Vestigial Plastids in Parasitic Plants: Evolutionary Remnants or Adaptive Innovations?

Throughout the evolutionary history of plants, chloroplasts originating from a cyanobacterial endosymbiosis have undergone remarkable adaptation and specialization, giving rise to a multitude of plastid types. The evolution toward parasitism in plants represents a particularly extreme case of such specialization. In holoparasitic species, the relaxation of selective pressures to maintain photosynthesis has led to the progressive loss of photosynthesis-related genes and the emergence of vestigial plastids. These organelles are often highly reduced in size, frequently devoid of thylakoids and can accumulate lipids, pigments, proteins and/or starch. The study of these reduced plastids in parasitic plants is conceptually challenging yet offers a unique opportunity to uncover fundamental principles of plastid biology during evolution. In this review, we discuss recent progress in understanding the biology of these vestigial plastids, exploring their unique morphology and functions, and how photosynthesis, photoprotection and retrograde signalling have evolved. Finally, we highlight the advantages of shifting such key physiological traits for adaptation into new ecological niches for the evolutionary success of holoparasitic plants.