Arsenic Uptake, Transport and Accumulation in Plants

Project Leader:
Mary Lou Guerinot Ph.D.
Associate Director, Toxic Metals Superfund Research Program
Professor, Biological Sciences
Dartmouth College

Project Co-Leaders:
Tracy Punshon Ph.D.
Research Assistant Professor, Biological Sciences, Dartmouth College
David E. Salt Ph.D.

Professor and 6th Century Chair, University of Aberdeen

The aim of this project is to understand the genetic control of arsenic uptake in plants, with the eventual goal of protecting plant foods from arsenic contamination. While arsenic as a contaminant of drinking water is a well-established public health priority, the issue of arsenic exposure via the diet is still emerging.

How does arsenic get into our food?

Arsenic occurs naturally in the environment, in minerals such as realgar and arsenopyrite and from there enters the soil, water and even the air. The historical use of arsenic-containing pesticides, long before modern environmental safety regulations were in place, is also an important source of arsenic in the environment. Arsenic--like other metals and metalloids--stays in the soil for long periods of time, where it can either be taken up by plants or washed down into the groundwater, and present a risk to human health. However, the most important factor determining whether arsenic in the soil gets into the plant-based food crops we eat every day, is the genetic makeup of the plant itself.

Arsenic can take numerous chemical forms that are broadly classed into inorganic or organic. Here the term 'organic' is used in its scientific sense to indicate arsenic binding to the elements associated with organic chemistry - carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Inorganic forms are more acutely toxic, but both forms are toxic to humans. Almost half of the arsenic in U.S. rice, for example, is in the inorganic or more toxic form, but the arsenic in rice from Bangladesh is almost all (80%) inorganic. Interestingly, different varieties of rice from all over the world grown on the same soil accumulated different amounts and different species of arsenic. The key to resolving arsenic accumulation in food plants lies in understanding the genes that control uptake from the soil and storage within the plant's edible portions.

Why does rice accumulate arsenic?

Rice has been described as a natural arsenic accumulator, but the reasons why it does so make it particularly unusual in comparison with other cerial plants. Under normal circumstances rice plants actively take up large amounts of silicon from the soil, unlike its close relatives wheat, barley and oats. They use silicon to strengthen their stems and the husks that protect the grain against pest attack. Scientists have imaged large 'silica bodies' in the leaves of rice plants. Arsenic and silicon are chemically very similar under the soil conditions found in flooded rice paddies, and as a result arsenic literally fits into the silicon transporters, and is integrated into the plant as it grows, finding its way into the grain--the part of the rice plant we eat. Scientists are avidly trying to unravel the complex chemistry of arsenic as it moves from the soil into the plant and the genes that control its movement. Our challenge is to understand the metabolism of arsenic in plants, in particular how it finds its way into the grain, and find ways to prevent cells from taking up arsenic.

Professor Mary Lou Guerinot is an eminent plant molecular geneticist, whose principal area is metal transport and regulation of gene expression by metals in plants. Professor Guerinot has identified many key genes, including one encoding a transporter essential for iron uptake from the soil. Iron deficiency anemia is the most prevalent mineral nutrient disorder in humans and Professor Guerinot's impressive body of work has made a highly significant impact on developing crops that offer sustainable solutions for malnutrition. In collaboration with Professor David Salt, Guerinot developed the ionomics database, a unique collection of the elemental profiles of over 11,000 lines in Arabidopsis, which also contains rice ionomic data. This open-access database allows researchers to literally browse through metals to find the genes associated with them, and is an invaluable tool in our research.

Dr. Tracy Punshon has worked for almost ten years in imaging various metal(loids) in biological materials, plants in particular, with synchrotron x-ray fluorescence microspectroscopy (SXRF). SXRF can be compared to an x-ray or CAT scan where the metals can be seen within the tissues without the need to section or destroy the sample. Dr. Punshon was one of the first biologists to apply SXRF to the analysis of plants, and alongside Professor Guerinot was the first to use SXRF as a way of characterizing genes.

This is an interdisciplinary project, which combines bioinformatics in the PiiMS database, molecular genetics of the model plant Arabidopsis and synchrotron imaging techniques to understand, and hopefully solve, the public health issue of arsenic in seed. It has also provided collaboration between projects in the Dartmouth Superfund Program--using the information from plant molecular genetics to inform gene searches in mammalian model organisms and clinical studies, as well as launching pilot projects to survey the forms and amount of arsenic in rice-based foods.

Recent Publications

Mary Lou Guerinot Pubmed

Tracy Punshon Pubmed

David Salt Pubmed