Cell size is determined by highly dynamic and probably flexible signalling pathways in plants and our understanding of its control is still limited. Nevertheless, recent studies on various mutants and transgenic plants mainly in Arabidopsis, maize and legume Medicago show clear correlation between the level of ploidy and cell size (Table 1, Fig. 4), providing strong genetic evidence to support the long-standing "karyoplasmic ratio" or "nuclear-cytoplasmic ratio" theory. A question still remains as to which, an increase in ploidy or an increase in cell size, comes first, since deducing causal relationships based on the observed mutant phenotypes is not always straight-forward. However, our recent study using Arabidopsis tetraploid plants clearly demonstrates that increasing ploidy first does have a positive impact on cell size (Breuer et al. 2007; Fig. 2), suggesting that increased DNA content can indeed support further growth of cells. It is worth noting that increasing ploidy also allows further growth of some plant organs, e.g. floral petals and sepals, but not of others, e.g. leaves, which appear to have some compensatory mechanism to maintain the overall organ size (Fig. 2).
Whether plant cells possess some active mechanism to judge the level of ploidy and to determine the extent of cell growth accordingly still remains elusive. It is generally thought that increasing ploidy multiplies the DNA template available for gene expression and this, in turn, enhances the metabolic activity of cells to support further growth. However, this model has been challenged by a recent finding in maize endosperm where down-regulation of the endocycle by over-expression of the dominant-negative CDKA does not change the overall level of starch and other storage proteins (Leiva-Neto et al. 2004).
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