How Teamwork Shapes a Leaf
An article from carl 03|2025
by Karin Hollricher
The structure of the top of a leaf is different to the structure of the underside, and it is the refined interaction between two molecules that causes the self-organised formation of these structures.
If we look at botany, we see just how many different leaf shapes have been brought about through evolution. The formation of extensive shapes that capture as much light as possible for photosynthesis was an important development. Most flowering plants have bi-facial leaves with a different structure on the top and bottom. For example, the cells on the top of the leaf, facing the sun, contain a lot of chloroplasts. On the underside, however, there are stomata that let water vapour and air pass through.
All leaves are made of meristem cells, which are plant stem cells. But how do planar surfaces form with unique polarity?
Marja Timmermans has been looking into this question for a long time. Around 20 years ago, together with her team at the Cold Spring Harbor Laboratory on Long Island, New York, she discovered that it is not only transcription factors that control the different development of the different sides of leaves, but that two small regulatory RNA molecules – miR166 [1] and tasiARF [2] – are required for this. Even now, Timmermans is still researching the functions of these small RNAs, by now at the Centre for the Molecular Biology of Plants at the University of Tübingen. To learn more, her team traced the fate of each individual cell of a developing leaf. By doing this, the researchers were able to document that the two small and mobile RNA molecules form opposite gradients in the developing leaf [3].
In 1952, Alan Turing, a pioneer in computer.development, used a mathematically formulated mechanism, the so-called reaction-diffusion system to explain how gradients can develop autonomously [4]. At least two partners are required for this – an activator A and its inhibitor I – which are synthesised at the same place in the developing organism. Partner A autocatalytically strengthens its own production as well as that of I. Another condition: A diffuses more slowly than I. Based on this constellation, the local increase in the activator can strengthen itself autocatalytically and the inhibitor, which is available in a smaller quantity because it is more mobile, can do little about it. But with increasing distance from the synthesis location of the molecules, the concentration of I begins to prevail due to its higher diffusion efficiency, slowing the effect of A.
Through cell biology experiments and simulations, Timmerman’s team was able to show that leaf development can be explained with the two small RNA molecules miR166 and tasiARF in a Turing mechanism. The publication states: “Our results show that mobile, small RNAs, that interact directly with transcription factors, can generate a Turing dynamic.” [3].
Left: typical flat leaf with stable bipolar genetic activity pattern
Centre: leaf with displaced polarity, such as specialised formation of flesheating plants
Right: a radial genetic activity pattern due to the loss of polarity leads to the formation of vine patterns.
Glossary
Transcription factors are molecules that bond to specific regions of genes and in doing so can strengthen or prevent them from being read.
[1] M. Juares et al., 2004, Nature 428, 84-88 / [2] F. Nogueira et al., 2007, Genes Dev. 21, 750-755
[3] E. Scacchi et al., 2024, Nat. Plants 10, 412-422 / [4] A. Turing, 1952, Phil. Trans. R. Soc. Lond. B 237, 37–72
[5] www.uni-tuebingen.de/universitaet/aktuelles-und-publikationen/pressemitteilungen/newsfullview-pressemitteilungen/article/im-pflanzenblatt-organisieren-die-zellen-selbst-eine-optimale-flaeche-fuer-die-fotosynthese/
Image credits: Emanuele Scacchi / Friedhelm Albrecht, Universität Tübingen