Like any other plant, Arabidopsis thaliana, or mouse-ear cress, needs nitrogen in order to survive and thrive. But like corn, beans, and sugar beets, it prefers nitrogen in the form of nitrate and grows better on nitrate-rich soils. For example, while pine and rice prefer to grow on an ammonium diet, this is another form of the main macronutrient nitrogen. When the concentration or availability of the various forms of nitrogen fluctuates, the plants must adapt quickly. "One of the most important questions is what role plant hormones play in adapting to nitrogen availability. How do the machines in a plant deal with their changing environment?" asks Eva Benková, developmental biologist and professor at the Institute for Science and Technology (IST) Austria.
Find the balance
In search of answers, Krisztina Ötvös, postdoc in Eva Benková's research team, together with colleagues from the Universidad Politécnica de Madrid, the Pontifical Catholic University of Chile, the Austrian Institute of Technology and the University of Montpellier, examined two extremes: They compared how Arabidopsis seedlings, the Cultured solely on ammonium, responded after the scientists transferred them to media containing either ammonium or nitrate.
When a plant lives in sub-optimal soil, it tries to maintain its root growth for as long as possible in order to achieve a more suitable form of nitrogen. The main processes that maintain root growth are cell proliferation in the meristem, a plant tissue made up of undifferentiated cells, and cell expansion. The plant needs to find a good balance between the two. With ammonium added, the form of nitrogen Arabidopsis is not so popular, the meristematic zone of the cress produced fewer cells. Instead, they lengthened very quickly. "After we switched the plants to nitrate, the meristem suddenly became larger, more cells were produced and the cell expansion had different kinetics," says Benková. "Now Arabidopsis could afford to put more energy into cell division and optimize root growth differently."
Control of the hormone flow
Whether the plant invests in cell proliferation or cell elongation depends on the auxin level. This plant hormone is essential for all development processes. It is transported from one cell to the next in a very controlled manner by special auxin transporters. The proteins that control the transport of auxin out of the cells, so-called efflux carriers, regulate the flow of auxin depending on which side of the cell they are sitting on. Benková and her team were particularly interested in the auxin transporter PIN2, which mediates the auxin flow at the root tip. The researchers were able to identify PIN2 as the main factor in the balance between cell division and cell elongation. "We have observed that the localization of PIN2 changes as soon as we move plants onto the nitrate. This changes the distribution of auxin."
The activity of PIN2, on the other hand, is influenced by its phosphorylation status. "What really surprised us was that a modification, the phosphorylation of a protein as large as an efflux carrier, can have such an important impact on root behavior," adds Benková. In addition, the amino acid of PIN2, which is the target of phosphorylation, is present in many different plant species, suggesting that PIN2 could be universally involved in other adaptation strategies for plant species to changing nitrogen sources. In a next step, the researchers want to understand the machinery that controls the change in the phosphorylation status.
A very close look
"The present study is the result of input from a wide variety of people, from cell biologists and computer scientists to people who work in advanced microscopy. It really is a multidisciplinary approach," emphasizes Eva Benková. For example, to examine the processes within the roots of Arabidopsis more closely, the biologists used a vertical confocal microscope – a tool that was specially adapted at IST Austria to the needs of the researchers. Instead of a horizontal table, the microscope uses a vertical one, which allows you to observe plant growth naturally – along the factor of gravity. With its high resolution, Benková and her team were able to observe in real time how the cells in the roots of Arabidopsis divide and expand. In an earlier project, researchers from IST Austria won the Small World in Motion video competition by Nikon, in which a growing root tip of Arabidopsis thaliana is followed live under the microscope.