Ampicillin Inhibits Chloroplast Division in Cultured Cells of the Liverwort Marchantia polymorpha

The peptidoglycan synthase PBP interacts with PLASTID DIVISION2 to promote chloroplast division in Physcomitrium patens

The peptidoglycan (PG) layer, a core component of the bacterial cell wall, has been retained in the Physcomitrium patens chloroplasts. The PG layer entirely encompasses the P. patens chloroplast, including the division site, but how PG biosynthesis cooperates with the constriction of two envelope membranes at the chloroplast division site remains elusive. Here, focusing on the PG synthase penicillin-binding protein (PBP), we performed cytological and molecular analyses to dissect the mechanism of chloroplast division in P. patens. We showed that PBP, acting in the final step of PG biosynthesis, is likely a chloroplast inner envelope protein that can aggregate at mid-chloroplasts during chloroplast division. Physcomitrium patens had five orthologs of PLASTID DIVISION2 (PDV2), an outer envelope component of the chloroplast division complex. Our data indicated that PpPDV2 proteins interact with PpPBP and are responsible for recruiting PpPBP to the chloroplast division site, in addition to PpDRP5B. Furthermore, we found that PBP deletion and carbenicillin application restrain constriction of the chloroplast division complex, rather than its assembly. This work provides direct molecular evidence for a link between chloroplast division of P. patens and PG biosynthesis and indicates that PG biosynthesis is required for the constriction of the chloroplast division apparatus in P. patens.

Genes Sufficient for Synthesizing Peptidoglycan are Retained in Gymnosperm Genomes, and MurE from Larix gmelinii can Rescue the Albino Phenotype of Arabidopsis MurE Mutation

The endosymbiotic theory states that plastids are derived from a single cyanobacterial ancestor that possessed a cell wall. Peptidoglycan (PG), the main component of the bacteria cell wall, gradually degraded during plastid evolution. PG-synthesizing Mur genes have been found to be retained in the genomes of basal streptophyte plants, although many of them have been lost from the genomes of angiosperms. The enzyme encoded by bacterial MurE genes catalyzes the formation of the UDP-N-acetylmuramic acid (UDP-MurNAc) tripeptide in bacterial PG biosynthesis. Knockout of the MurE gene in the moss Physcomitrella patens resulted in defects of chloroplast division, whereas T-DNA-tagged mutants of Arabidopsis thaliana for MurE revealed inhibition of chloroplast development but not of plastid division, suggesting that AtMurE is functionally divergent from the bacterial and moss MurE proteins. Here, we could identify 10 homologs of bacterial Mur genes, including MurE, in the recently sequenced genomes of Picea abies and Pinus taeda, suggesting the retention of the plastid PG system in gymnosperms. To investigate the function of gymnosperm MurE, we isolated an ortholog of MurE from the larch, Larix gmelinii (LgMurE) and confirmed its presence as a single copy per genome, as well as its abundant expression in the leaves of larch seedlings. Analysis with a fusion protein combining green fluorescent protein and LgMurE suggested that it localizes in chloroplasts. Cross-species complementation assay with MurE mutants of A. thaliana and P. patens showed that the expression of LgMurE cDNA completely rescued the albefaction defects in A. thaliana but did not rescue the macrochloroplast phenotype in P. patens. The evolution of plastid PG and the mechanism behind the functional divergence of MurE genes are discussed in the context of information about plant genomes at different evolutionary stages.

Classification, naming and evolutionary history of glycosyltransferases from sequenced green and red algal genomes

The Archaeplastida consists of three lineages, Rhodophyta, Virideplantae and Glaucophyta. The extracellular matrix of most members of the Rhodophyta and Viridiplantae consists of carbohydrate-based or a highly glycosylated protein-based cell wall while the Glaucophyte covering is poorly resolved. In order to elucidate possible evolutionary links between the three advanced lineages in Archaeplastida, a genomic analysis was initiated. Fully sequenced genomes from the Rhodophyta and Virideplantae and the well-defined CAZy database on glycosyltransferases were included in the analysis. The number of glycosyltransferases found in the Rhodophyta and Chlorophyta are generally much lower then in land plants (Embryophyta). Three specific features exhibited by land plants increase the number of glycosyltransferases in their genomes: (1) cell wall biosynthesis, the more complex land plant cell walls require a larger number of glycosyltransferases for biosynthesis, (2) a richer set of protein glycosylation, and (3) glycosylation of secondary metabolites, demonstrated by a large proportion of family GT1 being involved in secondary metabolite biosynthesis. In a comparative analysis of polysaccharide biosynthesis amongst the taxa of this study, clear distinctions or similarities were observed in (1) N-linked protein glycosylation, i.e., Chlorophyta has different mannosylation and glucosylation patterns, (2) GPI anchor biosynthesis, which is apparently missing in the Rhodophyta and truncated in the Chlorophyta, (3) cell wall biosynthesis, where the land plants have unique cell wall related polymers not found in green and red algae, and (4) O-linked glycosylation where comprehensive orthology was observed in glycosylation between the Chlorophyta and land plants but not between the target proteins.

