Plants are able to respond to their environment and changes in conditions. In the phototropism phenomenon, for example, plant parts will turn towards the sun. And gravitropism refers to the ability of plants to sense gravity. Roots will also migrate towards water due to hydrotropism. Phototropism and gravitropism have been investigated for over a century, but we don't know as much about hydrotropism, even though drought can destroy crops.
Plant roots will anchor into soil with the help of gravitropism, but the direction of their growth can change and move toward a water source when it must. A new study has shown that this directional change can be boosted during a drought, as the plant roots' natural ability to sense gravity is dampened. This work may help scientists develop plants that are better at withstanding drought. The findings have been reported in the Proceedings of the National Academy of Sciences (PNAS).
Researchers wanted to know more about hydrotropism, so they studied plants that grew in simulated conditions of zero-gravity. They created a platform that rotated slowly in directions that would continuously counteract gravitational forces. This made the effect of hydrotropism more significant.
"By eliminating gravity, we see that the hydrotropism becomes much stronger," explained senior study author Professor Jiří Friml of the Institute of Science and Technology Austria (ISTA). "Under normal conditions with sufficient water, gravitropism usually prevails. However, when you switch off gravitropism, the effects of hydrotropism become evident."
During a drought, plants will naturally start to suppress gravitropism, as the search for water becomes far more important than growing in a downward direction, said Friml.
Previous work has shown that a regulator protein called MIZ1 is crucial to hydrotropism. When plants were engineered to lack MIZ1, gravitropism was no longer suppressed during a drought, and plants roots kept growing downward instead of seeking water. MIZ1 therefore seems to help control gravitropism, and is a master regulator of hydrotropism in roots.
As the climate of the world changes, cycles of drought and flood are becoming more common, and it will be more important for us to fully understand how these changes will impact plant growth.
"By identifying corn or wheat variants with highly active MIZ1 protein, these can be introduced into the high-yield variants. Theoretically, you could end up with more hydrotropic plants, which may improve their water uptake during droughts," Friml suggested.
Sources: Institute of Science and Technology Austria, Proceedings of the National Academy of Sciences (PNAS)
