Different adaptation strategies to saline soils identified in neighbouring populations of the same plant species

Researchers from the Universitat Autònoma de Barcelona (UAB) have identified two divergent adaptive responses to soil salinity in populations of the same wild species found in the Catalonia’s coastal area, the Brassica fruticulosa, and have pinpointed the genes involved. The study will help to investigate the ways to improve resilience in agricultural species of the same plant family, such as rapeseed and mustard, in the face of a globally relevant stressor as is soil salinization.

The study was recently published in Proceedings of the National Academy of Sciences (PNAS) and is signed by researchers Sílvia Busoms, Glòria Escolà and Charlotte Poschenrieder from the UAB Plant Physiology Lab, in collaboration with researchers from the University of Nottingham (UK), the Max Planck Institute for Biology (Germany), and Charles University in Prague (Czech Republic).

Over the past few years, UAB researchers have worked in close collaboration with members of the University of Nottingham to develop a study model along the Catalan coast to understand the interaction between environmental factors such as salinity and the adaptation of wild populations of the Brassicaceae family. They developed several studies focused on populations of Arabidopsis thaliana, a model organism for biological research, but in this case, they focused on Brassica fruticulosa, a species genetically and morphologically closer to cultivated brassicas such as rapeseed (Brassica napus) and mustard (Sinapis alba).

This research has allowed them to demonstrate that in Catalonia coastal populations of B. fruticulosa use two different strategies to tolerate soil salinity: those from the north (Cap de Creus region) are able to restrict root-to-shoot sodium transport, preventing the damage of the aerial parts. In contrast, those from the centre accumulate sodium in the leaves, but they use efficient mechanisms of osmotic adjustment and compartmentalisation that allow them to tolerate high concentrations of this compound.

The fact that two populations of the same plant species located so close geographically have evolved differently under the same environmental conditions surprised the researchers. “In general, in all organisms it is expected that species adapting to similar stressors also evolve in a similar way. In our case, however, although in the coastal habitats of the Catalan coast soil salinity can be considered the main selective agent, there must be other factors that have altered the recent evolutionary process of this Brassicaceae species,” Sílvia Busoms points out. This divergence in plant populations so close to each other has rarely been described, not so much because it is an exception, but because in many cases the studies are carried out at the macro-scale.

The Tramontane wind may explain this divergence

In their study, researchers examined in detail the characteristics of the soils and the climatology of all the populations location. The only parameter that showed significant differences was evapotranspiration, which was higher in the north due to the Tramontane wind that regularly blows there. “When there is high evapotranspiration, plants absorb more water and at the same time more sodium if they do not have mechanisms to exclude it. Therefore, the strategies used by the plants of the central coastal areas may be insufficient in the conditions of the northern coast. In the study we hypothesise that although they are neighbouring populations, the northern B. fruticulosa evolved differently in order to tolerate both high salinity levels and high evapotranspiration,” explains Charlotte Poschenrieder.

To characterise the genetic basis of the two adaptive strategies identified, researchers first created the reference genome of B. fruticulosa, which will contribute to the expansion of the catalogue of reference genomes of eukaryotic species from the Catalan-speaking territories (within the Earth Biogenome Project) and will allow further research with this species. Subsequently, the sequencing of 18 populations and the subsequent genetic and transcriptomic analyses validated the two strategies observed and allowed researchers to propose candidate genes involved in the mechanisms of salinity tolerance.

Salinity is a threat to the planet's agricultural soils and its consequences are greater when it affects impoverished soils such as those found in the Mediterranean basin. A better understanding of the mechanisms of salt tolerance used by plants living there and which have adapted to these conditions is essential to improve the resilience of cultivars that must adapt to the new environmental conditions. “This study, therefore, establishes B. fruticulosa as a promising source of desirable alleles, and the population diversity present in Catalonia as a powerful model for the study of adaptations to saline soils,” researchers conclude.