Shoot Growth

Shoot meristems and developing leaf organs adapt exquisitely to the abundance of light, CO2, and water by modulating leaf production rate, leaf size and shape, anatomy, and physiology to confer competitive advantages in an environment where competition for light is fierce. Many genes and growth factor signaling pathways have been identified that contribute to specifying final leaf size and shape, but it is not yet known whether or how these mediate specific environmental cues as well.

Most leaves are determinate organs, but monocot and dicot leaves grow differently. In monocots, meristematic cells across the leaf base produce cells until the blade has reached its full longitudinal extent, and lateral growth of the blade does not occur. By contrast, growth is more complex in dicot leaves: all cells initially grow and divide, but quiescence sets in in a basipetal direction from the leaf tip to the base, and cell divisions cease early in leaf development. However, proliferation persists at a low rate in vascular tissues and in isolated cells (e.g., cells of the stomatal lineage), and endoreplication continues. Analysis of dicot leaf organ growth in mutants and transgenic plants has also revealed a compensatory mechanism: reduced proliferation can be mitigated by enhanced cell expansion, thereby maintaining a similar leaf area (Hemerly et al. 1995; Horiguchi et al. 2006). Such compensatory control of final leaf organ area has been suggested to result from an organ size control mechanism (Hemerly et al. 1995), but components of such a regulatory mechanism have remained elusive. Since the extent of cell expansion in leaf epidermal cells positively correlates with ploidy level (Melaragno et al. 1993), it has been proposed that the observed compensatory increase in cell expansion could depend on modulation of ploidy. In a recent study, expansion at the cellular and leaf organ level was analyzed in response to low light and water deficit (Cookson et al. 2006). Plants growing in low light produced smaller leaves comprised of fewer, but larger cells; while those growing in water deficit conditions produced smaller leaves comprised of smaller cells. However, the mean number of endoreplication cycles was reduced under both experimental conditions (Cookson et al. 2006). Taken together, these observations are not consistent with a role for endoreplication in governing final cell size in response to a nutrient and environmental cue such as light.

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