When inner and outer cell layers of sepals – the leaf-like plant organs that cover and protect the flower bud before it opens – all grow upward, the organs stay smooth and uniformly stiff, which is optimal for the plant, according to a new study.
The study, published Sept. 1 in the journal Current Biology, compared a wild type Arabidopsis plant with a mutant one that grew with wrinkled sepals. For the first time, the researchers were able to image both the inner and outer layers of the sepal to understand factors that controlled smoothness and wrinkles.
This discovery could help engineer living materials into different forms, such as the custom of growing “living bridges” from rubber tree roots in India, and breeding kale so it’s smooth instead of crinkly.
The researchers wanted to uncover how some natural shapes, such as fly wings and leaf blades, form to be smooth while others form the complex folding or looping shapes seen in the brain, intestines or orchid flower petals, according to the paper.
“In thinking about how we can use living plants as our next advanced materials, one of the key things is to be able to engineer organs to grow to the size and shape we want, as opposed to what the plant wants,” said Adrienne Roeder, professor in the Weill Institute for Cell and Molecular Biology and the Section of Plant Biology in the School of Integrative Plant Science, in the College of Agriculture and Life Sciences and the paper’s senior author.
“If we want a smooth surface, we know we need to engineer it in such a way that the growth of the inside and outside are the same,” Roeder said. “If we want to make a curved surface, then we can engineer that growth to go in different directions.”
The study focused on sepals, which need to be smooth to properly enclose and protect the developing flower bud, and for photosynthesis, just as leaves require flatness to absorb sunrays.
The research team identified a mutant Arabidopsis with wrinkled and irregular sepals – a conditioncaused by a gene that is overexpressed on both the outer and inner sepal layers. The researchers then live-imaged the plants every 24 hours to see how curvature was generated in the mutant and how the wild typestayed smooth. From the images, they measured cell growth and division rates, and the orientations that cells grew in. The data were then combined with computational modeling.
The researchers hypothesized that the folds were caused by mechanical buckling, while the model revealed that two things needed to happen for buckling to occur: first, the outer surface needed to overgrow, particularly in width; and second, the outer surface needed to be softer than the inner surface.
Then the team analyzed the live imaging.
“In the wild type, both the outer surface and the inner surface grow upwards, towards the tip of the sepal,” Roeder said. “In the mutant, the orientation of cells in the outer surface is turned to the side, so they’re growing wide instead of up.”
Meanwhile, the mutant sepal’s inner surface still grew upwards, creating a conflict in the ways the two surfaces grew, which helped generate compression and wrinkling, Roeder said.
In terms of stiffness, the outer and inner sepal surfaces were equally rigid in the wild type, while the outer side was soft and the inner stiff in the mutant.
To test if they could correct the problem in the mutant, and to see if the issue was a combination of orientation and stiffness, the team applied a genetic trick to reorient all the growth upward, and when they did, the sepals no longer buckled. The stiffness between the inner and outer cell layers also became uniform.
“It rescued both the stiffness and the growth orientation, and we got rid of the buckling,” Roeder said.
The study’s co-first authors are former postdoctoral researchers Avilash Singh Yadav and Lilan Hong. Co-authors include Arezki Boudaoud, a senior research scientist at the École Polytechnique in Paris, France, and Annamaria Kiss, a plant development research engineer at the École Normale Supérieure de Lyon, France.
The study was funded by the National Institutes of Health, the French National Research Agency, the 2023 Sam and Nancy Fleming Research Fellowship, the National Natural Science Foundation of China, the Cornell China Center, Zhejiang University and the National Science Foundation.
