An analysis of the expression, function, and evolution of the FtsZ plastid division genes

The photosynthetic capabilities of plants cells are dependent on the presence and maintenance of chloroplasts. The chloroplast complement of higher plant mesophyll cells, often very numerous, is maintained by division as cells differentiate and expand. Chloroplasts are evolutionarily derived from cyanobacteria through an endosymbiotic event and continue to bear several ancestral characteristics. The prokaryotic origins became even more evident when a chloroplast-targeted homologue of the bacterial cell division protein FtsZ was identified in plants. In bacteria, FtsZ is the first known protein to assemble at the division site. FtsZ has GTPase activity and polymerizes into a ring that encircles the cell at the cyoplasmic membrane surface. Following assembly of the FtsZ ring at the division site, several other proteins are recruited for assembly of a functional cell division apparatus.;In contrast to most bacteria that encode a single FtsZ gene, plants have multiple nuclear encoded FtsZ proteins that are localized to the chloroplast. The plant homologues have been grouped into two families, FtsZ1 and FtsZ2, based on sequence comparisons. Arabidopsis plants express three different FtsZ homologues: one FtsZ1 family member, AtFtsZ1-1, and two FtsZ2 family members, AtFtsZ2-1 and AtFtsZ2-2. Antisense repression experiments indicate members of both families are required for chloroplast division, suggesting that chloroplasts have evolved a more complex division apparatus than is present in bacteria.;In an effort to better understand the role of the FtsZ1 and FtsZ2 proteins in chloroplast division, experiments were designed to investigate their function, expression, and evolution. To investigate some of the FtsZ functions in plastid division, the FtsZ proteins were overexpressed in Arabidopsis and the effects on chloroplast division were observed. The results indicate a stoichiometric balance is required for division and that disruption of that balance inhibits chloroplast division. Experiments with promoter-GUS fusion constructs were used to determine where and when FtsZ is expressed in Arabidopsis. Some of the FtsZ expression patterns were confirmed by measuring the cDNA distribution patterns. The results indicate the three FtsZ homologues are coordinately expressed in several plant tissues including roots, meristems, and young leaves. FtsZ expression occurs in tissue regions with rapidly dividing chloroplast populations and is consistent with the role of FtsZ in chloroplast division.;In an effort to understand when and why chloroplasts evolved two FtsZ families, we performed phylogenetic analyses, compared genetic structures, and compared conserved protein sequences. Phylogenetic analyses indicate the FtsZ1 and FtsZ2 sequences diverged before the split between the chlorophycean and charophycean green algal lineages and possibly earlier. Genetic structure comparisons reveal intron positions are conserved within the FtsZ1 and FtsZ2 family members but differ between them, also supporting an early divergence. Comparison of conserved protein sequences indicated several regions in which the FtsZ1 and FtsZ2 family members differ. These conserved differences may define functional differences between the FtsZ1 and FtsZ2 proteins.