Previously overlooked asymmetry in Arabidopsis and tomato leaves
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 measureable left-right asymmetry in leaf
development, in species previously assumed to have symmetric leaves.
Kathy Munkvold, Ph.D.
301-251-0560 ext. 121
Associate Director of Public Affairs
Jennifer Mach, Ph.D.
Science Editor, The Plant Cell
American Society of Plant Biologists
Is your leaf left-handed?
Previously overlooked asymmetry in Arabidopsis and
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.
Recent research has shown that leaf initiation in
the meristem is specified by locally high concentrations of the plant hormone
In a study published in The Plant Cell, an international group
coordinated by Neelima R. Sinha, Ph.D., of the University of California at
Davis, examined how the pattern of auxin concentrations might affect the
symmetry of the leaf. She explains, "As leaves are initiated within a spiral
context, we might expect that they would be asymmetric and exhibit the same
handedness of the spiral, like propeller blades. Yet, superficially many leaves
appear symmetrical.” To examine whether the spiral pattern of leaves affected
symmetry, her team first modeled the anatomy of the forming leaves and the
location of the highest concentrations of auxin, finding that the two were not
perfectly aligned. Following up, they found that this difference caused
asymmetry at both the molecular level, altering gene expression, and the
anatomical level, altering leaf shape, in tomato and Arabidopsis thaliana leaves. Indeed, the authors found measurable
anatomical differences between the left and right sides of both young and
mature leaves, identifying a previously overlooked axis of asymmetry.
Dr. Sinha summarizes: "Our results show that asymmetry is
indeed very much present in the leaves around us and that the spiral, within
which they are initiated, influences their development from the earliest stages.
Quite literally, the handedness of the spiral in plants transmits its asymmetry
to leaves. By studying these asymmetries, we can begin to understand the
mechanisms by which plants produce such a staggering array of leaf shapes in
such regular arrangements.”
This work was funded through a National Science
Foundation grant (IOS-0820854)
The research paper
cited in this report is available at the following link:
Plant Cell (http://www.plantcell.org/)
is published by the American Society of Plant Biologists. For more information
about ASPB, please visit http://www.aspb.org/.
credit: Richard Smith
caption: Model of developing leaves
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