Genetic switching in plants can turn simple spoon-shaped leaves into complex leaflets

The variety of forms of living organisms is great. But how individual cells coordinate the formation of organs and tissues in complex organisms is still an open question.

Researchers at the Max Planck Institute for Plant Breeding Research in Cologne, Germany, have discovered a genetic mechanism that changes the direction of plant cell growth during leaf development and thereby determines leaf shape.

Miltos Tsiantis and his group from the Max Planck Institute for Plant Breeding Research want to discover how biological forms develop and the basis for their diversity. The researchers are using thale cress (Arabidopsis thaliana), as the genome and development of this small garden weed have been intensively studied for many years.

By comparing it to its close relative, hairy bittersweet (Cardamine hirsuta), which has leaves formed from individual leaflets rather than the simple spoon-shaped leaves of Arabidopsis, researchers want to find out how forms develop different leaves.

The findings are published in Proceedings of the National Academy of Sciences.

Leaf growth is controlled by the hormone auxin: Leaves, leaves or flowers develop in areas with a high concentration of auxin. The place where the hormone accumulates is determined by the activity of the protein PIN1, which transports auxin from the cells. PIN1 transporters are not evenly distributed on the surface of a cell, but can be concentrated, for example, on the top or bottom side. This asymmetry is crucial for the site where auxin acts.

The distribution of PIN1 can also be altered to create an on/off growth pattern, for example in the arrangement of leaves along a stem. This ability of PIN1 and auxin to organize plant growth has been known for some time.

“However, we know very little about how the different distributions of the PIN1 transporter are controlled and how different growth patterns are induced in cells, which then determine the shape of a leaf,” explains Tsiantis.

The researchers used state-of-the-art microscopes to visualize individual cells in plants and created images of leaf development, which allow them to measure the growth of each cell on the leaf’s surface. By using fluorescent proteins to label the products of the genes they are interested in, they can also observe which genes are active, when and where in the cells.

Working alongside Adam Runions of the University of Calgary, the researchers then use this biological data to generate computer models that allow them to simulate the genetic interactions that ultimately control growth patterns in leaves.

The genetic switch controls where auxin will accumulate

During their investigations of their two model plants, the team discovered a genetic switch involving a gene called CUC1. When activated, this switch can affect where in a cell the transporter PIN1, and subsequently the growth hormone auxin, accumulates.

CUC1 is not active in Arabidopsis simple leaves. In bittercress, however, CUC1 leads to leaflet formation. “We found that this CUC1-dependent switch directs cell growth to occur in a specific pattern, which in bitter cabbage allows the development of its complex leaf shape,” explain researchers Ziliang Hu and David Wilson-Sánchez, authors main study. . “When we activate CUC1 in Arabidopsis thaliana, it also forms more complex leaves.”

Their experiments not only help explain the different leaves of the two plant species studied, they also demonstrate how a genetic switch can affect the polarity and growth of individual cells in a coordinated manner, and thus lead to the formation of complex forms.

“With this work, we now have a much clearer picture of the fundamental mechanisms that operate in cells to generate plant forms and their diversity,” says Tsiantis.