Chloroplast division and morphology are differentially affected by overexpression of FtsZ1 and FtsZ2 genes in arabidopsis

In higher plants, two nuclear gene families, FtsZ1 and FtsZ2, encode homolo gs of the bacterial protein FtsZ, a key component of the prokaryotic cell d ivision machinery. We previously demonstrated that members of both gene fam ilies are essential for plastid division, but are functionally distinct. To further explore differences between FtsZ1 and FtsZ2 proteins we investigat ed the phenotypes of transgenic plants overexpressing AtFtsZ1-1 or AtFtsZ2- 1, Arabidopsis members of the FtsZ1 and FtsZ2 families, respectively. Incre asing the level of AtFtsZ1-1 protein as little as 3-fold inhibited chloropl ast division. Plants with the most severe plastid division defects had 13- to 26-fold increases in AtFtsZ1-1 levels over wild type, and some of these also exhibited a novel chloroplast morphology. Quantitative immunoblotting revealed a correlation between the degree of plastid division inhibition an d the extent to which the AtFtsZ1-1 protein level was elevated. In contrast , expression of an AtFtsZ2-1 sense transgene had no obvious effect on plast id division or morphology, though AtFtsZ2-1 protein levels were elevated on ly slightly over wild-type levels. This may indicate that AtFtsZ2-1 accumul ation is more tightly regulated than that of AtFtsZ1-1. Plants expressing t he AtFfsZ2-1 transgene did accumulate a form of the protein smaller than th ose detected in wild-type plants. AtFtsZ2-1 levels were unaffected by incre ased or decreased accumulation of AtFtsZ1-1 and vice versa, suggesting that the levels of these two plastid division proteins ale regulated independen tly. Taken together, our results provide additional evidence for the functi onal divergence of the FtsZ1 and FtsZ2 plant gene families.