Study on replication and morphology of chloroplasts in Arabidopsis thaliana

Abstract

In higher plants, only a few proplastids in shoot meristem cells undergo replication to develop into a large number of chloroplasts in mesophyll cells. The developmental process accompanies morphological changes of the organelle; small (~1 mm) and amorphous proplastids shape into large (4-6 mm) and uniformly spherical chloroplasts. In the present study, we analyzed evolutionary conserved nuclear genes of Arabidopsis thaliana that encode chloroplast proteins involved in chloroplast replication and morphology. FtsZ, a bacterial homolog of tubulins, forms a ring at midpoint of dividing chloroplasts as well as bacterial cells. Recent studies hypothesized that one family (FtsZ1) members of higher plant FtsZ proteins target into chloroplasts while another family (FtsZ2) members localize in cytoplasm. Contrary to this hypothesis, FtsZ2 fused with green fluorescent protein (GFP) was shown to target into chloroplasts by confocal laser scanning microscopy (CLSM). The FtsZ2-GFP distributed as fibers and dots that formed network structures within chloroplasts. Moreover, we identified a new nuclear gene from Arabidopsis that shows homology to eubacterial cell division site-determining factor MinE. GFP experiments demonstrated that MinE is also a chloroplast stromal protein. Transgenic Arabidopsis plants overexpressing MinE caused aberrant chloroplast replication and morphology, indicating that MinE is the third conserved factor involved in chloroplast division. Furthermore, we observed by CLSM that root leucoplasts frequently emanate tubular structures, called stromules. Some stromules connected with other leucoplasts while some extended towards cell nuclei. We are currently studying the effects of altered expression of chloroplast division genes on stromule proliferation.