Rapidly accumulating results from various genetic studies also have revealed that cell size and ploidy are not always tightly coupled and that in some cases modification of the endocycle leads to only minor or opposite changes in cell size (Table 1). For example, the size of root cells from different Arabidopsis ecotypes varies considerably but those differences in cell size have very little correlation with ploidy (Beemster et al. 2002). Likewise, mis-expression of CDK inhibitors or dominant-negative CDKs in Arabidopsis and tobacco results in the drastic reduction of ploidy while at the same time leads to an increase in cell size (Hemerly et al. 1995; Wang et al. 2000; De Veylder et al. 2001). These results suggest that ploidy is not the final determinant of cell size and some other ploidy-independent signalling pathways contribute to the determination of final cell size in plants. Possible candidates that may be involved in these pathways include signals that are imposed from the total organ-checkpoint control (Tsukaya 2006).
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