Extremophile plants, a model for understanding adaptation to environmental stresses

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. For a long time, response mechanisms to environmental stresses have been studied using the common plant Arabidopsis thaliana (ladies’ cress), which belongs to the family of mustard, rapeseed or even cabbage. It was chosen as a model organism because of the many advantages it has: rapid life cycle, abundant seed production, self-fertilization, relatively small genome… However, it is far from tolerating extreme environmental conditions! The team of José Dinneny, a professor at Stanford University in California, offers a different approach, by studying the response to stress not in a sensitive species, but in a resistant species.

“It was high time to choose the right models to understand these mechanisms,” agrees Alexandre Berr, researcher at the CNRS Institute of Molecular Biology of Plants (IBMP), who studies these extremophile 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, saline in this case (strongly linked to water stress and therefore to human activity and global warming), of four species with similar genomes: two naturally tolerant (Eutrema salsugineum et Schrenkiella parvula) and two other sensitive ones (Sisymbrium irio et Arabidopsis thaliana).

First observation: in a saline environment, while sensitive plants interrupt the growth of their roots, tolerant plants continue to grow… To understand this difference in behavior, the team focused on a “classic” mode of response of plants: the regulation of gene expression under the effect of a well-known plant hormone for controlling their growth under stress conditions, abscisic acid (ABA). ABA generally behaves as a growth inhibitor when conditions become less favorable, allowing the plant to save its resources while waiting for an improvement. In a singular way, in one of the two extremophile plants studied, Schrenkiella parvulait is the opposite, ABA causes an acceleration of growth.

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By relying on high-throughput sequencing to quantify variations in gene expression in response to ABA (RNA-Seq, RNA sequencing) and to identify regulatory sequences in genomes (DAP-seq or DNA affinity purification sequencing), scientists found notable differences in Schrenkiella parvula. They also highlighted the importance of other plant hormones such as auxin, known for its major role in controlling growth and development.

Without calling into question the interest of these discoveries, Alexandre Berr points out, however, that the direct link between the salt stress tolerance of Schrenkiella parvula and the uniqueness of its response to high concentrations of ABA remains to be established. “For example, it would have been interesting to quantify the ABA, a routine analysis, to find out if this plant synthesizes more of it than the others or accumulates it more quickly under stress conditions”, he notes.

Anyway, this study underlines the interest of extremophile models in improving the understanding of the mechanisms of response and tolerance of plants to environmental stresses. It also highlights the diversity of strategies of extremophilic plants: 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 know more to consider transferring these results by transgenesis or gene surgery to related cultivated plants.

Extremophile plants, a model for understanding adaptation to environmental stresses