Cutting edge research projects are currently in progress in the Pilon-Smits Lab. Many of these studies are discovering aspects of selenium hyperaccumulators that have never before been researched.  These studies involve the inner workings of selenium accumulation in hyperaccumulators and their effects on the local environment.

 

 

Hyperaccumulator Neighborhood

 

The presence of selenium hyperaccumulators is predicted to drive out species that are sensitive to selenium and attract species that are tolerant or resistant to the element.  Hyperaccumulators’ effects on neighboring plants could significantly alter the surrounding ecosystem’s plant diversity. Jason Reynolds, a graduate student in the Pilon-Smits Lab, is currently researching selenium hyperaccumulators and their effects on neighboring plants and animals. The areas including hyperaccumulators are predicted to have a different plant species composition and perhaps lower plant diversity than areas that do not. 

 

 The research is being conducted in an area where hyperaccumulators occur

naturally in abundance.   The study involves six sites naturally containing

selenium in the soil.  The sites are nearly identical in location, soil selenium

content, sunlight, and water received. The one major difference is that three

of the sites contain hyperaccumulators while three do not. The plots that

contain selenium hyperaccumulators will be compared with the plots that do

not, and the location of hyperaccumulators on each plot will be recorded and

mapped.  The species composition in areas containing hyperaccumulators and

areas not containing hyperaccumulators will be compared.  If the species

in the areas are wildly different, then this is in agreement with the hypothesis

that Se hyperaccumulators affect which plants thrive around them.

 

In an additional study, graduate student, Ali El Mehdawi is interested in the specific ways selenium hyperaccumulators affect neighboring plants. The study was designed to determine whether or not growing close to hyperaccumulators affects neighboring plants’ desirability to herbivores, particularly grasshoppers. 

 

Soils containing selenium hyperaccumulators and some without were sampled and tested. 

Soils with hyperaccumulators contained much higher levels of selenium

than the soils without hyperaccumulators.  The hypothesis was that

the high levels of selenium in the soil near the hyperaccumulators

would be less productive for growing selenium sensitive plants. 

Non-hyperaccumulating selenium tolerant plants, were predicted to

grow better in seleniferous soils. Hyperaccumulators and 

non-accumulatorswere sampled from the same seleniferous site. 

The hyperaccumulators collected were Prince’s Plume,

Stanleya pinnata, and two-grooved milkvetch, Astragalus bisculatus.

 The non-accumulators were dwarf sunflower, Helianthus pumilus,

and Alfalfa, Medicago sativa.   When grown near the hyperaccuularots, dwarf sunflower and alfalfa did not sprout easily.  Both plants were far less productive when grown near the hyperaccumulators then when grown near non-hyperaccumulators.  

 

The growth differences of heath aster, Symphyotrichum ericoides, and white sage, Artemesia ludoviciana, two selenium tolerant plants in the sunflower family, were compared near and far from the two hyperaccumulators. 

 

The collected plants were grown in a controlled setting to determine differences in growth based on plant proximity rather than other environmental factors.  Plants that grew in close proximity to hyperaccumulators collected up to forty times more selenium than the plants that grew separate from hyperaccumulators.  The plants that grew further from the hyperaccumulators also had two times more damage from herbivores as the plants that grew near the hyperaccumulators. This study suggests that accumulating selenium can benefit a plant's defenses to herbivory. 

 

To furthur the herbivore study, an experiment using grasshoppers collected from the same

site as the plants was initiated.  The grasshoppers were given the option of eating either the

plants grown close or far from the hyperaccumulators.  The grasshoppers always chose to

consume the lower selenium plants grown further from hyperaccumulators. 

When the grasshoppers had no choice to which plants they ate, there were almost

twice as many deaths in grasshoppers that ate the high selenium plants compared to

those who ate plants with lower levels of selenium. 

 

This study demonstrated positive effects that hyperaccumulators can have on their environment. 

Neighboring plants that are able to tolerate the high selenium levels bestowed upon them by

their hyperaccumulator neighbors can benefit from it. The selenium acts as a force field for the tolerant species, reducing their chance of being attacked by herbivores and other harmful organisms.   Understanding how hyperaccumulators benefit their surrounding environment will help researchers in the Pilon-Smits Lab share the importance of preserving these unique plants with the public. 

