From a monoplastidic ancestor to multifaceted polyplastidy in land plants

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The land plant cell, from root to leave tissue, houses dozens of plastids. The contrary is true for the majority of algae that are monoplastidic and house a single plastid per cell and nucleus. Polyplastidy is the norm for land plants, but an exception among algae. The transition from mono- to polyplastidy occurred a few times independently in evolution and appears to enable macroscopic phenotypes. The steps that allow escaping the monoplastidic bottleneck remain unexplored, but are essentially linked to the de-synchronization of the cell cycle and plastid division. The land plant ancestor was monoplastidic, as are extant streptophyte algae sister to the land plant ancestor. Traces of monoplastidy are evident in bryophytes such as Marchantia. This common liverwort undergoes a switch from poly- to monoplastidy during sporogenesis, such that the released sporocytes carry only one plastid. This, we speculate, is associated with a check-point in which inheritance of healthy plastids is guaranteed, also because meiosis only commences upon successful plastid division. We plan on exploring the genetics and mechanisms behind reduction and increase of plastid number per cell in the bryophyte. We will pay particular attention to the plastid division apparatus and involved proteins (MinD, MinE, FtsZ1-3, Drp5), and the peptidoglycan layer (and its related mur genes). We plan on examining the mono- to polyplastiy transition (monoplastidic bottleneck) by the interplay of the cell cycle and the plastid division apparatus. The release from the monoplastidic constrain was a critical step in evolving higher plants and the different types of plastids (pro-, chromo-, amyloplast, etc.) that embryophytes can mobilize. Marchantia offers a great model system, both because of its evident transition between poly- and monoplastidy as part of its life cycle (a unique window into organelle evolution and plastid inheritance), as well as the ease with which both the nuclear and plastid genomes can be manipulated.