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’.

Protonemacells of Physcomitrella patens
Protonema cells of Physcomitrella patens Image: Anja Martin, Ralf Reski / Wikipedia

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!

Further reading

Takayuki Hirano, Koji Tanidokoro, Yasuhiro Shimizu, Yutaka Kawarabayasi, Toshihisa Ohshima, Momo Sato, Shinji Tadano, Hayato Ishikawa, Susumu Takio, Katsuaki Takechi, Hiroyoshi Takano, 2016, 'Moss Chloroplasts are Surrounded by a Peptidoglycan Wall Containing D-Amino Acids', The Plant Cell, p. tpc.00104.2016 http://dx.doi.org/10.1105/tpc.16.00104

D. J. Cove, P.-F. Perroud, A. J. Charron, S. F. McDaniel, A. Khandelwal, R. S. Quatrano, 2009, 'The Moss Physcomitrella patens: A Novel Model System for Plant Development and Genomic Studies', Cold Spring Harbor Protocols, vol. 2009, no. 2, pp. pdb.emo115-pdb.emo115 http://dx.doi.org/10.1101/pdb.emo115

M. J. Prigge, M. Bezanilla, 2010, 'Evolutionary crossroads in developmental biology: Physcomitrella patens', Development, vol. 137, no. 21, pp. 3535-3543 http://dx.doi.org/10.1242/dev.049023

John M. Archibald, 2015, 'Endosymbiosis and Eukaryotic Cell Evolution', Current Biology, vol. 25, no. 19, pp. R911-R921 http://dx.doi.org/10.1016/j.cub.2015.07.055

Anna Karnkowska, Vojtěch Vacek, Zuzana Zubáčová, Sebastian C. Treitli, Romana Petrželková, Laura Eme, Lukáš Novák, Vojtěch Žárský, Lael D. Barlow, Emily K. Herman, Petr Soukal, Miluše Hroudová, Pavel Doležal, Courtney W. Stairs, Andrew J. Roger, Marek Eliáš, Joel B. Dacks, Čestmír Vlček, Vladimír Hampl, 2016, 'A Eukaryote without a Mitochondrial Organelle', Current Biology, vol. 26, no. 10, pp. 1274-1284 http://dx.doi.org/10.1016/j.cub.2016.03.053