Impact of plant ecotype on Bacillus-mediated growth promotion : extending beyond the standard model plant

Abstract: This study investigated how bacterial treatment modulates plant growth. Bacillus amyloliquefaciens UCMB5113 was used for treatment of several different natural accessions (ecotypes) as well as mutants of Arabidopsis thaliana. Treatment effect on plant root architecture was studied in particular. Increased quantitative and qualitative production has long been an underlying motivation for agricultural development. We are currently in a phase of environmental awareness that calls for agricultural practices that cause less strain on natural ecosystems. However, increased production and lower environmental impact are two goals that blend poorly. Bio-fertilization and bio-control have gained increased attention lately as approaches to reducing the use of synthetic pesticides and fertilizers in agriculture, thus lowering environmental impact. In short, the terms describe the use of natural organisms to combat pests and increase yield in crops. Soil-borne rhizobacteria have been found to host a number of candidates for biocontrol and biofertilization. Among these is the Bacillus amyloliquefaciens subsp. plantarum strain UCMB5113 which has been shown to possess the ability of promoting growth and increasing pest resistance in plants. The mode of action is largely unknown and most of the work concerning it is undertaken using A. thaliana, and particularly the natural accession Colombia (Col-0). In this thesis, a number of natural A. thaliana accessions (ecotypes) as well as mutants were treated with B. amyloliquefaciens in order to contribute to a better understanding of the properties underlying a fruitful interaction between these two organisms. Such knowledge could prove valuable in the identification or development of agricultural crops able to be cultivated effectively with UCMB 5113 bacteria. Treatment with B. amyloliquefaciens UCBM 5113 proved to significantly increase root hair growth in half of the tested A. thaliana accessions. The increase in total root hair length was a result of either higher number of root hairs, increased length of individual root hairs or a combination of both. The root hair-deficient mutant N2259 showed formation of root hairs following treatment with Bacillus, which could indicate an ability to restore calcium ion influx to root hair cells. When incubating UCMB 5113 with A. thaliana root exudates, some of the A. thaliana root exudates could be differentiated from one another based on the growth kinetics of Bacillus. Root exudates from Ag-0, Stw-0 and Cvi-0 led to the highest endpoint growth of Bacillus, while Col-0, Ler-0 and Edi-0 resulted in the lowest Bacillus growth. The importance of plant accession for volatile organic compound (VOC)-mediated bacterial plant interaction was studied in a concluding set of experiments, using five accessions. Can-0 increased total root growth as a result of exposure to VOC emitted by B. amyloliquefaciens UCBM 5113 while Mt-0 was repressed in total root growth. It was also observed that the substrate supporting the Bacillus growth affected the outcome of the interaction, with LB medium generally resulting in repression of root growth.