The first article, by Utpal Nath, Coen, and colleagues, looks at a quality of leaves often taken for granted—flatness. “Flatness is not a natural outcome of growth,” says Coen. “An organism has to coordinate lots of things to keep leaves flat.” According to the group's results, part of this coordination is accomplished by a transcription factor called CIN. The authors identified mutant snapdragon plants that lacked CIN and, consequently, had curly, crinkly leaves rather than their normally flat leaves.
The curvature occurred because the edges grew more than the center of cin mutant leaves. In several plant species, a wave of cell-cycle arrest moves from the leaf tip to the base. In wild-type snapdragon, this front advanced along the leaf edges slightly ahead of the center. In the cin mutant, however, the arrest front moved more slowly at the edges, which therefore grew more in relation to the leaf center. CIN expression in wild type lay just ahead of the division arrest front, and was stronger at the margins than at the center of wild-type leaves. Coen imagines that CIN sensitizes cells to an arrest signal, which must be particularly strong at the edges of wild-type leaves to ensure flatness.
Along with Anne-Gaelle Rolland-Lagan and J. Andrew Bangham (University of East Anglia, Norwich, UK), Coen has also deconstructed petal shape, again using snapdragon. The group used clonal analysis to examine asymmetry in snapdragon petals. Data from the clonal analysis was coupled to a growth model to calculate petal growth parameters: growth rate, growth direction (or angle to a reference axis), and anisotropy (the degree to which cells grow in a given direction). The sensitivity of petal shape to parameter values was examined by simulation, which revealed that overall direction of anisotropic growth, rather than regional variation in growth rate, is the major determinant of petal asymmetry. Similar clonal analysis and modeling techniques may be used to study growth of other structures, including fly wings and plant leaves. ▪