INVITED REVIEW
Plastid Division: Evolution, Mechanism and Complexity
JODI MAPLE and SIMON GEIR MØLLER*
Department of Mathematics and Natural Sciences, University of Stavanger, 4036 Stavanger, Norway
Received: 8 August 2006 Returned for revision: 18 September 2006 Accepted: 29 September 2006 Published electronically: 30 November 2006
† Background The continuity of chloroplasts is maintained by division of pre-existing chloroplasts. Chloroplasts
originated as bacterial endosymbionts; however, the majority of bacterial division factors are absent from chloro-
plasts and the eukaryotic host has added several new components. For example, the ftsZ gene has been duplicated
and modified, and the Min system has retained MinE and MinD but lost MinC, acquiring at least one new
component ARC3. Further, the mechanism has evolved to include two members of the dynamin protein family,
ARC5 and FZL, and plastid-dividing (PD) rings were most probably added by the eukaryotic host.
† Scope Deciphering how the division of plastids is coordinated and controlled by nuclear-encoded factors is key
to our understanding of this important biological process. Through a number of molecular-genetic and biochemi-
cal approaches, it is evident that FtsZ initiates plastid division where the coordinated action of MinD and MinE
ensures correct FtsZ (Z)-ring placement. Although the classical FtsZ antagonist MinC does not exist in plants,
ARC3 may fulfil this role. Together with other prokaryotic-derived proteins such as ARC6 and GC1 and key
eukaryotic-derived proteins such as ARC5 and FZL, these proteins make up a sophisticated division machinery.
The regulation of plastid division in a cellular context is largely unknown; however, recent microarray data shed
light on this. Here the current understanding of the mechanism of chloroplast division in higher plants is reviewed
with an emphasis on how recent findings are beginning to shape our understanding of the function and evolution
of the components.
† Conclusions Extrapolation from the mechanism of bacterial cell division provides valuable clues as to how the
chloroplast division process is achieved in plant cells. However, it is becoming increasingly clear that the highly
regulated mechanism of plastid division within the host cell has led to the evolution of features unique to the
plastid division process.
Key words: Arabidopsis, ARC, E. coli cell division, Min system, plastid division, FtsZ.
INTRODUCTION
Plastids are essential plant organelles in which photosyn-
thesis and many other fundamental intermediary metabolic
reactions are housed (reviewed in Tetlow et al., 2004). In
plants, an integral part of chloroplast development is div-
ision, as they are not created de novo but arise by division
from pre-existing plastids in the cytosol. Chloroplasts are
derived from cyanobacteria that were engulfed by a hetero-
trophic eukaryotic host cell (Gray, 1999; McFadden, 1999)
and, reminiscent of their prokaryotic ancestors chloro-
plasts, divide by binary fission. This led to a number of
groups investigating the possibility that essential bacterial
cell division genes have been conserved through the
evolution of the chloroplast division machinery. Using the
Escherichia coli and cyanobacterial cell division proteins
as input sequences, Arabidopsis has been found to encode
functional homologues of the bacterial cell division
proteins FtsZ (Osteryoung and Vierling, 1995; Osteryoung
et al., 1998), MinD (Colletti et al., 2000; Dinkins et al.,
2001), MinE (Itoh et al., 2001; Maple et al., 2002;
Reddy et al., 2002) and GC1/SulA (Maple et al., 2004;
Raynaud et al., 2004). Further components of the chloro-
plast division machinery were identified through cloning
of the disrupted loci in several of the accumulation and
replication of chloroplasts (arc) mutants, a collection of
ethylmethane sulfonate- and T-DNA-mutagenized seed-
lings with altered numbers of chloroplasts in mesophyll
cells (Pyke and Leech, 1991; Rutherford, 1996). The
identification of ARC5 (Gao et al., 2003), ARC6 (Vitha
et al., 2003) and ARC3 (Shimada et al., 2004) provided
insights into the complex evolution of the chloroplast
division machinery. More recently, the identification of
components that affect chloroplast division but are not
directly involved in the division machinery, such as the
MSL proteins (Haswell and Meyerowitz, 2006), has added
another level of complexity to our understanding of the
division process.
The continued identification of new plastid division
proteins means that it is now possible to begin to address
the evolution, mechanism and complexity of the plastid
division machinery. It is well established that the
chloroplast division machinery requires components of
both prokaryotic and eukaryotic origin, but how these are
coordinated is still unclear. Although the components of
prokaryotic origin have been shown to share many
functional features with their prokaryotic counterparts, the
recent identification and functional characterization of new
components is beginning to shed light on how the division
machinery has evolved to adapt to the environment of the
chloroplast. Furthermore, although in its infancy, studies
addressing the regulation of the chloroplast division
mechanism are starting to reveal how the division process
may be controlled and coordinated by the host cell.
* For correspondence. E-mail simon.g.moller@uis.no
# The Author 2006. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
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Annals of Botany 99: 565–579, 2007
doi:10.1093/aob/mcl249, available online at www.aob.oxfordjournals.org

COMPONENTS OF PROKARYOTIC ORIGIN
Bacteria have sophisticated molecular machineries dedi-
cated to regulate their growth and division with remark-
able accuracy. Because of the endosymbiotic origin of
chloroplasts, it was speculated relatively early that plastid
division might share common features with bacterial cell
division. In the last few years, substantial progress has
been made in several aspects of cell division, particularly
the molecular basis for Z-ring placement by the Min
system, the assembly of the division protein machinery
and the solution of the crystal structures of several division
proteins (reviewed in Rothfield et al., 2005). The super-
iorly defined status of the bacterial cell division system
has made it an invaluable tool to accelerate the investi-
gation of the molecular mechanism of plastid division, and
recent evidence has begun to shed light on the similarities
and differences between cell and chloroplast division.
