The secret behind flat leaves – University of Copenhagen

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03 November 2016

The secret behind flat leaves

Copenhagen Plant Science Centre

How does a set of plant cells grow from a bump into a flat leaf that can efficiently capture sunlight? A new publication based on findings by researchers at Copenhagen Plant Science Centre and the European Molecular Biology Laboratory literally sheds light on how different types of molecules on the top and bottom of a leaf keep each other in check, ensuring that the leaf grows flat.

Original text in Danish by Natasja Lykke Corfixen
Editing and translation by Jagger Andersen Kirkby

When the molecular balance is maintained, flat leaves are developed (left). If the balance is disturbed in the early stages of the leaf’s development, it will produce a structure that resembles a plant stem (right). Photo: Paz Merelo – EMBL.

How leaves go from being small bumps at the early stage to becoming flattened structures, a recognizable characteristic of most full-grown plants, has long confounded researchers.  In a project carried out by Stephan Wenkel’s research group at Copenhagen Plant Science Centre (CPSC), Department of Plant and Environmental Sciences at the University of Copenhagen, in collaboration with Marcus Heisler’s research group at the European Molecular Biology Laboratory (EMBL), the scientists have finally solved the riddle. Here they show how molecules in the top and bottom of a leaf keep each other at bay to ensure the correct development process.   

“The project began when I still lived in Germany, in collaboration with Marcus Heisler’s lab at EMBL. In 2010, we started discussing how a joint effort could help us discover new genes and transcription factors involved in leaf growth. This resulted in a publication in 2013 and the collaboration has now given us these results,” says Stephan Wenkel, Associate Professor at CPSC, one of the main researchers behind the project.

Oxygen reserves on Earth are produced by plants, and these highly specialized oxygen factories have adapted to and caused dramatic changes in the Earth’s biosphere over millions of years. The secret behind their successful colonization of the land lies, among other places, in the highly effective shape of the leaves, a shape that makes it possible for them to absorb as much sunlight as possible per unit area.

“The top side of the leaf consists of tissue, which plays a part in both photosynthesis and the fixation of CO2 from the atmosphere, and consequently, produces sugar. If we find out how a leaf is produced, we may be able to manipulate this process and thus, produce leaves that contain more photosynthetic tissue, and may even be larger,” explains Stephan Wenkel.

Molecular interaction controls the leaf development
The dissimilarity between the top and bottom sides of the leaf is established in an initial stage, which only contains a few cells. This separation is essential for the development of the flat structure of leaves. It has long been clear that a certain transcription factor that regulates the expression of genes is only produced in those cells that provide a basis for the leaf’s surface. The production in the cells in the bottom part, however, is impeded by certain microRNA.

When one transcription factor is removed (right), the production of REVOLUTA (red) is heavily reduced, as the creation of microRNA is no longer impeded (left). Photo: Paz Merelo – EMBL.

The new study has determined how it can be possible that the microRNA is not also produced in the surface cells of the leaves, despite a threefold interaction between several molecules. The associated transcription factor, along with one of two other transcription factors belonging to a different family, binds to the DNA-based starting point for the formation of the microRNA, and thus, impedes the production of it. 

“We have discovered that the transcription factor, REVOLUTA, which normally contributes to the formation of the top side of the leaf, regulates its own regulator. It does this by putting into motion the production of two other transcription factors, to which it binds itself and prevents the production of a specific microRNA in the leaf’s surface,” explains Stephan Wenkel.

In this way, the research has elucidated an important question in plant physiology, but has at the same time contributed to the creation of new questions. The transcription factors are also involved in the ability of the plant to extend its stem when it is exposed to shade. Yet, it remains unclear if the microRNA is also involved in this regulation.

The project was rendered possible with support from Deutsche Forschungsgemeinschaft (DFG), a Marie Curie International Reintegration Grant (IRG) and grants from the European Research Council (ERC).

The findings of this project resulted in the publication Regulation of MIR165/166 by class II and class III homeodomain leucine zipper proteins establishes leaf polarity in the journal Proceedings of the National Academy of Sciences of the United States (PNAS).