Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Biology

Supervisor

Macfie, Sheila M.

Abstract

Three soybean cultivars with contrasting retention of cadmium (Cd) in the root were grown in Cd-spiked nutrient solution and used to determine that symplastic compartmentalization of Cd in roots is probably responsible for retention of Cd in roots. Roots of the low Cd-accumulator AC Hime treated with 30 mM Cd exuded up to 10-fold higher concentrations of citric, succinic, fumaric and malic acids into the hydroponic solution when compared to control; concentrations of the same organic acids from the high Cd-accumulator Westag 97 increased by up to 3-fold. The same cultivars were grown in Cd-spiked soil and the physiological profiles of the rhizosphere bacteria were assessed using Biolog® EcoPlatesTM to test the hypothesis that bacterial community diversity increases in response to organic acids exuded by the plant. Bacteria in the rhizosphere of AC Hime, the cultivar with the highest Cd-induced exudation of organic acids, had a distinct carbon utilization pattern, illustrating important interactions among rhizosphere bacteria, plants and toxic metals as a result of metal contamination.

Summary for Lay Audience

Cadmium contamination in agricultural soils is increasing through anthropogenic activities such as mining, smelting and the disposal of municipal wastes. Crops grown on cadmium-contaminated soil may have reduced yield if they suffer from cadmium toxicity. Crops that can tolerate cadmium may pose a health risk if the cadmium reaches the edible parts. It is important to understand how plants can reduce the uptake and movement of cadmium from roots to aboveground parts, including leaves and seeds. This project studied soybean, whose seeds are used to make animal feed, tofu, oil and other soy products.

In this study, three cultivars (types) of soybean that vary in the amount of cadmium found in the seeds were used to determine what controls the movement of cadmium up from the roots. I discovered that the cultivar that kept more cadmium in the roots stored more cadmium inside the root’s cells (a region called the symplast). In all three cultivars, exposure to cadmium was associated with the production of molecules (called organic acids) that bind with cadmium. These molecules were released from roots into the surrounding environment, a region called the rhizosphere where many beneficial bacteria live. The cultivar with more cadmium stored in the root produced more organic acids and had a lower level of activity in the community of bacteria in its rhizosphere. However, the cultivar with less cadmium in the root had a stronger response to cadmium (a greater stimulation of organic acid production) and was associated with a higher level of activity in its community of bacteria. These results could help plant breeders to develop new cultivars with reduced cadmium uptake and movement. It also provides information about the relationship between plants and bacteria in contaminated soils.

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