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. Response mechanisms to environmental pollution have been studied in common plants for a long time Arabidopsis thaliana (Lady cress), which belongs to the mustard, rapeseed or 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 of four species with similar genomes to stress, in this case saltwater (strongly linked to water stress and therefore to human activities and global warming): two inherently tolerant (Eutrema salugineum and Schrenkiella parvula) and two more sensitive (Sisymbrium irio and Arabidopsis thaliana).
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 like 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 parvulaon the contrary, ABA causes growth acceleration.
Using high-throughput sequencing to quantify variations in gene expression in response to ABA (RNA-Seq, RNA sequencing) and for identifying regulatory sequences in genomes (DAP-seq or DNA affinity purification sequencing), scientists found remarkable differences in Schrenkiella parvula. They also emphasized the importance of other plant hormones such as auxin, known for its important role in controlling growth and development.
However, without questioning the interest of these discoveries, Alexandre Berr points out that the direct link between the stress tolerance of salt 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.
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