Plastid division

Abstract

Background and aims: Plastids undergo a process of binary fission in order to replicate. Plastid replication is required at two distinct stages of plant growth: during cell division to ensure correct plastid segregation, and during cell expansion and development to generate large populations of functional plastids, as in leaf mesophyll cells. This review considers some of the recent advances in the understanding of how plastids undergo binary fission, a process which uses several different proteins, both internal and external to the plastid, which have been derived from the original endosymbiont's genome as well as new proteins that have been recruited from the host genome.

Key points: Several of the proteins currently used in this process in higher plants have homologues in modern-day bacteria. An alternative mode of replication by a budding-type mechanism also appears to be used in some circumstances. The review also highlights how most of our knowledge of plastid division is centred on the chloroplast developing in leaf mesophyll cells and a role for plastid division during the development of other plastid types is poorly understood. Whilst models for a protein-based mechanism have been devised, exactly how the division process is controlled at the plastid level and at the plastid population level is poorly understood.

Fig. 1

The mutation of genes encoding plastid division proteins can have a dramatic effect on chloroplast morphology in leaf cells. In this figure, protoplasts have been made from leaf mesophyll cells of Arabidopsis and imaged. (A) Protoplasts containing populations of small green chloroplasts from leaf mesophyll cells of wild-type Arabidopsis plants. (B) Protoplasts from leaf mesophyll cells of the arc6 mutant of Arabidopsis showing many protoplasts with single, large green chloroplasts within each. Protoplasts lacking obvious green chloroplasts are derived from epidermal cells in the leaf.

Fig. 2

Green chloroplasts in the pericarp cells of ripening tomato fruit containing a plastid-targeted GFP and imaged by confocal microscopy. These chloroplasts have distinct vesicle-like structures containing GFP, which may be derived from stromules as well as showing heterogeneity in the shape of the plastid body. Although plastids divide by binary fission during the development of these cells, plastid population growth as a result of budding or vesicle production appears to occur as an alternative mode of replication.

Fig. 3

Imaging of plastids in a group of cultured tomato cells reveals a complex array of plastids. This cluster of about 12 cells are from a cell culture derived originally from a transgenic tomato seedling contain a transgene which targets GFP to the plastid and revealed by confocal fluorescence microscopy. Plastids routinely congregate around the nucleus (arrowed n) and are also dispersed throughout the cytoplasm in a heterogeneous array of shapes and sizes, with many congregating on the cross-cell walls joining cells together (arrowed cw). Larger plastids produce stromule tubules (arrowed s), whilst many small bodies containing GFP are also present. The detailed dynamics of such a population of plastids is largely unknown. Image by Linda Cholerton.