An Investigation of Plant-Microbe Interactions Under Cadmium Stress in Agar-Based Medium, Hydroponics, and Soil Studies

This thesis investigated plant-microbe-metal interactions at two scales: a single plant-microbe system and an agricultural rhizobacterial community. The first objective was to investigate the effectiveness of a plant growth-promoting rhizobacterium (PGPR) on mediating cadmium stress in a plant model system. Arabidopsis thaliana Col-0 was inoculated with Pseudomonas putida UW4, which in its wild type form has been reported to reduce plant stress by simultaneously metabolizing the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) with the enzyme ACC deaminase and stimulating plant growth through the production of indole-3-acetic acid (IAA). A mutant strain that lacks ACC deaminase and a no bacteria treatment were used as controls. When plants were grown on agar-based or hydroponic Murashige and Skoog (MS) medium containing cadmium both strains of PGPR had deleterious effects on plant growth. Further investigation revealed that the PGPR were unable to survive in MS medium without the presence of a plant. Loss of plant growth-promotion was hypothesized to be due to the unfavourable environment for the bacterium. To test this, agricultural soil was maintained for 28 days with either MS medium or distilled water and the bacterial community profile was analyzed using terminal restriction fragment length polymorphism (TRFLP) analysis. A decrease in fragment richness was observed in the MS medium treatment, which lends further support to the theory that certain environmental conditions can be detrimental to rhizobacteria. The final objective was to determine if the rhizosphere microbial communities varied among two pairs of high and low metal-accumulating plants (two cultivars of Triticum durum, Kyle and Arcola, as well as Brassica juncea and B. napus). Plants were grown in agricultural soil containing cadmium and the microbial community profiles were analyzed using TRFLP. When the plant’s metal-accumulating ability was well matched to the metal concentration in the soil a unique rhizobial community developed; when they were unmatched, the rhizobacteria did not differ from the bulk soil. As a whole, this thesis demonstrates the complex nature of plant-microbe-metal interactions and the need to continue to look at these systems. Knowledge gained will help in properly matching PGPR to field applications to increase the efficacy of bioremediation strategies, agricultural yields, and food safety