Is the Uncoupling of Cell Division from Cell Growth Responsible for Compensation

In yeast and metazoans, the acceleration or blocking of the progression of the cell cycle by experimental manipulation results in a decrease or increase in cell size, respectively. This uncoupling of cell division from cell growth occurs because growth is not dependent upon cell cycling (Johnston et al. 1977; Jorgensen and Tyers 2004). Does compensation in plants function in a similar manner? The answer is no. First, in a KRP2 overexpresser (KRP2 OE), where the mature leaves of which clearly show compensation, proliferating cells are already larger than wild-type cells at the earliest stage of leaf development (De Veylder et al. 2001). This observation appears to suggest, at a glance, that cell division is uncoupled from cell growth, but this is untrue. If the uncoupling of cell division from cell growth occurs, cell size should not be kept, but the cell size is maintained in a certain size also in the KRP2 OE (Ferjani et al. 2007). Therefore, uncoupling between the cell division and cell growth is not seen in the compensation.

Moreover, one must be cautious of interpreting the contribution of this increase in cytoplasmic volume in early development to the final cell size, since most of the volume of the plant cell is occupied by large vacuoles that develop post-mitotically. In a model that simulates cell growth and cell division in the context of leaf development, the in silico inhibition of cell cycle progression mimics the effects of KRP2 overexpression, increasing cell size during cell cycling. However, this size difference accounts for only a small proportion of the cell size at the end of post-mitotic cell expansion (Beemster et al. 2006). In fact, a transient increase in cell size due to a delay in cell cycling was observed only in the mitotic phase in triple mutants of cytokinin receptors (Nishimura et al. 2004). In this case, the increase in cell size dissipated in the post-mitotic phase of cell expansion. Moreover, our time-course analyses of cell expansion in compensation-exhibiting mutants revealed no abnormal enhancement of cell enlargement during the cell proliferation stage in most of the mutants examined, but abnormal enhancement did occur, just after entry into the post-mitotic stage (Ferjani et al. 2007). Thus, the uncoupling of cell growth from cell division does not account for the enhanced cell expansion during compensation.

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