Arabidopsis FtsZ2-1 and FtsZ2-2 Are Functionally Redundant, But FtsZ-Based Plastid Division Is Not Essential for Chloroplast Partitioning or Plant Growth and Development

Abstract

FtsZ1 and FtsZ2 are phylogenetically distinct families of FtsZ in plants that co-localize to mid-plastid rings and facilitate division of chloroplasts. In plants, altered levels of either FtsZ1 or FtsZ2 cause dose-dependent defects in chloroplast division; thus, studies on the functional relationship between FtsZ genes require careful manipulation of FtsZ levels in vivo. To define the functional relationship between the two FtsZ2 genes in Arabidopsis thaliana, FtsZ2-1 and FtsZ2-2, we expressed FtsZ2-1 in an ftsZ2-2 null mutant, and vice versa, and determined whether the chloroplast division defects were rescued in plants expressing different total levels of FtsZ2. Full rescue was observed when either the FtsZ2-1 or FtsZ2-2 level approximated total FtsZ2 levels in wild-type (WT). Additionally, FtsZ2-2 interacts with ARC6, as shown previously for FtsZ2-1. These data indicate that FtsZ2-1 and FtsZ2-2 are functionally redundant for chloroplast division in Arabidopsis. To rigorously validate the requirement of each FtsZ family for chloroplast division, we replaced FtsZ1 with FtsZ2 in vivo, and vice versa, while maintaining the FtsZ level in the transgenic plants equal to that of the total level in WT. Chloroplast division defects were not rescued, demonstrating conclusively that FtsZ1 and FtsZ2 are non-redundant for maintenance of WT chloroplast numbers. Finally, we generated ftsZ triple null mutants and show that plants completely devoid of FtsZ protein are viable and fertile. As plastids are presumably essential organelles, these findings suggest that an FtsZ-independent mode of plastid partitioning may occur in higher plants.

GTP-dependent Heteropolymer Formation and Bundling of Chloroplast FtsZ1 and FtsZ2

“…Interestingly, the apparent plasticity of FtsZ coassembly in vitro is consistent with genetic studies showing that chloroplast FtsZ filaments assemble under a wide range of in vivo stoichiometries, although significant deviations from the wild type stoichiometry alters FtsZ filament morphology and perturbs plastid division (17,19,20,24,61). Nevertheless, the ability of FtsZ1 and FtsZ2 to coassemble at different stoichiometries raises the intriguing possibility that the degree of heteropolymerization and/or bundling may be regulated in vivo as a means of regulating FtsZ filament morphology and hence Z ring constriction and dynamics.…”

“…15 and 16). Depletion or overexpression of either protein impairs division and results in enlarged chloroplasts, which are the phenotypic equivalent of the bacterial filamentation phenotype (17)(18)(19). These findings, along with data showing that the molar ratio between Arabidopsis FtsZ1 and FtsZ2 remains constant throughout leaf development (20), suggest that their stoichiometry is important for chloroplast division activity.…”

“…Such regulation may be necessary because plants have multiple plastid types that vary greatly in size and division activity; perhaps the relative levels of FtsZ1 and FtsZ2 vary in different plastid types or at different stages of development (61). In this context, it is interesting to note that complete FtsZ2 rings are occasionally observed in small plastids of ftsZ1 null mutants (25), but the converse is not true (19). This suggests that even though both proteins are capable of protofilament formation in vitro, FtsZ1 may have evolved as a bundling factor rather than as a distinct ring-forming polymer.…”

“…To produce recombinant Arabidopsis thaliana FtsZ1 (AtFtsZ1-1, At5g55280) and FtsZ2 (AtFtsZ2-1, At2g36250) resembling the mature in vivo forms, we expressed constructs lacking transit peptides as His-tagged fusion proteins in E. coli. A second FsZ2 isoform in Arabidopsis, AtFtsZ2-2, is functionally redundant with AtFtsZ2-1 (19); therefore, we used only AtFtsZ2-1 (henceforth FtsZ2) in our experiments. Because recombinant proteins were mostly in inclusion bodies, they were solublized and subjected to nickel affinity chromatography in urea, then refolded by dialysis, and further purified by size exclusion chromatography.…”

Rice Cytokinin GATA Transcription Factor1 Regulates Chloroplast Development and Plant Architecture

“…While Arabidopsis contains at least two FtsZ homologs (AtFtsZ1-1 and AtFtsZ2-1; Stokes et al, 2000), rice appears to contain only a single FtsZ copy of this ancestral bacterial division gene. Increased production of FtsZ genes results in a significantly reduced number of enlarged chloroplasts, indicating a severe inhibition of chloroplast division (Stokes et al, 2000;Yoder et al, 2007;Schmitz et al, 2009). Transgenic Arabidopsis plants overexpressing FtsZ1 showed increased chloroplasts in mesophyll cells, but these plants were chlorotic and died as seedlings (Stokes and Osteryoung, 2003).…”