Some plants that we call “extremophiles” tolerate or even appreciate very salty, very dry, or very cold environments in which most plant species would not survive. The mechanisms of response to environmental stress have long been studied using the widespread plant Arabidopsis thaliana (queen cress), which belongs to the mustard, canola, and cabbage family. It was chosen as a model organism because of its many advantages: fast life cycle, abundant seed production, self-pollination, relatively small genome… However, it is far from tolerant of extreme environmental conditions! The team led by José Dinneny, a professor at Stanford University in California, offers a different approach by studying the response to stress in a resistant species rather than in a susceptible species.
“It was high time to choose the right models to understand these mechanisms,” confirms Alexandre Berr, a researcher at the CNRS’s Institute of Molecular Biology of Plants (IBMP), who studies these extremophilic plants. Especially since genome sequencing has never been so technically and financially affordable. The other originality of this work was to compare the responses to stress, in this case saline (strongly linked to water stress and therefore human activities and global warming), of four species with similar genomes: two naturally tolerant ones (Eutrema salsugineum and Schrenkiella parvula) and two other susceptible ones (Sisymbrium irio and Arabidopsis thaliana).
On the same subject
No. 77 – October 2012
Unusual plants
First observation: while sensitive plants stop growing their roots in a saline environment, tolerant plants continue to grow… To understand this behavioral difference, the team focused on a “classical” mode of plant response: the regulation of gene expression under the action of a well-known plant hormone to control their growth under stress conditions, abscisic acid (ABA). ABA generally behaves as a growth retardant when conditions become less favorable, allowing the plant to conserve resources while it waits for improvement. In one of the two extremophile plants studied, Schrenkiella parvula, it is unique that ABA causes growth acceleration.
Using high-throughput sequencing to quantify variations in gene expression in response to ABA (RNA-Seq) and to identify regulatory sequences in genomes (DAP-seq or DNA affinity purification sequencing), scientists found striking differences in Schrenkiella parvula. They also emphasized the importance of other plant hormones such as auxin, which is known for its important role in controlling growth and development.
Without questioning the importance of these discoveries, however, Alexandre Berr points out that the direct link between the salt stress tolerance of Schrenkiella parvula and the uniqueness of its response to high concentrations of ABA has yet to be established. “For example, it would have been interesting to quantify ABA, a routine analysis to find out whether this plant synthesizes more of it than the others or accumulates faster under stressed conditions,” he notes.
In any case, this study underscores the interest of extremophile models in improving the understanding of the mechanisms of plant response and tolerance to environmental stress. It also shows the variety of strategies used by extremophile plants: to keep their roots covered with a protective layer, to stiffen their cells, or, as here, to redirect the pathways to respond to ABA. One must wait to know more before considering extending these findings to related crops through transgenesis or gene editing.
