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Investigation of cell wall integrity signalling in Chlamydomonas reinhardtii Cronmiller, Evan Thomas Stephen


A fundamental aspect of plant survival is the development of a resilient cell wall. Throughout the life cycle of the unicellular alga Chlamydomonas reinhardtii, the cell wall is continuously synthesized and remodelled. When cells are starved of nitrogen, they differentiate into gametes and enter the sexual (meiotic) phase. In doing this, mating gamete cells shed their cell walls via gametolysin, fuse their naked protoplasts, and build an exceptionally hardy zygote cell wall. Previous research on the sexual life cycle has focused on regulatory events upstream of the mating reaction, i.e. gametogenesis, and downstream of cell fusion during zygote development. As such, we don’t yet understand how cells are responding to wall shedding at the onset of the mating reaction. A recent transcriptome analysis has reported a set of genes that are specifically regulated by gametolysin-mediated wall shedding, consisting of two major functional categories – protein processing-related and cell wall-related. The research presented here attempts to understand the level at which these genes are regulated and establishes three hypotheses for what the ‘trigger’ of their regulation might be. Using a suite of target genes as representative factors of the gametolysin-mediated event, we performed promoter-reporter and mRNA expression analyses in several treatment conditions and cell lines to examine this regulation. We found both target genes and their respective promoters are strongly induced by gametolysin, indicating transcription-level regulation. When cells were pre-treated with a protein synthesis inhibitor prior to gametolysin, we observed differential effects on the transcription of our target genes, suggesting multiple regulatory proteins control this event. We also observed that an ER stress response likely helps to regulate cell wall recovery, as a stress response-impaired mutant showed diminished transcript expression following gametolysin treatment. Cell wall-defective mutants were found to have a negligible response to gametolysin, but expressed our target genes constitutively, indicating the need for cells to maintain a cell wall. Early investigations of our hypotheses for the ‘trigger’ of the gametolysin-mediated response suggests a minor role for osmotic stress signalling and that sensing the condition of the cell wall is likely the primary ‘trigger.’

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