After cells pass through the domain with high rates of cell growth and division, they cease dividing and cell size rapidly becomes larger. This transition from phase I to II is visually distinct in root meristems, whereas in leaf organs this transition is morphologically less conspicuous. Cell expansion in phase II is not driven by macromolecular synthesis but is the result of turgor-driven water uptake and concomitant cell wall loosening. The generation of increased osmotic pressure requires the activities of three major proteins or protein complexes in the tonoplast membrane: The V-type H+ATPase, H+pyrophosphatase and aquaporins (see Maeshima 2001 for review). This is balanced by cell wall loosening that permits the cell to expand mostly in one direction, and which involves several activities including expansins, xy-loglucan endotransglycolase/hydrolase (XET), endo-(1,4)-^-D-glucanase, and hydroxyl radicals (see Cosgrove 2005 for review). In quantitative terms, cell expansion contributes most to organ growth: during cell expansion, volume increases from 20- to 1000-fold. Thus, the extent of cell growth and division during phase I define the potential for organ growth by producing the cellular building blocks; during phase II, this latent ability is fulfilled during cell expansion.
The phase I/II boundary marks a transition of the cellular mechanism that mediates organ growth: from growth by cell production to organ growth by cell expansion. However, not all processes associated with organ and plant growth change at this transition. DNA synthesis persists during this transition, but in the absence of division, it leads to endoreplication. Therefore, DNA replication can be considered as the process that frames the entire organ growth process. In Arabidopsis, endoreplication can result in ploidy levels of up to 64C (with 1C being a haploid genome equivalent), indicating that cells undertake up to five additional rounds of DNA synthesis without dividing. In Arabidopsis leaves, cellular DNA content is positively correlated with mature, fully expanded cell size (Melaragno et al. 1993), however, in roots no such correlation was found (Beemster et al. 2002). DNA synthesis, and with it en-doreplication and cell expansion, is thought to cease when cells become fully differentiated and primary organ growth is completed.
Although expanding cells increase their size by a different mechanism than cells growing in the proliferative zone, they continue entering the DNA replication cycle as long as they undertake endoreplication cycles. The bulk of the volume increase in expanding cells is mediated by inflation of the vacuole, but it is likely that the cytoplasm must also increase in mass to insure that the necessary concentration of reactants is thermodynamically favorable. This raises the interesting, and as yet unresolved, question whether the onset of S-phase in endoreplicating cells is also coupled to proxies of cell growth such as the rate of mRNA translation.
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