Salt tolerance is a critical stress response in many plants and is controlled by a wide variety of interacting genes. Researchers studying sodium transporters in trees from high-salinity environments have characterized the evolution of these genes and determined that they are under strong positive selection in salty soils.
Soil is the growth medium and main source of water, nutrients, and minerals for most plant species. However, an under-appreciated fact of soils is their extreme variability.
Along with many soil properties that vary across the globe, such as pH and temperature, salinity is an important characteristic determining how well a given plant will thrive in a given soil.
This is because high salinity soil often translates to high salinity of available water, which is drawn into the plant via roots and distributed to each of its cells.
Given that plant cells themselves are water-based, they must maintain a balance between the amount of salt within their walls and in the water delivered to them. An imbalance in salt content can lead to osmotic challenges that cause decreased photosynthetic activity or even death of the plant.
A large proportion of soil salt is usually made up of sodium chloride (table salt), and consequently a major mechanism plants have evolved to maintain equilibrium is sodium transporters.
The main function of these proteins is to move a sodium ion out of the cell by importing a hydrogen ion in its place (or vice versa).
In high-salt conditions, this effectively eliminates the salt-forming sodium ion from the cell, helping keep its osmotic balance intact.
As we developed technologies to sequence large amounts of DNA over the last two decades, hundreds of genes related to sodium transport have been characterized in plants, bolstering the notion that they are critical for cell health.
This has led researchers to hypothesize that a high-salt environment might be imposing pressure on these sodium-hydrogen transporters and driving their evolution – in other words, Darwinian natural selection may act on these salt-tolerance genes when salt is stressful.
A recent study has examined this possibility in two closely related poplar tree species, Populus euphratica and Populus pruinosa, which live in the highly saline soils of northwest China.
The research team from Lanzhou University in China used modern statistical techniques to measure the magnitude of the positive selection signature on sodium transport genes compared to its role on some non-salt-tolerance genes in these organisms.
They concluded that sodium transporters were much more frequently targets of natural selection compared to non-salt-tolerance genes, suggesting that evolution has acted on the poplar species in such a way that has selected for salt tolerance.
This is what one might expect from a classical model of evolution by natural selection.
More interestingly, however, is that the strength of selection of two sister species was much different.
The poplars are estimated to have diverged just 0.66-1.37 million years ago. However, P. pruinosa had a much higher sodium transporter gene diversity and stronger selection on these genes compared to P. euphratica.
This indicates that, although natural selection was acting on salt-tolerance genes in both species, it may not have been acting in the same way.
Although this study was technically dense and used sophisticated evolutionary models to support its findings, the conclusions of the paper are quite intuitive:
First, in the high-salt environment of these trees, “failed” mutations that hurt a plant’s salt tolerance are purged, leaving a signature of positive selection on the remaining “useful” genes.
Second, these species don’t do much gene sharing (sex) and, however subtle, they do not “feel” evolutionary pressure in the same way, despite sharing the same environment.
This last point is notable because if we would like to, say, engineer a corn plant to tolerate the increasing saltiness of our fields, then we must account for the ongoing process of evolution by natural selection in order to keep our gene from being purged from the population.