The control of plastid differentiation

The type of plastid present in a given type of cell is dictated by the nature of that cell type. Exactly how this developmental system is controlled by the host cell is largely unknown. It is normally assumed that the differentiation of proplastids into mature chloroplasts is the default pathway of plastid development, occurring in much of the above ground tissues in most plants. The leaf is a good organ in which to consider variations in tissue-specific chloroplast development. The fine tuning of this developmental process is significant since different cell types in a leaf all contain chloroplasts but these chloroplasts vary significantly in size, the extent of chlorophyll accumulation and membrane synthesis as well as large differences in their abundance within the cell. The most authentic development occurs in palisade and spongy mesophyll cells, where chloroplasts pack the cytoplasm and individual chloroplasts are fully photosynthetic with extensive thylakoid membrane and high levels of chlorophyll. In all other types of leaf cells, chloroplast development is less extensive and could be considered repressed. Although all other cell types in the leaf such as bundle sheath cells, epidermal pavement cells, vascular tissue, stomata, and hair cells have chlorophyll-containing chloroplast, the chloroplasts are all smaller, less well developed, and less abundant per cell. The implication is that a cell-specific repressive signal perturbs normal chloroplast development in these cells, resulting in poorly developed chloroplasts. Although photosynthetically compromised, these chloroplasts perform a crucial role in cellular metabolism in these different cell types and without them cellular function would be highly compromised.

A fundamental point of control in chloroplast differentiation is the presence of light, which initiates a complex chain of events inducing gene expression and protein synthesis, which in turn generates the proteome and the resulting metabolome of the mature chloroplast. A tight interaction between the developing chloroplast and cellular differentiation is crucial during this stage and a key part of this is a retrograde signalling pathway from the developing chloroplast back to the nucleus, which induces patterns of expression for genes, which encode plastid-destined proteins. Details of these molecular processes have been discussed extensively in recent times (Moller 2004; Lopez-Juez and Pyke 2005; Lopez-Juez 2007) and are also considered in other chapters in this book and will be considered only briefly here.

The big question remains as to what are the major control genes, which enable chloroplast differentiation to occur in a light-induced manner in mesophyll cells but not to the same extent, for instance, in neighbouring epidermal cells. Mutant screens for chloroplast biogenesis genes have identified a vast array of lines, mutant in genes which are critical for normal chloroplast function and which result in pale compromised seedlings. Many studies have shown that perturbation of genes which have basic functions in the chloroplast, result in pale compromised chloro-plasts; for instance, mutation of the RpoTp RNA polymerase (Hricova et al. 2006) or components of the Clp protease core (Rudella et al. 2006). Sifting out from such collections, mutants that represent the major control genes in this system is very difficult, although directed efforts in this direction are being made (Gutierrez-Nava et al. 2004). Indeed one might forecast that mutation in a global master switch for plastid development would be embryo lethal and therefore unlikely to figure in screens for pale mutants. Lopez-Juez (2007) considers the possibility of global master switches, which facilitate chloroplast development from proplastids. Several candidates are possible although none have compelling evidence to merit them being in complete charge. Maybe the most likely candidates at present are GLK genes, which encode transcription factors and appear to be conserved in all land plants but not in single-celled photosynthetic organisms. Maize and Arabi-dopsis contain two GLK genes and when both are mutated, chloroplast development and thylakoid biogenesis is dramatically perturbed (Fitter et al. 2002; Yasamura et al. 2005). Intriguingly, GLK genes are not sufficient to overcome the general repression of chloroplast development in non-green tissues, as GLK over-expressing plants fail to develop green roots, for example, and thus function only in the correct developmental context.

Progress in understanding how plastids develop and the precise differences between differentiated plastid states will likely come from proteomic analyses of distinct cell types and the plastids within them (Kleffmann et al. 2006). Such technology has the potential to pinpoint subtle differences between plastids that currently are unknown. For instance, differences between chloroplasts in bundle sheath and mesophyll cells in leaves as highlighted by proteomic analysis, reveals subtle differences in addition to the basic known differences in photosynthetic metabolism (Majeran et al. 2005). It seems likely that progress using such strategies may well reveal that even the chloroplast actually represents a collection of subtly different organelles reflecting their precise development in different types of cell.

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