|Samac, Deborah - Debby|
Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2003
Publication Date: 8/1/2003
Citation: Tesfaye, M., Dufault, N.S., Dornbusch, M.R., Allan, D.L., Vance, C.P., Samac, D.A. 2003. Influence of enhanced malate dehydrogenase expression by alfalfa on diversity of rhizobacteria and soil nutrient availability. Soil Biology and Biochemistry. 35:1103-1113. Interpretive Summary: Plants release large amounts of the carbon captured through photosynthesis into the soil in the form of root exudates. These root exudates stimulate growth of microorganisms on the root surface and in the soil close to roots. We compared the microbes from roots of transgenic alfalfa plants producing enhanced amounts of organic acids in root exudate to microbes from roots of untransformed alfalfa plants. The density of microbes removed from alfalfa roots that could be grown on a laboratory medium was identical between the two plants. Because most microbes in soil cannot be cultured easily, the identity of members of the microbial populations was determined by comparing DNA sequences from the root-associated bacteria to known bacteria. Several groups of bacteria were more numerous on roots of transgenic alfalfa than the untransformed alfalfa, while other groups were less numerous. These results indicate that increasing organic acids in root exudates can favor the growth of certain bacteria. Microbes from transgenic roots were able to use a greater number of nutrients than microbes from untransformed roots. This indicates that there is greater diversity in the microbial populations on transformed roots. Altering root exudates also changed the mineral nutrient concentrations of the soil around roots. Soil around roots of transgenic plants had more available mineral nutrients for plant growth than soil from around untransformed roots. These plants are useful for examining how root exudates affect soil microbes and soil properties. The ability of plants secreting enhanced amounts of organic acids to liberate more soil nutrients will allow these plants to be grown with fewer soil amendments, decreasing production costs and increasing yields.
Technical Abstract: Transgenic alfalfa over-expressing a nodule-enhanced malate dehydrogenase (neMDH) cDNA and untransformed alfalfa plants were grown at the same field site and rhizosphere soils collected after 53 weeks of plant growth. These alfalfa lines differ in the amount and composition of root organic acids produced and exuded into the rhizosphere. Nucleotide sequencing of PCR-based 16S ribosomal DNA (rDNA) clone libraries and Biolog GN microtiter plates were employed to assess the activity of naturally occurring rhizobacteria in the two alfalfa rhizospheres. Selected macro- and micro-elements in the two alfalfa rhizosphere soils were also measured. Analysis of 240 16S rDNA clone sequences indicated the existence of about 11 bacterial phyla and their major subdivisions in the 2 alfalfa rhizosphere samples. There were qualitative changes in the abundance of bacterial phylogenetic groups between rhizosphere soils of transgenic and untransformed alfalfa. Carbon substrate utilization profiles suggested that rhizosphere samples from transgenic alfalfa showed significantly higher functional diversity compared to rhizosphere samples from untransformed alfalfa. The concentrations of nitric acid extractable P, K, Mn, Zn, and Cu increased significantly in the transgenic alfalfa rhizosphere compared to the untransformed alfalfa rhizosphere. These observations indicate that organic acids produced by plant roots significantly influence rhizosphere microbial diversity and availability of macro- and micro-nutrients and demonstrate the utility of such transgenic plants as tools for studying the potential impact of plant exudates on soil microbial ecosystems.