People are notorious for introducing toxins into once pristine environments;

however, many natural environments, free of human influence, contain high

levels of toxic metals and other substances in their soils.  In these areas

where many plants have a difficult time growing, “super plants”  flourish.  

 

"Super plants" are so called for their ability to thrive in conditions that can be difficult for other plants to grow.  In the science community, these plants are called hyperaccumulators.   Because hyperaccumulators are capable of growing in soils with high metal and other elemental concentrations, these plants are often found blossoming in areas where few other plants choose to grow.  Plant processes allow hyperaccumulators to concentrate toxins in their tissues to amounts that would be deadly to non-accumulating plants. These unique mechanisms make hyperaccumulators an interesting subject for plant physiologists.

 

Researchers in the Pilon-Smits Lab study hyperaccumulators with hopes of eventually applying their discoveries to increasing phytoremediation efficiency.  The high concentration of toxins in their tissues causes hyperaccumulators to grow more slowly than other nonacumulating plants.  Their slow growing-habits make hyperaccumultors less than ideal for phytoremediation. Studying the methods that hyperaccumulators use to take up high quantities of pollutants will help researchers apply these methods to faster growing plants that are more suitable for phytoremediation.

 

Researchers in the lab focus their studies on the processes that

hyperaccumulators use to take up selenium and the effects

these plants have on their surrounding environment. Selenium

is both a naturally occurring and a man-made element with the

chemical symbol Se.  Understanding the processes that selenium

hyperaccumulators use to take up high concentrations of selenium

is especially important in an area that has high amounts of

naturally occurring selenium in the soil such as Northern Colorado.

 

Shale, a type of rock from the cretaceous period, is abundant in

Fort Collins which is part of the Colorado Rocky Mountain

Front Range. The cretaceous shale is naturally rich in selenium. Over

time, wind and rain erodes the shale into soil, eventually

transferring selenium into the soil.  In these areas, selenium

hyperaccumulators can be found in abundance. 

 

Selenium is not necessary for plants to grow, in fact, too much                                 

selenium is deadly for most plants.  Selenium hyperaccumulators                                       MGMB Geologic Research/ Mapping - Geologic Timeline.

are able to concentrate selenium levels up to one hundred times the amount of non-accumulators.

How are these plants able to concentrate so much selenium?

 

Transporters

 

Plants are able to take up metals and other elements from the soil

into their tissues by proteins called transporters.  Each transporter

allows only certain molecules into the high security interior of the

plant.  Imagine the children’s game in which a box is presented with

holes cut in the top in the shape of a triangle, square, and circle. 

The circle piece will fit only through the circle cut-out in the box. 

Transporters and their matching elements work the same way. Just

as the circle will only fit in the circle cut-out, a sulfate ion can only

enter the plant through a sulfate transporter. 

 

Plants need many different elements to be taken into their tissues

to survive.  Transporters allow plants to take in the necessary elements and leave out harmful ones.  However, in some instances, elements can trick the transporter to let them in, even if they are not quite the right shape.  Picture a circle with a hole in the center still fitting into the circle cut-out. The imposter ion is close enough chemically to the transport’s desired ion that the plant may let it slip inside. For example, selenium tricks plants into taking them up through sulfur transporters.

 

Plants move the selenium through their tissues like a highly organized assembly line.  At certain stations on the line, the selenium may be altered into different forms by the same processes that the plant uses to alter sulfur.  This process uses plant proteins called "enzymes."

Selenium in its most common natural state, selenate, is taken up by the plant by sulfate transporters.  The plant uses its interior assembly line to convert selenate into various forms of organic selenium.  Some of these forms are volatile and may be emitted from the plant into the atmosphere as a gas. Other forms remain inside the plant until the plant dies and reintroduces the selenium back into the soil.  High concentrations of selenium associated with hyperaccumulators have noticeable effects on the surrounding environment.  Hyperaccumulators are closely examined by many members of the Pilon-Smits Lab.  Researchers hope to gain a better understanding of the internal working of these fascinating plants and how their unique physiology helps shape the environments in which they live. 

 

Selenium in the Environment

 

Selenium is an essentail element for human life.  The importance of this element only adds to the Pilon-Smits Lab researcher's facination of the plants that accumulate this precious metal.  Dr. Pilon-Smits states the importance of selenium to the environment in her article "Selenium Accumulation in Flowers and its effects on Pollination" published in The New Phytologist. Selenium is an essential element for all mammals, including people.  People need selenium to live healthy lives. Moderate amounts of selenium have even been shown to reduce the chance of cancer. Despite its positive effects, too much selenium can be toxic.  A surplus of selenium can cause problems for all walks of life, from loss of hair and nails, to possible death with extreme concentrations. High concentration of toxic elements, such as selenium, can alter the environment in which hyperaccumulators grow.

 

Plants that are extremley sensitive to selenium cannot grow in soils influenced by hyperaccumulators to hold high concentrations selenium. Similarly, herbivores and pathogens that are sensitive to selenium cannot feed on high-selenium plants. Because high levels of selenium are toxic to most animals, selenium in plants acts as a protectant against herbivores and pathogens. However, certain insects and pathogens that occur in selenium-rich areas have developed selenium tolerance and can feed on hyperaccumulators without suffering ill effects. Some plants that are tolerant to selenium have even been found to  benefit from growing next to hyperaccumulators in selenium-rich areas. These plants maintain higher selenium levels in their leaves which protects them from herbivores. It is clear that hyperaccumulators have effects on their surrounding environment. The Pilon-Smits Lab is one of the only labs researching and publishing findings on how selenium hyperaccumulators affect their surounding environment. 

 

 

Sources

 

Quinn CF, Prins CN, Gross AM, Hantzis L, Reynolds RJB, Freeman JL, Yang SI, Covy PA, Bañuelos GS, Pickering IJ, Fakra SF, Marcus MA, Arathi HS, Pilon-Smits EAH (2011) Selenium Accumulation in Flowers and its Effects on Pollination. New Phytoogist 192: 727-737

 

Mehdawi AF, Quinn CF, Pilon-Smits EAH (2011) Selenium Hyperaccumulators Facilitate Selenium-Tolerant Neighbors via Phytoenrichment and Reduced Herbivory. Current Biology 21: 1440-1449

 

Unlabeled photos sourced from Wikipidia Commons.