The Chloroplast Envelope of Angiosperms Contains a Peptidoglycan Layer

Plastids in plants are assumed to have evolved from cyanobacteria as they have maintained several bacterial features. Recently, peptidoglycans, as bacterial cell wall components, have been shown to exist in the envelopes of moss chloroplasts. Phylogenomic comparisons of bacterial and plant genomes have raised the question of whether such structures are also part of chloroplasts in angiosperms. To address this question, we visualized canonical amino acids of peptidoglycan around chloroplasts of Arabidopsis and Nicotiana via click chemistry and fluorescence microscopy. Additional detection by different peptidoglycan-binding proteins from bacteria and animals supported this observation. Further Arabidopsis experiments with D-cycloserine and AtMurE knock-out lines, both affecting putative peptidoglycan biosynthesis, revealed a central role of this pathway in plastid genesis and division. Taken together, these results indicate that peptidoglycans are integral parts of plastids in the whole plant lineage. Elucidating their biosynthesis and further roles in the function of these organelles is yet to be achieved.

Automated summary

Plastids in plants are believed to have originated from cyanobacteria and retain bacterial characteristics. Recent research has found that peptidoglycans, a component of bacterial cell walls, exist in the envelopes of moss chloroplasts. The question arises whether similar structures are also present in chloroplasts of angiosperms. This study visualized peptidoglycan's canonical amino acids around chloroplasts in Arabidopsis and Nicotiana, supported by additional detection of peptidoglycan-binding proteins. Experiments with D-cycloserine and AtMurE knock-out lines in Arabidopsis confirmed the central role of this pathway in plastid genesis and division, indicating that peptidoglycans are integral parts of plastids in the entire plant lineage. The biosynthesis and further functions of these organelles remain to be elucidated.