Coordination of meiotic recombination and prophase I progression in plants: the role of retinoblastoma (RBR)

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It is predicted that as a result of population increase, industrialization and climate change global demand for food will double by 2050 (Reaping the Benefit: Royal Society Review 2009). To meet this challenge it will be necessary to develop new crop varieties that are improved in various ways, for example, increased nutritional value and yield and tolerance to biotic and abiotic stresses. Although GM has its part to play, the development of new varieties will remain highly dependent on methodologies derived from traditional breeding methods which are reliant on meiotic recombination to generate variation through the formation of genetic crossovers (COs) which results in new combinations of genes. Understanding the factors that control meiotic recombination is of great significance for the improvement of crop-breeding since it is now clear that many species notably cereals, possess large regions on their chromosomes that rarely recombine. This presents a significant barrier for the introgression of new genetic traits. Hence, to overcome this problem we need to know how the frequency and distribution of COs are controlled. In addition an estimated 50% of plants are polyploid, which creates an additional level of meiotic regulation that we need to understand. Studies indicate that the controlled formation of COs is dependent on the interplay between the proteins that catalyse recombination and those that regulate the extensive remodelling of chromosomes during prophase I of meiosis. How these processes are coordinated remains poorly understood. Recently, we obtained the first evidence in any organism, that the retinoblastoma protein Rb (RBR in plants) plays an essential role in the control of meiotic recombination. The function of Rb in mitotic cell-cycle control and as a tumour-suppressor has been extensively studied. However, it is generally difficult to study its role in development in adult organisms as loss of Rb results in lethality during embryogenesis. Using a specific rbr mutant we have been able to overcome this problem. Our studies reveal an important coordinating role for RBR in meiosis through a direct interaction with the meiotic chromosomes at the sites of recombination. We now propose to investigate how RBR exerts this coordinating role. In particular we aim to establish how RBR links with components of the cell-cycle machinery to ensure that chromosome remodelling occurs in a timely fashion in relation to meiotic recombination and investigate if it functions as "sensor" to link meiosis with changes in temperature. Although studies will primarily be conducted in the model plant Arabidopsis, as this is the most experimentally tractable system, these will be complimented by additional work in crop species. Our experimental strategy will be based around molecular cytogenetics using antibodies that recognize key meiotic proteins combined with high resolution light microscopy to study meiotic prophase I in wild-type plants and a range of meiotic mutants, including a line lacking RBR. Interactions between RBR, meiotic proteins and cell-cycle components will be studied using mass-spectrometry to analysis protein complexes that have been precipitated from meiocytes using an anti-RBR antibody. Yeast two-hybrid analysis will be used as an alternative strategy and to confirm putative interactions. A further aspect of the analysis will be to investigate the role of RBR in coordinating chromosome pairing and recombination in polyploid organisms. Specifically, we will investigate the relationship between RBR and the function of the Ph1 locus in wheat which is important for the control of chromosome pairing in this hexaploid species. We anticipate these studies will provide important new insights into the control of recombination during meiosis that will be of benefit for plant breeding by providing approaches to enable changes in CO frequency and distribution.