Reduced Genomes from Parasitic Plant Plastids: Templates for Minimal Plastomes?

Plastids are the characteristic cell organelles of plants. While movement, loss, and replacement of whole plastids have occurred in single-celled algae and some parasites derived thereof, land plants have shown more moderate twists in plastid evolution. Here, the most poignant deviations are the reduction in size and coding capacity of plastid genomes as a consequence of a heterotrophic lifestyle in haustorial parasites and mycoheterotrophic plants, which will be broadly summarized in this article as “parasitic plants”. While the loss of photosynthesis genes can be easily explained with the vanishing of a photoautotrophic lifestyle, other gene losses are more difficult to reconcile with persisting regulatory and metabolic functions of the reduced plastids. An assessment of plastid gene essentiality using tobacco plastome mutants revealed that the catalog of losses even includes genes for the gene expression apparatus that are essential for cell viability under heterotrophic conditions. We will discuss whether these genes really are dispensable and to what degree minimal parasitic plant plastomes could be blueprints for artificial plastid genomes.