The chloroplasts of eukaryotes evolved from endosymbiotic cyanobacteria, and both divide by binary fission (Gray, 1999; McFadden, 2001). In almost all bacteria, filamentous temperature-sensitive Z (FtsZ), which is a tubulin-like protein that self-assembles into filaments (Mukherjee & Lutkenhaus, 1994), represents the major component of the complex division apparatus, the divisome. FtsZ assembles into a ring-like structure at the future division site and serves as a scaffold to which other proteins are recruited successively to achieve cell division (Aarsman et al., 2005; Goehring & Beckwith, 2005). Plant FtsZ proteins are required for plastid division (Osteryoung et al., 1998; Strepp et al., 1998). During evolution, their genes were translocated to the nucleus where they experienced duplication (Vaughan et al., 2004). Plant FtsZ proteins cluster in two clearly separated subfamilies (FtsZ1 and FtsZ2) (Stokes & Osteryoung, 2003; Rensing et al., 2004). Despite its evolutionary descent from bacterial cytokinesis, which is well defined, the plastid division machinery remains only fragmentarily characterized (Margolin, 2005). Understandably, certain morphological innovations connected with the evolutionary adaptation of the endosymbionts to their new environment had to be complemented by novel strategies of division, while some other long-established strategies progressively became obsolete. In particular, the necessity to generate force from the outside of a membranous envelope in the process of constriction required the development of distinct, though closely neighbouring, annular systems, the inner FtsZ ring and the outer plastid division ring(s), the latter being regarded as a donation from the endosymbiontic host (Miyagishima et al., 2001; Kuroiwa et al., 2002). Also, the abolishment of a rigid peptidoglycan cell wall in modern chloroplasts rendered the murein-synthesizing or -modifying functions of the prokaryotic divisome dispensable (Iino & Hashimoto, 2003; Sato et al., 2005). Ultimately, except for FtsZ itself, none of the proteins localizing to the divisome of heterotrophic prokaryotes has been retained in plants harbouring chloroplasts (Miyagishima et al., 2005). This raises the question of whether a complex structure representing an FtsZ-based divisome actually exists in plants. The issue of putative FtsZ interaction partners has been addressed in Arabidopsis thaliana, albeit by employing heterologous overexpression of A. thaliana genes in yeast and tobacco (Nicotiana tabacum) cells (Maple et al., 2005). Interaction of AtFtsZ2-1 with AtFtsZ1-1 and AtARC6 (ARC, accumulation and replication of chloroplasts) was, however, observed in both systems. Intriguingly, ARC6 of Arabidopsis had previously been identified as an evolutionary descendant of Filamentation 2/Z-ring interacting protein N (Ftn2/ZipN) a component exclusive to the cyanobacterial divisome (Vitha et al., 2003).
The duplication and diversification of ftsZ genes in the course of plant evolution and the obvious concomitant elimination of most prokaryote-derived divisomal FtsZ interaction partners suggest that functionality of FtsZ-based systems in plant cells relies largely on the interplay of the distinct plant FtsZ isoforms themselves.
In the moss Physcomitrella patens, two FtsZ isoforms have been described in each of the two subfamilies (Rensing et al., 2004). Both members of the FtsZ2 subfamily (PpFtsZ2-1 and 2-2) reside exclusively within plastids, where in a fairly similar manner they polymerize into network-like structures on transient moderate overexpression as C-terminal green fluorescent protein (GFP) fusions. A proposed function of this ‘plastoskeleton’ is the maintenance of chloroplast integrity (Kiessling et al., 2000; McFadden, 2000; Reski, 2002). Recent evidence suggests that prokaryotic FtsZ might have comparable functions in the cytoplasm of filamentous cyanobacteria (Klint et al., 2007). At the amino acid level, the protein sequences of FtsZ2-1 and FtsZ2-2 are 94.7% identical. Despite this high degree of similarity, functional redundancy of FtsZ2-1 and FtsZ2-2 does not exist; the loss of FtsZ2-1 function cannot be compensated by FtsZ2-2 (Strepp et al., 1998). PpFtsZ1-2 was found to be dually targeted to chloroplasts and the cytoplasm and assembled into rings in both cell compartments. Immunolocalization experiments revealed the formation of FtsZ1-2 ring structures at the division sites of cells and chloroplasts, indicating that this protein might be involved in chloroplast and cell division (Kiessling et al., 2004).
Here it is shown that in vivo assembly of a fourth isoform, FtsZ1-1, is distinctly different from that of the other FtsZ proteins in P. patens, lending further support to the hypothesis of functional differences among FtsZ isoforms.
Addressing the assumption that the apparent functional diversification of FtsZ isoforms was an evolutionary prerequisite for the establishment of a eukaryotic interaction system derived from (and probably replacing) the prokaryotic divisome, it is demonstrated that the four FtsZ isoforms are indeed capable of specific in vivo interaction in P. patens. The results suggest that FtsZ functions related to cell division and maintenance of chloroplast integrity require cooperative action of distinct FtsZ isoforms in P. patens.