When Katherine Osteryoung started her postdoctoral fellowship in Elizabeth Vierling's laboratory at the University of Arizona, Tucson, she wanted to work on heat shock proteins, which cells make in response to stress. Chloroplast division was the farthest thing from her mind. But science, which is no stranger to serendipity, had other plans.

Plastids, including chloroplasts, are descendants of cyanobacterial endosymbionts. Like their ancestors, they divide by fission, ensuring continuity through succeeding generations and amplification during specialization of tissues such as the photosynthetic cells of leaves. But how plastids split is still a mystery. A number of years ago, two filamentous rings were discovered in the constriction furrow of dividing chloroplasts, one inside the organelle next to the inner of the two bounding membranes and the other directly opposite in the cytoplasm.1 The rings may help power constriction, perhaps by acting as purse strings. The presence of similar filaments...

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