To Be or Not to Be: Does Bacterial Peptidoglycan Remain in the Chloroplast Envelope of Vascular Plants?

Abstract

Given that plant plastids retain many bacterial characteristics, it is believed that they originated from cyanobacteria through endosymbiosis. Peptidoglycan, an essential bacterial cell wall component, was once thought to be absent in plastids. However, recent findings have demonstrated its presence in the chloroplast envelope of mosses. Phylogenomic comparisons between bacterial and plant genomes led to the question of whether similar structures exist in angiosperm chloroplasts. With click chemistry and fluorescence microscopy, canonical peptidoglycan amino acids surrounding the chloroplasts of A. thaliana and N. benthamiana were visualized. Transient expression studies and fluorescence microscopy visualization of several peptidoglycan-binding proteins from bacteria and animals acting as intrabodies supported this finding. The effects of peptidoglycan-digesting enzymes and D-cycloserine (DCS) on A. thaliana were also exam- ined in this study. The results demonstrated that plastid morphology is altered by substances that interfere with the peptidoglycan structure. Additional research on A. thaliana ddl and murE knockout lines, which are both involved in peptidoglycan biosynthesis, highlighted the importance of this pathway in plastid genesis and division. Overall, this study provides evidence that peptidoglycan is present in the chloroplast enve- lope of angiosperms and plays a significant role in chloroplast genesis, indicating conservation and adaptation of this structural element in the evolution of angiosperms.