The glycosyltransferase repertoire of the spikemoss Selaginella moellendorffii and a comparative study of its cell wall

Spike mosses are among the most basal vascular plants, and one species, Selaginella moellendorffii, was recently selected for full genome sequencing by the Joint Genome Institute (JGI). Glycosyltransferases (GTs) are involved in many aspects of a plant life, including cell wall biosynthesis, protein glycosylation, primary and secondary metabolism. Here, we present a comparative study of the S. moellendorffii genome across 92 GT families and an additional family (DUF266) likely to include GTs. The study encompasses the moss Physcomitrella patens, a non-vascular land plant, while rice and Arabidopsis represent commelinid and non-commelinid seed plants. Analysis of the subset of GT-families particularly relevant to cell wall polysaccharide biosynthesis was complemented by a detailed analysis of S. moellendorffii cell walls. The S. moellendorffii cell wall contains many of the same components as seed plant cell walls, but appears to differ somewhat in its detailed architecture. The S. moellendorffii genome encodes fewer GTs (287 GTs including DUF266s) than the reference genomes. In a few families, notably GT51 and GT78, S. moellendorffii GTs have no higher plant orthologs, but in most families S. moellendorffii GTs have clear orthologies with Arabidopsis and rice. A gene naming convention of GTs is proposed which takes orthologies and GT-family membership into account. The evolutionary significance of apparently modern and ancient traits in S. moellendorffii is discussed, as is its use as a reference organism for functional annotation of GTs.

An integrated physiological and genetic approach to the dynamics of FtsZ targeting and organisation in a moss, Physcomitrella patens

Plant FtsZ (filamentous temperature-sensitive Z) proteins are regarded as descendants of prokaryotic cell division proteins. We could show previously that four FtsZ isoforms of the moss Physcomitrella patens assemble into, and interact in, distinct structures inside the chloroplasts and in the cytosol. Their organisation and localisation patterns indicate an involvement in chloroplast and cell division and in the maintenance of chloroplast shape and integrity. The cellular processes of chloroplast division and maintenance of chloroplast shape were disturbed either by application of the beta-lactam antibiotic ampicillin or by a mutation that presumably affects signal transduction of the plant hormone cytokinin. When cells of these plants were analysed microscopically, there was no indication that cytosolic functions of FtsZ proteins were affected. Furthermore, FtsZ proteins continued to build three-dimensional plastoskeleton networks, even in considerably enlarged or malformed chloroplasts. On the other hand, macrochloroplast formation promoted the localisation of FtsZ proteins in filaments that emanate from the plastids and, therefore, most likely represent stromules. Annular FtsZ structures that are regarded as essential components of the division apparatus were absent from macrochloroplasts of ampicillin-treated cells. Thus, the distribution of FtsZ proteins after inhibition of chloroplast division further strengthens our hypothesis on the functions of distinct isoforms. In addition, the results provide further insight into the regulation of protein targeting and dynamics of plastoskeletal elements.

Genes for the peptidoglycan synthesis pathway are essential for chloroplast division in moss

The general consensus is that a cyanobacterium phagocytosed by a host cell evolved into the plastids of red and green algae, land plants, and glaucophytes. In contrast to the plastids of glaucophytes, which retain a cyanobacterial-type peptidoglycan layer, no wall-like structures have been detected in plastids from other sources. Although the genome of Arabidopsis thaliana contains five genes that are essential for peptidoglycan synthesis, MurE, MurG, two genes for D-Ala-D-Ala ligase (Ddl), and the gene for translocase I (MraY), their functions have not been determined. We report that the moss Physcomitrella patens has nine homologous genes related to peptidoglycan biosynthesis: MurA, B, C, D, E, and F, Ddl, genes for the penicillin-binding protein Pbp, and dd-carboxypeptidase (Dac). Corroborating a computer prediction, analysis of the GFP fusion proteins with the N terminus of PpMurE or of PpPbp suggests that these proteins are located in the chloroplasts. Gene disruption of the PpMurE gene in P. patens resulted in the appearance of macrochloroplasts both in protonema and in leaf cells. Moreover, gene knockout of the P. patens Pbp gene showed inhibition of chloroplast division in this moss; however, no Pbp gene was found in A. thaliana.

Effects of antibiotics that inhibit the bacterial peptidoglycan synthesis pathway on moss chloroplast division

Moss chloroplasts should prove useful for studying the cyanobacteria-derived system in chloroplasts. To determine the effects of antibiotics that inhibit bacterial peptidoglycan synthesis, the numbers of chloroplasts in treated Physcomitrella patens cells were counted. Ampicillin and D-cycloserine caused a rapid decrease in the number of chloroplasts per cell. Fosfomycin affected half of the cells, while vancomycin affected a few cells. Conversely, bacitracin had no effect. With the decrease in chloroplast number, macrochloroplasts appeared in antibiotic-treated cells. Removal of the antibiotics resulted in the recovery of chloroplast number, suggesting that the decrease in number was directly dependent on the antibiotic treatment. Microscopic observations showed that the decrease in the number of chloroplasts resulted from cell division without chloroplast division. These results suggest that enzymes derived from the bacterial peptidoglycan synthesis pathway are related to moss chloroplast division.