A mysterious cloak: the peptidoglycan layer of algal and plant plastids

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 organization 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. Our view on its distribution keeps on changing and little is known regarding its molecular relevance, especially for 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 and 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 protein (PBP) orthologs in seed plants, which were previously thought to be absent from this clade. The 5-7 nm thick, and seemingly unchanged, PG layer armoring 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 PG layer of bryophyte and tracheophyte plastids. There are several issues to be explored regarding the composition, exact function, and biosynthesis of the PG layer in land plants. These issues arise from the fact that land plants seemingly lack certain genes that are believed to be crucial for PG layer production, even though they probably synthesize a PG layer.