Imaging-based screen identifies novel natural compounds that perturb cell and chloroplast division in Chlamydomonas reinhardtii

ABSTRACT Successful cell division requires faithful division and segregation of organelles into daughter cells. The unicellular alga Chlamydomonas reinhardtii has a single, large chloroplast whose division is spatiotemporally coordinated with furrowing. Cytoskeletal structures form in the same plane at the midzone of the dividing chloroplast (FtsZ) and the cell (microtubules), but how these structures are coordinated is not understood. Previous work showed that loss of F-actin blocks chloroplast division but not furrow ingression, suggesting that pharmacological perturbations can disorganize these events. In this study, we developed an imaging platform to screen natural compounds that perturb cell division while monitoring FtsZ and microtubules and identified 70 unique compounds. One compound, curcumin, has been proposed to bind to both FtsZ and tubulin proteins in bacteria and eukaryotes, respectively. In C. reinhardtii, where both targets coexist and are involved in cell division, curcumin at a specific dose range caused a severe disruption of the FtsZ ring in chloroplast while leaving the furrow-associated microtubule structures largely intact. Time-lapse imaging showed that loss of FtsZ and chloroplast division failure delayed the completion of furrowing but not the initiation, suggesting that the chloroplast-division checkpoint proposed in other algae requires FtsZ or is absent altogether in C. reinhardtii . SIGNIFICANCE STATEMENT Successful cell division requires the coordination of both organelle inheritance and cytokinesis. The unicellular alga Chlamydomonas reinhardtii , which spatiotemporally coordinates the division of its chloroplast with cytokinesis, is an excellent model to study the regulation. We screened libraries of natural compounds for perturbations of cell and/or chloroplast division, identifying 70 unique chemicals. By time-lapse microscopy using one of the hits, curcumin, we demonstrate that although chloroplast division failures delay the completion of cytokinesis, it does not impair initiation. These findings suggest that the chloroplast-division checkpoint proposed in other algae requires FtsZ or is absent altogether in C. reinhardtii .