Tunicated corpuscle theory

You can’t escape your heritage; we all go through life with an assortment of ‘baggage’ that betrays our origins, whether it’s our accent, cultural preferences or clothing. And, as regards the latter, the same can be said for chloroplasts, those tiny photosynthetic, cytoplasmic, ‘corpuscles’.

Although there is overwhelming evidence for the Serial Endosymbiotic Theory – e.g. that the chloroplasts of eukaryotic cells were once free-living prokaryotic organisms – it’s always encouraging to get more. It’s therefore rather pleasing to learn that Takayuki Hirano et al. have demonstrated the presence of a peptidoglycan ‘wall’, or tunic, around the chloroplasts of the moss Physcomitrella patens.

It seems that this outer ‘tunic’ is a legacy of the external structure of the putative prehistoric, protochloroplastic prokaryotic predecessor. Probably located between the inner and outer membranes of the moss chloroplast envelope (as you’d expect from such an engulfment origin for this organelle), the tunic crucially contains D-amino acids. Whilst L-amino acids are the predominant form occurring in biological molecules, D-amino acids are basic components of bacterial – i.e. prokaryotic – peptidoglycan. Furthermore, and again consistent with a prokaryotic history, defects in the moss genes for production of the peptidoglycan result in imperfections in chloroplast division (amongst other roles, bacterial peptidoglycan functions in cell division). This bacterial peptidoglycan biosynthesis pathway is not found in angiosperms, suggesting that loss of plastid peptidoglycan occurred during flowering plant evolution.

How long before we discover the ‘outer tunic’ of the mitochondrion, another organelle that is likely to have an endosymbiotic serial sequestration origin? But, it’s no good looking for it in the oxymonad Monocercomonoides sp.. Why? Because it doesn’t have mitochondria!