Plastid Autonomy vs Nuclear Control Over Plastid Function

Plastids stem from free-living cyanobacteria. The transition from endosymbiont to organelle involved strong reductive evolution. Modern-day plastid genomes possess only a small fraction of the genes present in their cyanobacterial progenitors. In addition to genome reduction, plastids underwent modifications that facilitated recruitment of host-derived proteins and metabolites; both processes contributed to organellogenesis and a shift in control over plastid function from the organellar genome to that of the host. It is likely that most of the modifications to the early plastid happened before the major radiations that led to today's algae and plants. Plastids nevertheless exhibit substantial variation in form and function. In this chapter, we highlight some of the evolutionary implications of the differences in the genetic capacities of plastids across the breadth of plant and algal diversity. We focus on the transition from genetic semiautonomy, which is of relevance in the context of the endosymbiotic spread of plastids and kleptoplasty, to the high degree of nuclear control over plastid function seen in land plants. Genomic and transcriptomic investigations of diverse plants and algae have revealed important differences in the coding capacity of plastid genomes in different lineages, raising questions about how the plastid's own genetic capabilities impact its physiology as well as that of its host.