Moss Chloroplasts Are Surrounded by a Peptidoglycan Wall Containing D-Amino Acids.

Takayuki Hirano,Shinji Tadano,Yasuhiro Shimizu,Koji Tanidokoro,Katsuaki Takechi,Toshihisa Ohshima,Hayato Ishikawa,Susumu Takio,Yutaka Kawarabayasi,Momo Sato,Hiroyoshi Takano

doi:10.1105/tpc.16.00104

Abstract

It is believed that the plastids in green plants lost peptidoglycan (i.e., a bacterial cell wall-containing d-amino acids) during their evolution from an endosymbiotic cyanobacterium. Although wall-like structures could not be detected in the plastids of green plants, the moss Physcomitrella patens has the genes required to generate peptidoglycan (Mur genes), and knocking out these genes causes defects in chloroplast division. Here, we generated P patens knockout lines (∆Pp-ddl) for a homolog of the bacterial peptidoglycan-synthetic gene encoding d-Ala:d-Ala ligase. ∆Pp-ddl had a macrochloroplast phenotype, similar to other Mur knockout lines. The addition of d-Ala-d-Ala (DA-DA) to the medium suppressed the appearance of giant chloroplasts in ∆Pp-ddl, but the addition of l-Ala-l-Ala (LA-LA), DA-LA, LA-DA, or d-Ala did not. Recently, a metabolic method for labeling bacterial peptidoglycan was established using ethynyl-DA-DA (EDA-DA) and click chemistry to attach an azide-modified fluorophore to the ethynyl group. The ∆Pp-ddl line complemented with EDA-DA showed that moss chloroplasts are completely surrounded by peptidoglycan. Our findings strongly suggest that the moss plastids have a peptidoglycan wall containing d-amino acids. By contrast, no plastid phenotypes were observed in the T-DNA tagged ddl mutant lines of Arabidopsis thaliana.

Highlights

The acquisition of two energy-supplying organelles, mitochondria and chloroplasts, were important events in the evolution of eukaryotic cells
These results suggest that the bacterial peptidoglycan biosynthesis pathway has a conserved function in plastid division in the basal streptophytes, including charophycean algae, bryophytes, and lycophytes, but not in angiosperms (Figure 1B)
Even in the wild-type chloroplasts, we did not observe peptidoglycan-like structures between the two envelopes. These results suggest that P. patens uses the D-Ala:D-Ala ligase (DDL) gene and D-amino acids in the plastid peptidoglycan biosynthesis pathway for plastid division, similar to the bacterial peptidoglycan system, and that new methods are necessary to observe plastid peptidoglycan

Summary

Introduction

The acquisition of two energy-supplying organelles, mitochondria and chloroplasts, were important events in the evolution of eukaryotic cells. It is widely accepted that a symbiotic association between a cyanobacterium and a mitochondriate host led to the origin of the plastids of primary photosynthetic eukaryotes, such as glaucophytes, red algae, and green plants (Keeling, 2010). Almost all free-living bacteria, including cyanobacteria, use peptidoglycan as part of their cell wall (Leganés et al, 2005). Peptidoglycan is a continuous covalent macromolecule composed of a sugar-amino acid polymer containing D-amino acids (Typas et al, 2011). Bacterial peptidoglycan confers mechanical resistance to osmotic pressure, maintains cell shape, and functions in cell division. The evolution of plastid morphology and division is

Methods

Results

Conclusion