种之间的交叉水平也有相当大的差异。为了探索这一现象，我们以前使用拟南芥来鉴定交叉水平的自然变异。这些实验确定了一种名为HEI10的主要基因，该基因在拟南芥种群和控制交叉水平之间存在差异。我们还发现HEI10具有高度的剂量敏感性。我们发现，通过增加HEI10基因的拷贝数和表达水平，我们可以有效地提高总交叉水平。在育种方面，这将使育种者能够用较少的个体产生更多样化的种群。这将在时间和费用方面提供相当大的优势。因此，在拟议的工作中，我们描述了一个基础和应用实验相结合的计划，旨在改进HEI10作为重组的主开关的使用，并将其部署在重要的作物物种(番茄)中。这个项目有三个主要目标。首先，我们将使用模式植物拟南芥来详细研究HEI10基因如何用于促进重组。我们将进一步增加HEI10的剂量，以测试何时发生重组反应饱和。我们将进一步改变HEI10在减数分裂中的表达时间。额外的遗传修改将与HEI10相结合，以便以有利于育种的方式偏向交叉模式。其次，为了了解HEI10如何在分子水平上控制重组，我们将使用一种专门的技术沿着染色体绘制HEI10结合位点的图谱。这将提供HEI10结合位置的高分辨率地图，我们将与现有的重组热点地图进行比较。使用一种新的技术，我们还将绘制减数分裂不同阶段的HEI10结合图，以研究其与基因组结合的动力学。这一点很重要，因为HEI10在减数分裂过程中显示出与染色体的动态结合。第三，我们将用额外的HEI10基因拷贝转化番茄，并测试是否会增加交叉。这一点很重要，因为重组限制了番茄的育种和性状改进，例如，当从野生近缘关系中进行导入时。由于减数分裂和HEI10高度保守，拟议的研究结果将很容易适用于不同的作物物种。这项研究将对植物有性繁殖过程中的性状重组机制提供更详细的了解，并将为开发加速作物育种的新工具奠定基础。

Society is facing a major challenge to improve crop yield and quality, in order to meet the needs of the growing global population. One means to address this challenge is to breed and further improve crop germ plasm. For example, disease and stress resistance traits from wild populations can be beneficial to introduce into elite varieties. In this case beneficial traits must then be introgressed over many generations to achieve an improved strain. This process of crop breeding largely relies on the trait re-assortment in offspring that occurs naturally during reproduction. Specifically, diversity is generated during a specialised cell division called meiosis, which results in production of gametes (sex cells). One mechanism that creates genetic diversity during meiosis is called crossover. Here homologous chromosomes physically pair and exchange reciprocal regions, which generates new combinations of genetic variation. However, crossover patterns along chromosomes are highly skewed in many of the most important crop species, including wheat, maize, barley and tomato. As a consequence, some regions of the genome are inaccessible for breeding, despite containing important genes with effects on agronomically important traits. When compared across plants, animals and fungi, crossover levels are typically low, with most species showing ~1-2 crossovers per chromosome per meiosis. However, there is also considerable variation in crossover levels within and between species. To explore this phenomenon we previously used Arabidopsis to identify natural variation in crossover levels. These experiments identified a major gene called HEI10 that varies between Arabidopsis populations and controls crossover levels. We also discovered that HEI10 is highly dosage sensitive. We found that by increasing the HEI10 gene copy number and expression levels we could efficiently increase total crossover levels. In the context of breeding this would allow breeders to generate more diverse populations using smaller numbers of individuals. This would provide considerable advantages in terms of time and expense. Therefore, in the proposed work we describe a combined program of basic and applied experiments designed to refine the use of HEI10 as a master-switch for recombination, and deploy it in an important crop species (tomato). This project has three major objectives. First, we will use the model plant Arabidopsis to investigate in detail how the HEI10 gene can be used to increase recombination. We will further increase HEI10 dosage to test when saturation of the recombination response occurs. We will further vary the timing of HEI10 expression during meiosis. Additional genetic modifications will be combined with HEI10 in order to bias the pattern of crossovers in ways that would be beneficial for breeding. Second, to understand how HEI10 controls recombination at the molecular level, we will use a specialised technique to map HEI10 binding sites along the chromosomes. This will provide a high-resolution map of where HEI10 binds, which we will compare to our existing maps of recombination hotspots. Using a novel technique we will also map HEI10 binding at different stages during meiosis, in order to investigate the dynamics of its binding to the genome. This is important as HEI10 foci shows dynamic binding to chromosomes during meiosis. Third, we will transform tomato with additional HEI10 gene copies and test whether crossover increases. This is important as recombination limits tomato breeding and trait improvement, for example when introgressions are made from wild relatives.As meiosis and HEI10 are highly conserved, the outcomes of the proposed research will be readily applicable to diverse crop species. The proposed research will provide a more detailed understanding of the mechanisms of trait re-assortment during plant sexual reproduction and will lay a foundation to develop novel tools to accelerate crop breeding.