Plant roots grow differently when searching for their preferred nutrients

Jan. 5 (UPI) -- Scientists have discovered the hormonal secrets to a plant's ability to track down its preferred nutrients.

All plants need nitrogen, but the macronutrient can be found in a variety of forms -- different kinds of plants have evolved to prefer different kinds of nitrogen.

For example, maize, beans and sugar beets prefer nitrate, while pine and rice like ammonium, a compound formed by nitrogen and hydrogen.

Because soil composition, including its nutritional components, don't exist in stasis, plants must be able to adapt.

"One of the most important questions is, what is the role of plant hormones in adaptation to the nitrogen availability?" study co-author Eva Benková said in a news release.

"How do the machineries within a plant cope with their changing environment?" said Benková, a developmental biologist and professor at the Institute of Science and Technology in Austria.

In a new study, published Tuesday in the EMBO Journal, researchers detailed the strategies mouse-ear cress and their roots use to locate their preferred form of nitrogen.

Like maize and sugar beets, mouse-ear cress, Arabidopsis thaliana, prefer nitrate.

For the study, researchers raised mouse-ear cress in ammonium-rich soil plots and then transferred them to either ammonium-rich or nitrate-rich soil plots.

Scientists used a special vertical confocal microscope to observe the behavior of different types of cells in the roots of the different plants.

The experiments showed plants in suboptimal soil privilege root lengthening, rather than thickening. Instead of broadly enhancing the size of their roots, a process characterized by cell proliferation in root tissue called meristem allows malnourished plants to enhance cell extension.

In the ammonium-rich soil, the cress plants produced fewer cells in the meristematic zone of their roots.

"Once we moved the plants to the nitrate, suddenly the meristem became bigger, more cells were produced and there was a different kinetics in cell expansion," said Benková. "Now Arabidopsis could afford to put more energy into cell division and optimized its root growth differently."

Researchers found the balance between the two root growth strategies was dictated by a plant hormone called auxin, which is carried from cell to cell by special transporters.

Proteins called efflux carriers determine which side of the cell auxin will exit -- thus, controlling the direction in which the hormone flows.

More specifically, the auxin transporter PIN2 dictates the flow of auxin from the root tips, and researchers found the transporter plays a key role in determining whether roots thicken or lengthen.

"What really surprised us was that one modification, the phosphorylation of such a big protein like an efflux carrier, can have such an important impact on the root behavior," Benková said.

Phosphorylation describes the addition of a phosphorous atom to a molecule. The process can alter the function of a cell's proteins.

Researchers found the phosphorylation target in PIN2 is an amino acid that is common in a variety of plant species, which suggests this newly identified mechanism for root growth is universal.

In followup studies, researchers hope to highlight the cellular processes that alter the phosphorylation status of PIN2.