Organelle Development and Inheritance are Driven by Independent Nuclear and Organellar Mechanisms in Malaria Parasites

The apicoplast organelle of Plasmodium falciparum is essential for parasites replication, however the details of its biogenesis, inheritance and regulation throughout the cell cycle are unknown. Here, we report the development of a dynamic imaging platform coupled with an analytical pipeline that enables us to follow and measure subcellular structures throughout the 48-hour cell cycle of live parasites. We reveal a predetermined sequence of four discrete morphological steps in organelle development, which are tightly correlated with nuclear replication. We show that one of these steps, which we term the Crown morphology, is required for nucleus-apicoplast attachment. During Crown, apicoplast is stretched over multiple nuclei, fastened by centriolar tubulin. A complementary molecular approach was used to discover the basic ploidy of apicoplast and mitochondrial genomes, their replication rates and association with nuclear DNA replication. We inhibited nuclear DNA replication and found that it completely blocks apicoplast biogenesis in its most initial stages, demonstrating dependency on S-phase initiation. Conversely, specific inhibition of apicoplast genome replication resulted in an almost-undisturbed organelle development and division. However, it affected the Crown step, preventing association to tubulin-nuclear structures, leading to failure in accurate organelle sorting into daughter cells. Collectively, these experiments reveal a central cellular pathway linking apicoplast development to the parasites cell cycle, and a second independent organellar mechanism responsible for segregation into daughter cells.