Connecting the Plastid: Transporters of the Plastid Envelope and Their Role in Linking Plastidial with Cytosolic Metabolism

Plastids have a multitude of functions in eukaryotic cells, ranging from photosynthesis to storage, and a role in essential biosynthetic pathways. All plastids are of either primary or higher-order endosymbiotic origin. That is, either a photosynthetic cyanobacterium was integrated into a mitochondriate eukaryotic host cell (primary endosymbiosis) or a plastid-bearing eukaryotic cell merged with another eukaryotic cell (secondary or higher-order endosymbioses), thereby passing on the plastid between various eukaryotic lineages. For all of these endosymbioses to become functional, it was essential to establish metabolic connections between organelle and host cell. Here, we review the present understanding of metabolite exchange between plastids and the surrounding cytosol in the context of the endosymbiotic origin of plastids in various eukaryotic lineages. We show that only a small number of transporters that can be traced down to the primary endosymbiotic event are conserved between plastids of diverse origins.