HEI10: a master switch for recombination in plants

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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.