The protein phosphatase family BSL is highly conserved and universally present in all green plants, from single-celled algae to multicellular land plants. Genetic evidence shows that it plays an essential, albeit completely unknown, role in plant cell biology. The model plant Arabidopsis has four BSL genes, but genetic redundancy coupled with embryonic lethality of mutants has seriously hampered functional studies of the family. In contrast, the green alga Chlamydomonas reinhardtii has only a single BSL homolog and the recent isolation of a temperature-conditional mutation (bsl1-1) in this gene generated a unique tool for rapid switching of BSL activity to study its molecular/celular functions. Detailed analysis of the bsl1-1 mutant identified a role for BSl1 in cell division in Chlamydomonas, specifically in mitotic progression. This phenotype is also consistent with the reduced meristem size observed in BSL knock-down in Arabidopsis and suggests an evolutionarily conserved role for the BSL phosphatase in cell cycle regulation. In the proposed project I will first perform a comprehensive proteomic/biochemical analysis of BSL1 in Chlamydomonas to identify the mechanistic basis for its key role in mitosis. This analysis with include (i) a phosphoproteomic screen to look for substrates and downstream effectors of the BSL1 phosphatase, and (ii) a characterization the BSL1 interactome using a variety of immunoprecipitation-based methods. I will then, during the return phase, translate the findings from Chlamydomonas into Arabidopsis by studying the role of BSL during root meristem development using a combination of microscopic and genetic approaches. Given the conservation of the BSL family throughout the viridiplantae, the results from this study are likely to have a broad impact on the plant biology research community.
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