Sauret-Güeto et al. analysed petal development in Arabidopsis thaliana using a combination of experimental and computational approaches. They tracked petal growth by inducing GFP expression in sectors of the emerging petal at specific times of development and imaging the clones that arose from the induced cells over time; the shapes and positions of the clones thus provided information on how the petal grows. The authors found that although growth rates are largely even among different sectors of the petal the clones become elongated, indicating that there is more growth along the axis that runs from the base to the tip of the petal. Also, the clones 'fan' outwards towards the distal edge.
Next, the authors tested models that might account for these observations. Previous studies of leaf development describe a 'polarity field' that runs along the leaf and converges at the distal tip. This field provides directionality in leaf development, and specific growth rates that are parallel and perpendicular to this field give rise to the leaf shape. For petals, Sauret-Güeto et al. found a good fit to the experimental data using a model in which there is a polarity field that diverges at the distal edge. This divergence could be caused by an 'organizer' of polarity at the distal margin; this organizer probably acts by influencing auxin dynamics. They went on to show that the transcription factor JAGGED (JAG) is crucial for patterning distal growth and controlling the extent of the distal organizer.
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