Marco Polo had it right. It was around 1295, and his caravan to China was struggling across the barren and frightening lands near the Gobi Desert. Back then, travelers were rare and truck stops were even rarer. Travelers had to pack everything with them, including extra horses and meat on the hoof, which meant that caravans included herds of sheep, cattle and horses. As Marco Polo's procession crossed that vast range country, the animals grazed whatever they could find, and they were suffering some health problems. He observed these disturbing symptoms, and he recorded them in his diary: “a poisonous plant … which if eaten by [horses] has the effect of causing the hoofs of the animal to drop off.”
It was the first recorded description of selenium toxicity. Marco Polo didn’t have any spectroscopy equipment with him to analyze the forage, and besides, selenium hadn’t been discovered yet (that wouldn’t happen until 1817), but like any blogger today, he recorded what he saw: the classic symptoms of chronic selenium toxicosis.
Then he did the next logical thing: He trailed his caravan out of that region to a place where pastures were greener and safer, like China.
This month, however, we’re less interested in the veterinary details of selenium toxicity than in the unusual plants that cause it. We’ll focus on the selenium accumulator plants that thrive in arid regions – the toxic plants that distressed Marco Polo, the toxic plants notorious in cowboy movies for causing “blind staggers” and “alkali disease.”
Unraveling nature's biochemical mystery
First, of course, some background about selenium. Although mammals require selenium only in exceedingly small amounts – usually just 0.1 parts per million (ppm) of their diet – it has some extremely important metabolic functions. Selenium occurs in more than 20 different mammalian proteins that are involved in everything from protection against oxidation to activation of iodine hormones. Ironically, most plants do not require selenium as a nutrient, but nonetheless they absorb selenium through the roots because, chemically, it resembles sulfur.
This resemblance is actually quite important. Remember the periodic table? (Of course you do.) Recall that elements in the same column tend to share certain atomic characteristics, such as electronic configurations and bond energies. Well, just above selenium in the periodic table is sulfur, so these two elements can sometimes get interchanged in chemical reactions. For example, two common amino acids that contain sulfur are cysteine and methionine. If selenium replaces sulfur, these amino acids become selenocysteine and selenomethionine, respectively. Animal proteins typically contain selenocysteine – there is even a DNA sequence that encodes for it – while forages and plants used in human foods typically contain selenomethionine.
In practice, however, we’re really interested in how much selenium occurs in plants. For most plants, this usually reflects the amount of available selenium in the soil. Soil selenium can occur in a number of possible forms: selenate, selenite, selenide, elemental selenium and also in organic complexes. Selenate is the form that is most soluble and available to plants. It is also more common in alkaline soils (soil pH greater than 7). The less-available forms like selenide and selenite predominate in acidic soils (soil pH less than 7). This helps explain why some regions suffer from selenium deficiency: Forages growing in acidic soils will routinely contain selenium levels lower than 0.1 ppm. Forages growing in alkaline soils usually contain selenium levels higher than 0.1 ppm – high enough to meet nutritional requirements and prevent deficiencies in our livestock. And for the record, we currently believe that selenium toxicity can occur if the selenium levels are higher than 3 to 5 ppm (depending on the livestock species).
A closer look at plant-animal interactions
Which brings us to the plants that march to their own drummer: “selenium accumulator plants.” The scientific literature uses terms that are a bit confusing, containing references to hyperaccumulator plants, selenium indicator plants, selenium converter plants, etc. But whatever we call them, they certainly are different. These plants don’t just accumulate a little extra selenium; they sock it away like Scrooge and his bank account. Accumulator plants can have selenium levels of 1,000 ppm and even as high as 15,000 ppm. That’s impressive.
Which plants? Actually, quite a few plant groups contain selenium accumulators (for brevity, I’ll just list the genus). Examples: Astragalus (various species of milkvetch), Oonopsis (goldenweed), Castilleja (Indian paintbrush) and Stanleya (prince’s plume). Some are legumes, some are not. These selenium accumulators all grow in arid or desert regions across the western U.S. and Canada – usually as isolated plants or shrubs, or as clumps of plants that rise from the dry gravel or sand. And they’re not just in North America. At least two species – Morinda reticulata (mapoon) and Neptunia amplexicaulis (a legume) – grow in Queensland, Australia. And of course, at least one species attracted the attention of Marco Polo in Central Asia.
These plants don’t accumulate such high levels of selenium because they grow on selenium-toxic soils like industrial waste dumps. No, these plants grow naturally in dry range country. Nearby plants on the same soils contain selenium levels typical of alkaline soils: 0.1 to 3 ppm. But selenium accumulator plants are designed differently.
First, they are ultraefficient in finding and absorbing selenium from the soil. Their roots actively absorb selenate preferentially over sulfate.
Second, once they absorb selenium from the soil, they sequester it in safe compounds that protect them from toxicity. Safe compounds? Well, plants can suffer from selenium toxicity too. For example, after a normal (non-accumulator) plant absorbs selenium and uses it to make selenomethionine, this amino acid is then inserted into proteins instead of regular methionine. But from a metabolic point of view, selenomethionine is not methionine, and the chemical differences between these two amino acids can reduce the effectiveness of proteins. If too many proteins are affected, the plant can’t accomplish its metabolic tasks, and it dies. It’s as if we built a bridge using aluminum girders instead of the steel girders required in the original blueprints. The bridge may hold together under normal use, but under the stress of heavy traffic or high winds, it might collapse.
In contrast, selenium accumulator plants avoid this toxicity problem by incorporating all that extra selenium into unusual compounds like methylselenocysteine and selenocystathionine instead of selenomethionine. These compounds can’t be inserted willy-nilly into the plant’s functional proteins, so they don’t interfere with the plant’s metabolic processes. Instead, these compounds accumulate to astronomical levels in the leaves and shoots.
But now we must ask, what’s in it for the plant? Why does the plant go to all this trouble to accumulate 1,000 ppm selenium or more? Frankly, we don’t know (aside from it being a fascinating topic for an article). But scientists speculate that it’s part of the biochemical warfare in the plant world. Because high levels of selenium are toxic to most living things, plants that accumulate high levels of selenium protect themselves from other predatory living things – like insects, pathogenic bacteria and fungi, and grazing ruminants. I don’t know what these plants taste like to an insect, but they are definitely not palatable to sheep and cattle. Also, some of those excess selenium compounds break down into dimethyl selenide, which is a pungent gas. So these plants can stink like garlic or sulfur – not exactly appetizing to livestock.
But if you are herding sheep or cattle across a wide, dry plain, with your animals hungry and only a few green plants sticking up from the bare ground – well, those plants can seem mighty attractive to your animals. And at 1,000 ppm selenium, it doesn’t take many plants to cause toxic symptoms. Which brings us full circle back to Marco Polo. I’m willing to bet that after he wrote those observations, he didn’t eat a salad that night.