The plastid FtsZ ring(s)
The identification of a homologue of the bacterial cell
division protein FtsZ in the nuclear genome of
Arabidopsis (Osteryoung and Vierling, 1995) led to the
concept that chloroplast division utilizes at least part of
the ancestral bacterial division machinery. Since then,
FtsZ genes have been identified in many plant and algae
species (reviewed in Gilson and Beech, 2001), indicating
that the FtsZ protein is a universal component of the
plastid division machinery.
The FtsZ protein of E. coli was initially identified as
part of a screen for temperature-sensitive E. coli mutants,
designated fts ( filamentous temperature sensitive), which
show a filamentous phenotype because they are unable to
divide under non-permissive temperatures (Bi and
Lutkenhaus, 1991; Dai and Lutkenhaus, 1991). FtsZ is by
far the most conserved bacterial cell division protein, with
FtsZ proteins from widely divergent bacteria showing
approx. 50 % amino acid identity. FtsZ is essential
throughout the process of cell division (Ma et al., 1996;
Bramhill, 1997; Lutkenhaus and Addinall, 1997; Rothfield
and Justice, 1997; Margolin, 2000; Addinall and Holland,
2002; Romberg and Levin, 2003). FtsZ self-assembles into
a contractile ring (Z-ring) beneath the cytoplasmic mem-
brane at the division site (Addinall et al., 1996; Ma et al.,
1996), and the formation of the Z-ring after daughter
chromosome segregation is believed to be the earliest
event in the cell division process. The Z-ring is essential
for the hierarchical recruitment of at least ten other pro-
teins (ZipA, FtsA, FtsE, FtsX, FtsK, FtsQ, FtsL, FtsW,
FtsI and FtsN) that are recruited to the septum to form the
divisome. All of the components of the divisome are
required for the progression and completion of cell div-
ision (Ma et al., 1996; Addinall and Lutkenhaus, 1996;
Lutkenhaus and Addinall, 1997; Wang et al., 1997; Din
et al., 1998; Chen et al., 1999; Ma and Margolin, 1999;
Weiss et al., 1999; Goehring at al., 2006; Vicente et al.,
2006).
The precise role of FtsZ during cell division is not
known. It is possible that FtsZ function is limited to its
role as a scaffold protein during divisome assembly, but
the FtsZ ring remains associated with the septum through-
out division and could therefore play an active role in the
ingrowth of the new septum. In support of this model, in
the prokaryote mycoplasma, which lacks a cell wall and
contains a small genome, FtsZ is the only conserved cell
division protein and is thought to be sufficient to complete
the division process (Margolin, 2001).
In contrast to bacteria that contain a single ftsZ gene,
paralogues of ftsZ have arisen in higher plants and algae
which contain several FtsZ genes that are clustered into
two families termed FtsZ1 and FtsZ2 (Osteryoung et al.,
1998; Osteryoung and McAndrew, 2001; Stokes and
Osteryoung, 2003). The mature Arabidopsis AtFtsZ1-1
and AtFtsZ2-1 proteins show approx. 40 % similarity and
30 % identity to the E. coli FtsZ protein, respectively. The
duplication of the FtsZ gene is believed to have occurred
in the cyanobacterial progenitor of chloroplasts between
the divergence of red and green algae (Stokes and
Osteryoung, 2003).
In Arabidopsis, both AtFtsZ1-1 and AtFtsZ2-1 are
imported into chloroplasts and co-localize to a ring-like
structure at the chloroplast division site on the stromal side
of the chloroplast envelope (Fig. 1B; Fujiwara and
Yoshida, 2001; McAndrew et al., 2001; Vitha et al.,
2001). Similar to the bacterial homologues, AtFtsZ1-1 and
AtFtsZ2-1 can both form dimers and heterodimers in
planta (Fig. 1A; Maple et al., 2005), and AtFtsZ1-1 has
been shown to polymerize in vitro (El-Kafafi et al., 2005).
The exact composition of the Z-ring is still uncertain, and
determination of this will help shed light on the precise
function of the two FtsZ proteins. The interaction between
the FtsZ proteins in both bacteria and plants requires the
central region [E. coli: amino acids 100–326 (Wang et al.,
1997) and Arabidopsis: amino acids 145–302 (Maple
et al., 2005)] and like the bacterial FtsZ proteins
AtFtsZ1-1 and AtFtsZ2-1 can polymerize into additional
structures including filaments and mini-circles in planta
(Erickson et al., 1996; Maple et al., 2005). This suggests
that the Arabidopsis Z-ring(s) may resemble the bacterial
Z-ring. In prokaryotes, the estimated number of FtsZ mol-
ecules per cell (estimates between 4000 and 15 000;
Lutkenhaus, 1993; Lu et al., 1998; Rueda et al., 2003)
indicates that the Z-ring is probably composed of protofila-
ments organized into a spiral structure rather than closed
rings. However, in plants, this model will be further com-
plicated by the presence of two forms of functional FtsZ
protein.
Disruption of FtsZ protein levels in chloroplasts of
Arabidopsis, Physcomitrella patens and tobacco has con-
firmed that they are essential division proteins (Osteryoung
et al., 1998; Strepp et al., 1998; Stokes et al., 2000). In
Physcomitrella, a knockout of FtsZ2-1 causes inhibition of
chloroplast division and cells contain only one large chlor-
oplast (Strepp et al., 1998). The same phenotype is
observed in Arabidopsis where depletion of either
AtFtsZ1-1 or AtFtsZ2-1 causes inhibition of chloroplast
division, resulting in one large chloroplast per mesophyll
cell (Osteryoung et al., 1998), demonstrating that rather
than being redundant each FtsZ protein has a distinct
Maple and Møller — Evolution and Mechanism of Plastid Division566