ON THE COVER

It is now widely accepted that the plastids of green plants evolved from an endosymbiotic cyanobacterium. Although most free-living bacteria have peptidoglycans in their cell walls, it is believed that the plastids of green plants lost the endosymbiotic peptidoglycan during evolution. However, in the moss Physcomitrella patens, Hirano et al. (pages 1521–1532) show that knockout transformants for a homolog of the bacterial peptidoglycan-synthetic gene encoding d-alanine (d-Ala):d-Ala ligase display disrupted chloroplast division, and the normal phenotype is recovered by the addition of d-Ala-d-Ala. The cover photo shows a reconstructed three-dimensional image of apical (lower right) and subapical protonemal cells. Using a sensitive metabolic labeling method for peptidoglycan with a d-Ala-d-Ala dipeptide probe and click chemistry, the authors reveal that the plastid peptidoglycan (green) completely surrounded the chloroplasts (magenta) of the moss. These findings suggest that the plastids of basal land plants have a peptidoglycan wall containing d-amino acids.