How does ASYNAPSIS 5 mediate Synaptonemal Complex formation and crossover control in plants?

The majority of flowering plants (including crops) propagate through sexual reproduction where the male gamete (pollen) fertilises the female gamete (ovule). The gametes contain half the number of chromosomes of the parents that are restored to normal levels upon fertilisation. The halving of chromosomes occurs during meiosis, a specialised cell division characterised by homologous recombination. Homologous recombination shuffles segments of chromosomes, thus producing new combinations of genes in daughter cells. It is therefore a critical process for plant breeders in bringing advantageous traits (eg. yield) together for crop improvement. Efforts to increase the efficiency of gene shuffling in crops has been limited by internal constraints acting on this process; primarily the level of recombination and its tendency to be localized to certain chromosome regions and occur infrequently in others. An additional factor is many crop plants such as wheat, oil seed rape and potato are polyploids, that is they have multiple sets of chromosomes that are similar or identical. However, although such plants have adapted to solving this particular problem, they are still susceptible to meiotic failure that can yield in loss of fertility. In addition, this process is extremely sensitive to high temperature stress that is an increasing challenge due to global warming. Therefore, understanding how homologous recombination during meiosis is controlled at the molecular level will provide strategies for future-proofing plant breeding.

We have already identified many of the genes essential for homologous recombination in plants, but our previous analysis predicted the existence of an essential unknown gene. We have now identified this gene in the model plant Arabidopsis thaliana and named it ASYNAPSIS5 (ASY5) based on its phenotype. Our preliminary data shows that it is a fundamental gene for promoting homologous recombination in flowering plants. Therefore, the aim of this proposal is to characterise the role of the ASY5 protein by molecular, cytological, genetical and biochemical techniques. We will first determine how ASY5 interacts with already known genes/proteins and then quantify how ASY5 mediates homologous recombination in asy5 mutants. This will be followed by an investigation into how ASY5 stabilises meiosis in the polyploid plant Arabidopsis lyrata as well as identifying which gene variants are more effective at promoting this process. Finally, we will determine the heat sensitivity of ASY5 in A. thaliana to discover the underlying molecular mechanism of why it fails at higher temperatures with the aim of identifying variants that are more heat tolerant.

Meiosis is a specialised cell division required to halve the chromosome numbers during sexual reproduction. Meiosis is characterised by chromosome pairing, biogenesis of the chromosome axis/synaptonemal complex and homologous recombination, leading to the formation of crossovers. Crossovers are the points of genetic exchange between chromosomes and are required to tether homologous chromosomes together at meiotic metaphase I, thereby ensuring accurate chromosome segregation into daughter cells. We have previously identified many of the major genes controlling meiotic recombination in Arabidopsis thaliana, and now we have identified ASYNAPSIS5 (ASY5) as a novel plant meiotic axis/synaptonemal complex protein required for wild-type levels of crossovers. The aim of this proposal is to determine how ASY5 mediates biogenesis of the synaptonemal complex and influences crossover frequency and distribution in Arabidopsis using immunocytochemistry coupled with super-resolution microscopy and genetical approaches such as segregation of fluorescently tagged markers in pollen and seed. Molecular and biochemical analyses will be used to elucidate the interaction of ASY5 with known components of the chromosome axis, synaptonemal complex and recombination machinery. This will be followed by an investigation into how ASY5 stabilises meiosis in the polyploid plant Arabidopsis lyrata through genotype/phenotype association analysis and identify which alleles are more effective at promoting meiotic stability in tetraploids. Finally, we will determine the heat sensitivity of ASY5 in A. thaliana through a combination of immunolocalisation on meiotic chromosome spreads as well as RT-PCR of splice variants to discover the underlying molecular mechanisms of why the synaptonemal complex fails at higher temperatures, with the aim of identifying variants that are more heat tolerant.