Wild Relatives Confer Salt Tolerance

close up of brown and green plants against blue sky

Biological Strategy

Halophytes

UBC Botanical Garden

Image: Erik-Jan Leusink / Unsplash / Free non-commercial use

Protect From Chemicals

Chemicals are everywhere in the bodies of living organisms and their external environments. While most chemicals are valuable or benign, some are toxic, including those used for defense (such as the mucus that protects clownfish from an anemone’s stinging tentacles). Even naturally-occurring chemicals, such as arsenic, must be managed to reduce their impact. Some living systems have strategies to break down harmful chemicals, alter them into less toxic forms, physically prevent chemicals from harming sensitive tissues, and more. For example, some herbivorous mammals can digest toxic compounds in plants because they have a particular enzyme that helps them process poisonous plant compounds.

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Adapt Phenotype

Living systems evolve over time in response to selective pressures. Selective pressures are environmental factors that reduce the reproductive success of some individuals in a population. Genetic makeup (genotype) is one way in which living systems mitigate selective pressures, but another is adaptable phenotypes. A phenotype is an observable characteristic that can be thought of as the expression of the genotype, combined with modifications caused by the environment or developmental conditions. Individuals, populations, or ecosystems that have phenotypes capable of reducing the effects of selective pressures can survive. Some plants, for example, modify their leaf shape in response to changing environmental conditions. A single olive tree has variable leaf shapes on sunny compared to shady areas of the tree, yet the next year, those same buds may develop differently shaped leaves.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Salt tolerance evolved differently across plant species

Halophytes, or salt-tolerant plants, are found in a number of environments: seashores, mangrove swamps, salt marshes, and saline semi-deserts. As the amount of salt in agricultural soils increases, it’s important to understand how different plants cope with salt, a mechanism known as salt tolerance.

Some plants are much more tolerant of salt than others. Many of the crops we use in agriculture, such as wheat, rice, and corn, have a very low salt tolerance, although their wild relatives (non-domesticated species that are closely-related to food crops) are oftentimes much more tolerant. So, why haven’t food crops evolved to adopt this tolerance?

It is hypothesized that that salt tolerance is an “evolutionary labile” trait, which means that it is less correlated with other traits and therefore doesn’t face the same evolutionary “pressures” that would require it to be expressed in each generation. These traits often vary widely in how they are expressed between individuals and between related groups of organisms. Another way to think of labile traits is that they are not “locked in” within evolutionary lineages. Some examples of evolutionary labile traits are reproductive timing and migration timing. This is in contrast to non-labile traits, which are much more ‘constrained’ by evolution and are less likely to change, for example, leaf size and color.

Preserving wild relatives of food crops allows us to study and understand how salt tolerance has evolved in different species. As sites of both public engagement and plant conservation, botanical gardens are working to educate the public on the value of conserving a variety of different plant species. You can learn more by reading this open-access chapter on Public Education and Outreach Opportunities for Crop Wild Relatives in North America, co-authored by UBC Botanical Garden’s Dr. Tara Moreau.

Adapted from the UBC Botanical Garden “Botany Photo of the Day“

Last Updated February 17, 2020

References

“Despite concerted effort to develop salt-tolerant cereal crops, few commercially viable salt-tolerant crops have been released. This is puzzling, given the number of naturally salt-tolerant grass species. To better understand why salt-tolerance occurs naturally but is difficult to breed into crop species, we take a novel, biodiversity-based approach to its study, examining the evolutionary lability of salt-tolerance across the grass family. We analyse the phylogenetic distribution of naturally salt-tolerant species on a phylogeny of 2684 grasses, and find that salt-tolerance has evolved over 70 times, in a wide range of grass lineages. These results are confirmed by repeating the analysis at genus level on a phylogeny of over 800 grass genera. While salt-tolerance evolves surprisingly often, we find that its evolution does not often give rise to a large clade of salt-tolerant species. These results suggest that salt-tolerance is an evolutionarily labile trait in grasses” (Bennett 2013:1)

Journal article