No abstract

In most algae, the chloroplast division rate is held constant to maintain the proper number of chloroplasts per cell. By contrast, land plants evolved cell and chloroplast differentiation systems in which the size and number of chloroplasts change along with their respective cellular function by regulation of the division rate. Here, we show that PLASTID DIVISION (PDV) proteins, land plant-specific components of the division apparatus, determine the rate of chloroplast division. Overexpression of PDV proteins in the angiosperm Arabidopsis thaliana and the moss Physcomitrella patens increased the number but decreased the size of chloroplasts; reduction of PDV levels resulted in the opposite effect. The level of PDV proteins, but not other division components, decreased during leaf development, during which the chloroplast division rate also decreased. Exogenous cytokinins or overexpression of the cytokinin-responsive transcription factor CYTOKININ RESPONSE FACTOR2 increased the chloroplast division rate, where PDV proteins, but not other components of the division apparatus, were upregulated. These results suggest that the integration of PDV proteins into the division machinery enabled land plant cells to change chloroplast size and number in accord with the fate of cell differentiation.

The phylogenetic positions of bryophytes and charophytes, together with their genome features, are important for understanding early land plant evolution. Here we report the complete nucleotide sequence (105,340 bp) of the circular-mapping mitochondrial DNA of the moss Physcomitrella patens. Available evidence suggests that the multipartite structure of the mitochondrial genome in flowering plants does not occur in Physcomitrella. It contains genes for 3 rRNAs (rnl, rns, and rrn5), 24 tRNAs, and 42 conserved mitochondrial proteins (14 ribosomal proteins, 4 ccm proteins, 9 nicotinamide adenine dinucleotide dehydrogenase subunits, 5 ATPase subunits, 2 succinate dehydrogenase subunits, apocytochrome b, 3 cytochrome oxidase subunits, and 4 other proteins). We estimate that 5 tRNA genes are missing that might be encoded by the nuclear genome. The overall mitochondrial genome structure is similar in Physcomitrella, Chara vulgaris, Chaetosphaeridium globosum, and Marchantia polymorpha, with easily identifiable inversions and translocations. Significant synteny with angiosperm and chlorophyte mitochondrial genomes was not detected. Phylogenetic analysis of 18 conserved proteins suggests that the moss-liverwort clade is sister to angiosperms, which is consistent with a previous analysis of chloroplast genes but is not consistent with some analyses using mitochondrial sequences. In Physcomitrella, 27 introns are present within 16 genes. Nine of its intron positions are shared with angiosperms and 4 with Marchantia, which in turn shares only one intron position with angiosperms. The phylogenetic analysis as well as the syntenic structure suggest that the mitochondrial genomes of Physcomitrella and Marchantia retain prototype features among land plant mitochondrial genomes.

Chloroplasts evolved from a cyanobacterial endosymbiont, and chloroplast division requires the formation of an FtsZ division ring, which is descended from the cytokinetic machinery of cyanobacteria. As in bacteria, the positioning of the chloroplast FtsZ ring is regulated by the proteins MinD and MinE. However, chloroplast division also involves mechanisms invented by the eukaryotic host cell. Here we show that a plant-specific protein MULTIPLE CHLOROPLAST DIVISION SITE 1 (MCD1) regulates FtsZ ring positioning in Arabidopsis thaliana chloroplasts. Our analyses show that both MCD1 and MinD are required for chloroplast division, localizing at the division sites and punctate structures dispersed on the inner envelope. MinD overexpression inhibited FtsZ ring formation whereas MCD1 overexpression did not. Localization studies suggest that MCD1 is required for MinD localization to regulate FtsZ ring formation. Furthermore, the interaction between MCD1 and MinD in yeast two-hybrid assays suggests that MCD1 recruits MinD by direct interaction. These results point out differences in the MinD localization mechanism between chloroplasts and bacterial model systems and suggest that the plant cell evolved a component to modulate the cyanobacteria-derived Min system so as to regulate chloroplast FtsZ ring positioning.