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Old 06-23-2012, 04:38 PM
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Is your leaf left-handed? Previously overlooked asymmetry in Arabidopsis and tomato leaves

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Research published in the Plant Cell shows that the spiral pattern of leaf formation from the point of growth affects the developing leaf's exposure to the plant hormone auxin; This exposure leads to measurable left-right asymmetry in leaf development, in species previously assumed to have symmetric leaves.


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This is a model of developing leaves. Credit: Richard Smith
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The front of a leaf is different from the back of a leaf and the tip is different from the base. However, a leaf from a tomato or an Arabidopsis plant superficially appears to be bilaterally symmetrical, or the same on the left and right sides. Don't let its appearance fool you; there is an underlying asymmetry between the left and right sides of such leaves—it just took a while for scientists to discover it. The story begins with the mechanism by which leaves form along a stem. In broad-leafed plants, dicots, leaves form from the meristem, an actively dividing tissue at the top of the plant, so that as you look down the stem, the oldest leaves are at the bottom. Leaves don't just become arranged by random chance either—phyllotaxis, the arrangement of leaves or flowers along a stem, affects key plant characteristics, such as how much light can filter through to lower leaves. Leaves can form opposite each other, or in alternation, or in whorls; often leaves form in spirals where the next leaf is offset by roughly 137 degrees, known as the "golden angle", which is related to the Fibonacci sequence.



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Recent research has shown that leaf initiation in the meristem is specified by locally high concentrations of the plant hormone auxin.
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