The roots of corn seedlings grow downwards with gravity. Credit: Darkves/Getty Images
New research has discovered how the plant hormone auxin guides roots to grow downwards in the direction of gravity, a process known as gravitropism.
The researchers hope that a better understanding of how auxin shapes the way plant root systems function and grow can help in the design of technologies which improve crop yields during droughts to address food insecurity.
Charles Darwin was one of the first to write about gravitropism when he observed that plant roots grow downwards and shoots grow upwards. This allows a plant’s stem to grow upwards towards the light it needs for photosynthesis, while the roots anchor it and collect water and nutrients from the soil.
Although there has since been much research into gravitropism, there are still some aspects of the phenomenon that scientists are still unsure of.
“We already knew that auxin is important for root gravitropism, but for a long time, we did not know what acts downstream of auxin,” says Dr Rahul Bhosale, lead author of the study from the University of Nottingham’s School of Biosciences in the UK.
“That is what we have uncovered in this new research which is important for fundamental understanding of how the root system works.”
The researchers examined how auxin activates a specific gene in plants which strengthens the cell wall, but only on the lower side of the root. This creates a reinforcement that prevents any growth below while allowing cells above to expand, making the root bend downward.
“Our research resolves this longstanding question by showing that auxin promotes cell wall biosynthesis, strengthening the walls to block growth on the lower side,” says Bhosale.
“This dual mechanism explains auxin’s seemingly opposite roles in promoting and inhibiting cell elongation.”
How roots bend in response to gravity stimulus in normal and mutant plants. Credit: University of Nottingham
These findings build on the team’s previous work on the hormone abscisic acid (ABA) published earlier this year in Current Biology.
Their study found ABA influences the angles at which rice and corn roots grow by promoting the production of auxin. This helps plants respond to droughts and water shortages by directing the roots into the deeper subsoil when the topsoil is dry.
As climate change leads to record high temperatures and low rainfall across the globe, droughts pose a dangerous risk to much of the world’s agricultural and food supplies. Creating drought-resistant crops is now a priority for agricultural scientists.
“Understanding the role of hormones in this much detail opens up possibilities for engineering crops that are stress resistant and can overcome obstacles in the soil,” says Bhosale.
The new findings have been published in Science Advances.