 

Hyperaccumulator Helpers  

Not all selenium hyperaccumulators, even plants of the same species, accumulate equal amounts of selenium. Selenium accumulation depends on several variables; differences in soil concentration, the surrounding vegetation, and possibly, endophyte concentration.  Microbial endophytes are organisms such as fungi or bacteria that live inside plants.  In many instances they contribute to their host plant’s health. They have even been found to protect the plant from things that stress it, such as drought or nutrient deficiencies.

 

The Pilon-Smits lab is researching microbial endophytes effects on

hyperaccumulators. The study will explore how endophyte presence in

selenium hyperaccumulators  may contribute to the plant’s ability to

concentrate high levels of selenium. 

 

Two selenium hyperaccumulators native to Fort Collins are

Prince’s plume, Stanleya pinnata, a member of the mustard family,

and two-grooved milkvetch, Astragalus bisulcatus, a member of the

pea family.  These plants were used as the basis for the endophyte

research.  In collaboration with researchers from Prague,

over seventy different bacterial endophytes were isolated from the two            

hyperaccumulators.  Of these strains, eight were chosen to test for                Endophytes: Natural Pest Repellents. yardcare.com.

their ability to promote plant growth and accumulation of selenium. 

The endophyte effects on the two hyperaccumulators are being tested, as well as effects on related agricultural crop plants.  The hypothesis tested is that endophytes are a major player in the hyperaccumulator's ability to concentrate such high levels of selenium. In another approach, different plants of the same species of hyperaccumulaters will be tested and compared for their levels of selenium accumulation, as well as the endophytes that reside in them.  If a correlation is found between the presence of a certain endophyte and plant selenium concentration, then that endophyte may play a role in selenium hyperaccumulation.  Identifying the relationships between endophytes and hyperaccumulators will drive researchers in the Pilon-Smits Lab one step closer to fully understanding the inner workings of these remarkable plants. 

 

Hyperaccumulators and Selenium

 

Numerous studies show that selenium hyperaccumulators concentrate selenium to levels up to one thousand times greater than non-hyperaccumulator plants when growing in seleniferous soils, but is the presence of selenium beneficial for these plants to survive? The importance of selenium to selenium hyperaccumulators is one of the many questions that puzzle researchers in the Pilon-Smits Lab. Graduate student Ali El Mehdawi is currently researching the answer to the vital selenium puzzle.   

 

To test the question whether or not selenium hyperaccumulators benefit from growing in soils with high levels of selenium, hyperaccumulators were sampled from their natural environment just east of Horsetooth Reservoir. Samples of two-grooved milkvetch and Prince’s plume were seeded and grown in pots; one with soil containing selenium, and the other without. The growth of the two pots were then compared.  The hyperaccumulators planted in soil with selenium grew two to three times larger than the plants in the selenium-free soil. 

 

The large difference in growth between the hyperaccumulators grown in selenium soils and

non-selenium soils demonstrates the importance of selenium to hyperaccumulators.  Not only do hyperaccumulators tolerate selenium soil, but they actually prefer it to other soils.  This is an exciting discovery that allows researchers to grow hyperaccumulators more efficiently and healthily.

 

Sulfur vs. Selenium

 

In the case of selenium hyperaccumulators the question remains: How can the plants manage such a high concentration of selenium without accumulating large concentrations of sulfur as well? Graduate students in the Pilon-Smits Lab are currently working on answering this question.

 

It has been hypothesized that selenium hyperaccumulators have sulfur/selenium transporters that prefer taking up selenium rather than sulfur, in contrast to non-accumulators whose transporters cannot distinguish between the two elements. The ability to distinguish between sulfur and selenium would allow hyperaccumulators to concentrate high levels of selenium in their tissues without also accumulating high sulfur levels.

 

Elements that a plant recognizes as similar are typically moved through the plant in the same way.  In most species, sulfur and selenium are viewed by the plant asessentially the same element. Consequently, they should move through the plant and be stored in similar places.  If a plant moves selenium differently from sulfur, then the plant is recognizing the elements as two different elements and treating them accordingly. Indeed, graduate student Jon Harris, together with undergraduate student Katie Schneberg discovered that hyperaccumulator Stanleya pinnata enriches itself with selenium relative to sulfur, while related non-hyperaccumulator Brassica juncea (Indian mustard) does not. Also, at high sulfate levels, the uptake of selenate by the hyperaccumulator was not affected, while it was completely inhibited in the mustard plants.

