Live imaging of chloroplast FtsZ1 filaments, rings, spirals, and motile dot structures in the AtMinE1 mutant and overexpressor of Arabidopsis thaliana.

Chloroplast division involves the tubulin-related GTPase FtsZ that assembles into a ring structure (Z-ring) at the mid-chloroplast division site, which is where invagination and constriction of the envelope membranes occur. Z-ring assembly is usually confined to the mid-chloroplast site by a well balanced counteraction of the stromal proteins MinD and MinE. The in vivo mechanisms by which FtsZ nucleates at specific sites, polymerises into a protofilament and organizes a closed ring of filament bundles remain largely unknown. To clarify the dynamic aspects of FtsZ, we developed a living cell system for simultaneous visualisation of various FtsZ configurations, utilising the Arabidopsis thaliana overexpressor and mutant of the MinE (AtMinE1) gene, which were modified to weakly express green fluorescent protein (GFP) fused to AtFtsZ1-1. Time-lapse observation in the chloroplasts of both plants revealed disorderly movement of the dots and short filaments of FtsZ. The short filaments often appeared to emanate from the dots and to converge with a long filament, producing a thick cable. In the AtMinE1 overexpressor, we also observed spirals along the longitudinal axis of the organelle that often rolled the closed rings together. In the atminE1 mutant, we visualised the 'isolated' rings with a maximum diameter of approximately 2 mum that did not encircle the organelle periphery, but appeared to be suspended in the stroma. Our observations further demonstrated heterogeneity in chloroplast shapes and concurrently altered configurations of FtsZ in the mutant.