Some plants, which we call “extremophiles”, tolerate or even appreciate very salty, very dry or very cold environments, where most plant species would not survive. The mechanisms of response to environmental stresses have been studied for some time using the common plant Arabidopsis thaliana (women’s cress), which belongs to the family of mustard, rapeseed or even cabbage. It was chosen as a model organism for the many advantages it has: fast life cycle, abundant seed production, self-fertilization, relatively small genome … However, it is far from tolerating extreme environmental conditions! José Dinneny’s team, a professor at Stanford University in California, proposes a different approach, studying the stress response not in a sensitive species, but in a resistant species.
“It was time to choose the right models to understand these mechanisms, approves Alexandre Berr, researcher at the CNRS Institute of Molecular Biology of Plants (IBMP), who studies these extremophilic plants. Especially since genome sequencing has never been so technically and financially convenient. The other originality of this work was to compare the responses to stress, in this case saline (strongly linked to water stress and therefore to human activity and global warming), of four species with similar genomes: two naturally tolerant (Eutrema salsugineum And Schrenkiella parvula) and two other sensitive ones (Sisymbrium irio And Arabidopsis thaliana).
First observation: in a saline environment, while sensitive plants stop the growth of their roots, tolerant plants continue to grow … To understand this difference in behavior, the team focused on a “classic” way of responding to plants: the regulation of gene expression under the effect of a known plant hormone for the control of their growth under stress conditions, the abscisic acid (ABA). ABA generally acts as a growth inhibitor when conditions become less favorable, allowing the plant to save its resources while waiting for improvement. In a singular way, in one of the two extremophilic plants studied, Schrenkiella parvulaon the contrary, ABA causes an acceleration of growth.
Relying on high-throughput sequencing to quantify changes in gene expression in response to ABA (RNA-Seq, RNA sequencing) and to identify regulatory sequences in genomes (DAP-seq or sequencing for DNA affinity purification), the scientists found significant differences Schrenkiella parvula. They also highlighted the importance of other plant hormones such as auxin, known for its important role in controlling growth and development.
Without questioning the interest of these findings, Alexandre Berr points out, however, that the direct link between saline stress tolerance of Schrenkiella parvula and the uniqueness of its response to high ABA concentrations remains to be established. “For example, it would have been interesting to quantify ABA, a routine analysis, to find out if this plant synthesizes more than others or if it accumulates more quickly under stressful conditions,” he notes.
In any case, this study highlights the interest of extremophilic models in improving the understanding of the response and tolerance mechanisms of plants to environmental stresses. It also highlights the diversity of extremophilic plant strategies: preserving their roots with a protective layer, stiffening their cells or, as here, diverting the response pathways to ABA. It will be necessary to wait to learn more to consider transferring these findings by transgenesis or gene surgery to related crop plants.