 

Jennifer Cappa, a graduate student in the Pilon-Smits Lab, is interested in researching how the distribution of selenium differs from sulfur distribution throughout the plant, and how this changes from hyperaccumulators to non-accumulators. Nine different, but closely related, plants were collected for a recent study designed to test the  selenium and sulfur distribuition in different plants. The collected plants were members of the mustard family, Brassicaceae. One of the collected species was a hyperaccumulator, and the rest were not. The plants were grown in soils containing selenium in a greenhouse to have control over the growing conditions.  Once the plants had matured, the concentrations of selenium and sulfur in the plants’ leaves, roots, and fruits were measured.  In the non-accumulators, the plants moved  sulfur and selenium equally, suggesting that the plant did not recognize a difference between the two elements. However, in the hyperaccumulators, selenium and sulfur were moved differently, with selenium being stored more readily than the sulfur. 

 

This discovery suggests that hyperaccumulators have an extra “selenium sense” compared to non-hyperaccumulators and they purposefully store more selenium than sulfur in their tissues. They are able to recognize a difference between selenium and sulfur that other members of the same family cannot.  This exciting discovery allows researchers in the Pilon-Smits Lab to further isolate how hyperaccumulators differ from non-accumulators. Hyperaccumulators’ ability to recognize additional elements compared to their closely related neighbors instills an interesting question to how hyperaccumulators evolved.  

 

In an additional study, the evolution of hyperaccumulators is being questioned.  How did plants in the same family evolved from not being able to accumulate selenium to accumulating massive amounts of the element?  Various plants from the mustard family were sampled and their ability to accumulate and tolerate selenium was compared. The genetic codes of the plants were compared and the resulting  information was used to construct an evolutionary tree that showed where each tested mustard plant evolved from.  From overlapping the plant’s selenium properties onto the evolutionary tree, the  history of when hyperaccumulation evolved can be determined. Discovering the genetic differences between these species may lead to a better understanding of what makes a hyperaccumulator a hyperaccumulator.   Understanding where hyperaccumulators differ from their neighboring nonaccumulators will help researchers in the Pilon-Smits Lab construct a history for these remarkable plants.  Such a history may lead to discoveries about previous environmental conditions and plant species that have yet to be studied. 

Leading information on selenium hyperaccumulators continues to be discovered and developed in the Pilon-Smits Lab.  The Lab aims to educate both the scientific and the overall communities on the import roll hyperaccumulators play in the environment.

 

 

Sources

 

Cappa JJ, Pilon-Smits EAH (2014) Evolutionary aspects of hyperaccumulation. Planta 239:267–275

 

El Mehdawi AF, Cappa JJ, Fakra SC, Self J, Pilon-Smits EAH (2012) Interactions of Selenium and Non-accumulators during Co-cultivation on Seleniferous or Non-Seleniferous Soil – The Importance of Having Good Neighbors. New Phytologist 10.1111/j.1469-8137.2011.04043.x

 

Harris J, Schneberg KA, Pilon-Smits EAH (2013) Sulfur - selenium - molybdenum interactions distinguish selenium hyperaccumulator Stanleya pinnata from non-hyperaccumulator Brassica juncea (Brassicaceae). Planta 239:479-491

 

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

 

El-Mehdawi AF, Quinn CF, Pilon-Smits EAH (2011) Effects of selenium hyperaccumulation on plant-plant interactions: Evidence for elemental allelopathy? New Phytologist 191: 120-131

 

Pilon-Smits, E. Reynolds, J. Microbial endophytes of selenium-hyperaccumulating plants: who is in there? Do they have special properties? Do they contribute to hyperaccumulation?. Novakova lab at ICT Prague. 2013.

 

Reynolds, J. IOS Preliminary Proposal: Ecological Reverberations of Plant Selenium Hyperaccumulation. 2013.

 

Interview conducted by Claire Tortorelli with Jason Reynolds. 2013.

 

Un-sourced photos property of Wikipidia Commons or the Pilon-Smits lab