Root growth mechanism reveals how plants survive drought

Jiří Friml and Lanxin Li, of the Institute for Science and Technology, in Klosterneuburg, Austria, grew seedlings of the model plant, Arabidopsis thaliana, to understand how plants control root growth in more detail. 

They made changes to the growth medium of the seedlings and observed the impacts in real-time, using microfluidic chips and a microscope and software system they developed to enable live imaging and tracking of vertically growing roots. 

Previous research had shown that a group of compounds called rapid alkinization factors (RALFs) and the hormone, auxin, play central roles in plant responses to external stimuli.

The group, which also included Saqer Alotaibi, of Taif University, Saudi Arabia, observed that root tip growth was inhibited within one minute of applying RALF1, a member of the RALF chemical family. 

This was accompanied, as expected, with a space within the cell walls, called the apoplast, becoming more alkaline. Auxin has separately been shown to rapidly inhibit root growth through this same mechanism.

The researchers then showed that RALF1’s rapid root growth inhibition was auxin-independent by demonstrating that it still occurred in mutant A. thaliana seedlings bred to be irresponsive to auxin. However, after one hour, these same plants began growing normally.

“This showed us that there are two distinct phases of RALF1 action,” says Friml. “There is a very rapid one, and a slower one that relies on auxin to function, and which sustains root inhibition over a longer period.”

The group also showed that RALF1 triggers auxin production, which then plays a role in limiting root growth. This two-phase effect, where RALF1 enables plants to respond rapidly to environmental change and to alter growth in the long-term was previously unknown.

“This potentially provides tools to develop crops that can increase the size of their roots independent of environmental conditions, which could be extremely important for agriculture in the future,” says plant biologist, Daniel Scherer, de Moura of São Paulo University, Brazil, who was not involved in the study.

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