HEI10: a master switch for recombination in plants

社会正面临着提高作物产量和质量的重大挑战，以满足不断增长的全球人口的需求。应对这一挑战的方法之一是培育和进一步改良作物种质。例如，来自野生种群的抗病性和抗逆性性状可有益于引入优良品种。在这种情况下，有益的性状必须经过许多代的基因渗入，以获得改良的菌株。作物育种的这一过程很大程度上依赖于后代在繁殖过程中自然发生的性状重组。具体来说，多样性是在称为减数分裂的专门细胞分裂过程中产生的，这导致配子（性细胞）的产生。在减数分裂过程中产生遗传多样性的一种机制称为交叉。在这里，同源染色体物理配对并交换相互区域，从而产生新的遗传变异组合。然而，在许多最重要的作物物种中，包括小麦、玉米、大麦和番茄，沿染色体的沿着交叉模式是高度偏斜的。因此，基因组的某些区域无法进行育种，尽管它们含有对农艺学重要性状有影响的重要基因。当在植物、动物和真菌之间进行比较时，交换水平通常较低，大多数物种在每次减数分裂中每个染色体显示约1-2次交换。然而，物种内部和物种之间的交叉水平也存在相当大的差异。为了探索这一现象，我们以前使用拟南芥，以确定自然变化的交叉水平。这些实验确定了一个名为HEI 10的主要基因，该基因在拟南芥种群之间变化并控制交换水平。我们还发现HEI 10具有高度的剂量敏感性。我们发现，通过增加HEI 10基因拷贝数和表达水平，我们可以有效地增加总的交换水平。在育种的背景下，这将允许育种者使用较少数量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，我们描述了一个基本的和应用的实验，旨在完善HEI 10作为重组的主开关的使用相结合的程序，并将其部署在一个重要的作物物种（番茄）。该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HEI 10基因如何用于增加重组。我们将进一步增加HEI 10剂量，以测试何时发生重组反应饱和。我们将进一步改变减数分裂期间HEI 10表达的时间。额外的遗传修饰将与HEI 10组合，以便以有利于育种的方式偏向杂交模式。其次，为了了解HEI 10如何在分子水平上控制重组，我们将使用一种专门的技术来绘制HEI 10沿染色体沿着的结合位点。这将提供HEI 10结合的高分辨率地图，我们将与我们现有的重组热点地图进行比较。使用一种新的技术，我们还将映射HEI 10结合在减数分裂的不同阶段，以研究其结合到基因组的动态。这是重要的，因为HEI 10焦点在减数分裂期间显示与染色体的动态结合。第三，我们将用额外的HEI 10基因拷贝转化番茄，并测试交叉是否增加。这一点很重要，因为重组限制了番茄育种和性状改良，例如当基因渗入来自野生亲缘植物时。由于减数分裂和HEI 10高度保守，拟议研究的结果将很容易适用于不同的作物物种。这项研究将更详细地了解植物有性生殖过程中性状重配的机制，并为开发新的工具加速作物育种奠定基础。

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.