Proplastids

All plastids within a plant are ultimately derived from those progenitor plastids, which are found in meristem cells called proplastids. These in turn have been derived from the few proplastids, which were present in the zygote and derived potentially from both the maternal egg cell and the paternal pollen grain. However, most Angiosperms have mechanisms to exclude or degrade proplastids in the pollen line and hence the plastids present in the majority of plants are inherited maternally (Mogensen 1996; Corriveau and Coleman 1998; Zhang et al. 2003). In those species in which biparental inheritance occurs, plastids within the zygote constitute a mixed population derived from both parent egg and pollen. However, many factors appear to bias the relative proportion of maternally and paternally-derived plastids and plastid populations in resulting plants can be highly variable with respect to the origins of plastids within them (Mogensen 1996).

Considering the fundamental importance of proplastids to plastid biology, the knowledge of proplastid cell biology and their fine ultrastructure is limited, mostly because of the difficulties with analysing small organelles with no pigment in small regions of dense tissue. Knowledge of the physical appearance of proplas-tids has been derived largely from electron micrographs (Chaley and Possingham 1981; Akita and Sagisaka 1995; Robertson et al. 1995; Gunning 2004), which show proplastids as small organelles containing limited internal structure that are dispersed throughout the cytoplasm. Most proplastids contain rudimentary pieces of thylakoid membrane, but are unpigmented although those in shoot apical meristems appear to contain more thylakoid in a more organized state than those in the root apical meristem (Gunning 2004). In addition, ingrowths from the inner plastid envelope membrane into the proplastid stroma can also be seen occasionally, as well as ribosomes. Starch grains may be present, especially in proplastids of seeds where starch was laid down in the proplastid during seed development (Gunning 2004). In wheat plumules and potato stolons, starch content of proplastids is variable with some containing significant starch grains and others with no starch. This difference in starch content appears to result from differences in the capacity for starch synthesis since immunogold labelling of the enzyme starch synthase reveals two types of proplastids: those with and those without the enzyme (Akita and Sagisaka 1995).

Estimating proplastid numbers is difficult and to date no studies have definitively counted proplastid populations in meristem cells. However, various studies of shoot meristem cells estimate that they contain 10-20 proplastids per cell (Cran and Possingham 1972; Lyndon and Robertson 1976; Pyke and Leech 1992). Using modern fluorescent protein technology, imaging of proplastids in meristems and during cytokinesis should be feasible, although proplastid dynamics during meris-tematic cell divisions have yet to be studied in detail. Proplastids with fluorescent marker proteins on board, such as GFP, can be imaged in root meristems (Kohler and Hanson 2000) and those in shoot apical meristems can be observed also (Trynka and Pyke, unpublished), although experiments to determine population sizes and segregation patterns in different parts of the meristem could prove technically demanding.

Differences in proplastid number according to cell position within the meristem or in organs derived from it may well exist (Lyndon and Robertson 1976), but whether such differences are significant to cellular function are unclear and they may simply reflect differences in proplastid division rate compared to local rates of cell division. Differences in proplastid DNA content and morphology have been shown to exist between cell layers within a meristem, suggesting that tissue-specific characteristics of proplastids within a meristem may be important (Fujie et al. 1994). During the cell divisions of embryogenesis and the cell divisions within meristems, proplastids must divide to ensure continuity within cell lines and to ensure that all cells within the plant contain plastids. Little is known of a distinct mechanism by which a correct proplastid segregation is achieved at cytokinesis (Sheahan et al. 2004) and it would appear that aplastidic daughter cells are prevented simply because proplastids are generally distributed in the cytoplasm, thus ensuring segregation into both daughter cells, but also because they locate more particularly in positions close to the nucleus prior to the onset of mitosis. Positioning in the peri-nuclear cytoplasm during protoplast division is driven by the actin cytoskeleton leading to entrapment of plastids close to the nucleus (Sheahan et al. 2004). Whether a similar process happens during cytokinesis in meristems is unknown. Nuclear mutations, which affect proplastid division and give rise to populations of few, enlarged proplastids in meristematic cells (Robertson et al. 1995) do not result in the appearance of aplastidic cells in meristems, which implies that giant proplastids can still maintain a mechanism by which they segregate correctly. Giant plastids in tomato fruit cells appear able to replicate by a budding/fragmentation mechanism (Forth and Pyke 2006) and therefore it is feasible that the generation of small budded proplastids could ensure correct segregation in meristematic cells containing giant proplastids.

Efforts to study the extent of proplastid metabolism and DNA transcription and translation have been limited but those which have examined proplastid transcription at the tissue level have shown such activity to be low and that the initiation of a differentiation pathway, such as chloroplast differentiation, is necessary to upregulate transcriptional activity (Harak et al. 1995; Mache et al. 1997; Sakai et al. 1998; Baumgartner et al. 1989). Indeed, expression of nuclear genes for pro-plastid ribosomes is required prior to the expression of those genes, which are plastid encoded. Overall proplastids remain an exasperating organelle, occupying a pivotal place in plastid cell biology but yet about which there is so much still to learn.

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