The plastid of Toxoplasma gondii is divided by association with the centrosomes

Apicomplexan parasites harbor a single nonphotosynthetic plastid, the apico plast, which is essential for parasite survival. Exploiting Toxoplasma gond ii as an accessible system for cell biological analysis and molecular genet ic manipulation, we have studied how these parasites ensure that the plasti d and its 35-kb circular genome are faithfully segregated during cell divis ion. Parasite organelles were labeled by recombinant expression of fluoresc ent proteins targeted to the plastid and the nucleus, and time-lapse video microscopy was used to image labeled organelles throughout the cell cycle. Apicoplast division is tightly associated with nuclear and cell division an d is characterized by an elongated, dumbbell-shaped intermediate. The plast id genome is divided early in this process, associating with the ends of th e elongated organelle. A centrin-specific antibody demonstrates that the en ds of dividing apicoplast are closely linked to the centrosomes. Treatment with dinitroaniline herbicides (which disrupt microtubule organization) lea ds to the formation of multiple spindles and large reticulate plastids stud ded with centrosomes. The mitotic spindle and the pellicle of the forming d aughter cells appear to generate the force required for apicoplast division in Toxoplasma gondii. These observations are discussed in the context of a utonomous and FtsZ-dependent division of plastids in plants and algae.