How roots bend in response to gravity stimulus in normal and mutant plants. Credit: University of Nottingham.

Scientists have uncovered how the plant hormone auxin helps roots bend and grow towards gravity even after encountering obstacles in soil. 

Experts from the University of Nottingham’s School of Biosciences and Shanghai Jiao Tong University (SJTU), identified how auxin activates a specific gene, which strengthens cell walls on the lower side of the root. This reinforcement prevents growth below while allowing cells above to expand, making the root bend downward.

Root gravitropism, which shapes root angle and overall root system, occurs because cells on the upper and lower sides of roots expand differently in response to gravity. 

Dr Rahul Bhosale, associate professor in the School of Biosciences, who co-led the study with Dr Guoqiang Huang at SJTU, said: “Until now it was unclear how auxin inhibits cell expansion on the lower side of roots.

“Our research resolves this longstanding question by showing that auxin promotes cell wall biosynthesis, strengthening the walls to block growth on the lower side.

“This dual mechanism explains auxin’s seemingly opposite roles in promoting and inhibiting cell elongation.”

Clear picture

This research expands the team’s earlier work showing that the hormone abscisic acid (ABA), which helps plants respond to drought, changes root growth angles by affecting auxin levels during water stress.

Together, these discoveries give a clear picture of how roots sense their environment and adjust their growth direction.

Bhosale added: “We already knew that auxin is important for root gravitropism, but for a long time, we did not know what acts downstream of auxin.

“That is what we have uncovered in this new research, which is important for fundamental understanding of how the root system works.

“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.”

 The findings have been published in Science Advances.