A homologue of the bacterial cell division site-determining factor MinD mediates placement of the chloroplast division apparatus

Background: Chloroplast division in plant cells occurs by binary fission, y ielding two daughter plastids of equal size. Previously, we reported that t wo Arabidopsis homologues of FtsZ, a bacterial protein that forms a cytokin etic ring during cell division, are essential for plastid division in plant s, and may be involved in the formation of plastid-dividing rings on both t he stromal and cytosolic surfaces of the chloroplast envelope membranes. In bacteria, positioning of the FtsZ ring at the center of the cell is mediat ed in part by the protein MinD, Here, we identified AtMinD1, an Arabidopsis homologue of MinD, and investigated whether positioning of the plastid-div ision apparatus at the plastid midpoint might involve a mechanism similar t o that in bacteria. Results: Sequence analysis and in vitro chloroplast import experiments indi cated that AtMinD1 contains a transit peptide that targets it to the chloro plast. Transgenic Arabidopsis plants with reduced AtMinD1 expression exhibi ted Variability in chloroplast size and number and asymmetrically constrict ed chloroplasts, strongly suggesting that the plastid-division machinery is misplaced. Overexpression of AtMinD1 inhibited chloroplast division. These phenotypes resemble those of bacterial mutants with altered minD expressio n. Conclusions: Placement of the plastid-division machinery at the organelle m idpoint requires a plastid-targeted form of MinD. The results are consisten t with a model whereby assembly of the division apparatus is initiated insi de the chloroplast by the plastidic form of FtsZ, and suggest that position ing of the cytosolic components of the apparatus is specified by the positi on of the plastidic components.