A Cornell-led study found that differences in the direction and stiffness of cell growth between inner and outer plant tissues can cause flower organs to wrinkle, a discovery that could help scientists engineer living materials into desired shapes.
The research, published Sept. 1 in Current Biology, compared normal Arabidopsis plants with a mutant that developed wrinkled, irregular sepals — the leaf-like organs that cover and protect a flower bud before it opens.
Researchers imaged both the inner and outer layers of sepals, measured cell growth, division and orientation, and combined those observations with computer modeling. Cornell said it was the first time both layers had been imaged together to study the mechanics controlling smoothness and folds.
Two conditions produce buckling
The model indicated that buckling required the outer surface to overgrow, particularly across its width, while also remaining softer than the inner surface.
In a normal sepal, cells on both surfaces grew upward toward the tip and had similar stiffness. In the mutant, outer cells turned sideways and widened while cells on the inner surface continued growing upward.
Adrienne Roeder, a professor in Cornell's plant biology section and the paper's senior author, said the conflicting growth directions created compression and wrinkling. The mutant's outer layer was also softer than its inner layer.
The team tested the explanation by using a genetic technique to redirect all growth upward. That change eliminated the buckling and made stiffness uniform across the inner and outer layers.
Findings may guide living materials
Sepals need smooth surfaces to enclose developing flower buds and perform photosynthesis efficiently. The work also addresses a broader question: why some biological structures, including leaves and fly wings, remain smooth while brains, intestines and some flower petals develop folds and curves.
Roeder said controlling how inner and outer tissues grow could allow researchers to design flat or curved living surfaces. Possible applications include growing structures from plant roots, similar to living bridges made from rubber-tree roots in India, or breeding kale with smooth rather than crinkled leaves.
Former Cornell postdoctoral researchers Avilash Singh Yadav and Lilan Hong were co-first authors. Other collaborators included Arezki Boudaoud of École Polytechnique in Paris and Annamaria Kiss of École Normale Supérieure de Lyon.
Funding came from the National Institutes of Health, the French National Research Agency, the National Science Foundation and several fellowships and research organizations in the United States, France and China.
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