The first plastid evolved from an endosymbiotic cyanobacterium in the common ancestor of the Archaeplastida. The transformative steps from cyanobacterium to organelle included the transfer of control over developmental processes, a necessity for the host to orchestrate, for example, the fission of the organelle. The plastids of almost all embryophytes divide independently from nuclear division, leading to cells housing multiple plastids. Hornworts, however, are monoplastidic (or near-monoplastidic), and their photosynthetic organelles are a curious exception among embryophytes for reasons such as the occasional presence of pyrenoids. In this study, we screened genomic and transcriptomic data of eleven hornworts for components of plastid developmental pathways. We found intriguing differences among hornworts and specifically highlight that pathway components involved in regulating plastid development and biogenesis were differentially lost in this group of bryophytes. Our results also confirmed that hornworts underwent significant instances of gene loss, underpinning that the gene content of this group is significantly lower than other bryophytes and tracheophytes. In combination with ancestral state reconstruction, our data suggest that hornworts have reverted back to a monoplastidic phenotype due to the combined loss of two plastid division-associated genes, namely, ARC3 and FtsZ2.

The first plastid evolved from an endosymbiotic cyanobacterium in the common ancestor of the Archaeplastida. The transformative steps from cyanobacterium to organelle included the transfer of control over developmental processes; a necessity for the host to orchestrate, for example, the fission of the organelle. The plastids of almost all embryophytes divide independent from nuclear division, leading to cells housing multiple plastids. Hornworts, however, are monoplastidic (or near-monoplastidic) and their photosynthetic organelles are a curious exception among embryophytes for reasons such as the occasional presence of pyrenoids. Here we screened genomic and transcriptomic data of eleven hornworts for components of plastid developmental pathways. We find intriguing differences among hornworts and specifically highlight that pathway components involved in regulating plastid development and biogenesis were differentially lost in this group of bryophytes. In combination with ancestral state reconstruction, our data suggest that hornworts have reverted back to a monoplastidic phenotype due to the combined loss of two plastid division-associated genes: ARC3 and FtsZ2.

The plastids of algae and plants originated on a single occasion from an endosymbiotic cyanobacterium at least a billion years ago. Despite the divergent evolution that characterizes the plastids of different lineages, many traits such as membrane organisation and means of fission are universal - they pay tribute to the cyanobacterial origin of the organelle. For one such trait, the peptidoglycan (PG) layer, the situation is more complicated, and little is known about its distribution and molecular relevance in green algae and land plants. Here, we investigate the extent of PG presence across the Chloroplastida using a phylogenomic approach. Our data support the view of a PG layer being present in the last common ancestor of land plants and its remarkable conservation across bryophytes that are otherwise characterized by gene loss. In embryophytes, the occurrence of the PG layer biosynthetic toolkit becomes patchier, but the availability of novel genome data questions previous predictions regarding a functional coevolution of the PG layer and the plastid division machinery-associated gene FtsZ3. Furthermore, our data confirm the presence of penicillin-binding proteins (PBPs) in seed plants, which were previously thought to be absent from this clade. The thicker and seemingly unchanged PG layer armouring the plastids of glaucophyte algae might still provide the original function of structural support, but the same can likely not be said about the only recently identified and ultrathin PG layer of bryophyte and tracheophyte plastids. In combination with the apparent lack of some genes thought critical for PG layer biosynthesis in land plants that, however, likely have a PG layer, this leaves many issues with respect to the composition, exact function, and biosynthesis in land plants to be explored.

The ability for photosynthetic organisms to protect themselves from high levels of ultraviolet (UV) radiation has been a critical for their survival and evolution. To overcome this problem, some of these organisms either occupy habitats where low levels of UV radiation occur or have acquired various receptor mechanisms that protect their nucleic acids, photosynthetic machineries, and their epidermis. In green plants (Viridiplantae), photoprotective responses are predominantly mediated via photoreceptors. Initially discovered and isolated in the cress Arabidopsis thaliana, UV RESISTANCE LOCUS 8 (UVR8) is a photoreceptor that is specific to UV-B light and is mainly involved in switching on the UV-B-specific photoprotective and photomorphogenic responses in green plants. Here, I review biochemical, physiological and phylogenomic data to examine the evolution and diversity of the UVR8-mediated signalling response from algae to angiosperms. I speculate that UVR8 was necessary to act as an enabler for major transitions in green plant evolution – from chlorophyte freshwater adaptations to streptophyte terrestrialization – and discuss causal links that could motivate further research.