Phosphorus form-related metabolic responses in roots of Triticum aestivum and the impact of beneficial soil microorganisms

Phosphate fertilizers are a finite resource, thus sustainable crop production will most likely depend on the utilization of P sources naturally found in soil, in particular predominant organic P forms. To date we know little of the biochemical sensing and adaptation of crops to different organic and inorganic P that potentially affects its assimilation. Furthermore, the use of inoculants of beneficial soil microorganisms has become of increasing interest due to their ability to mobilize P from organic and inorganic sources naturally occurring in soil. However, many factors including nutrient type and status, plant species and the presence of other microbes have a positive or detrimental affect on microbial fitness and activity and as a result on crop P uptake and growth. Therefore, the aim of this PhD thesis was to improve our understanding of the biochemistry of phosphorus sensing, mobilization and uptake in wheat from various sources, and subsequently address the impact of mycorrhizal fungi as well as plant growth promoting rhizobacteria on these processes. Mesocosm studies provided detailed evidence that wheat root metabolism and the secretion of root exudates are sensitive to organic and inorganic P forms. In addition and with respect to P uptake and growth, wheat responsiveness to mycorrhizal fungi and plant growth promoting rhizobacteria colonization was highly dependent on the present P form. However, long-term changes in root metabolism were mainly driven by the P source. Based on these results, it is still necessary to ascertain if these metabolic changes are general responses of wheat or cultivar specific. Further, it is essential to link these responses to P uptake mechanisms and determine their effect on rhizosphere microorganisms in order to develop cultivars that not only have enhanced soil P exploitation and utilization capacities, but also positively respond to beneficial rhizomicroorganisms

Heterotrophic and Autotrophic Microbial Populations in Cold Perennial Springs of the High Arctic ▿ †

The saline springs of Gypsum Hill in the Canadian high Arctic are a rare example of cold springs originating from deep groundwater and rising to the surface through thick permafrost. The heterotrophic bacteria and autotrophic sulfur-oxidizing bacteria (up to 40% of the total microbial community) isolated from the spring waters and sediments were classified into four phyla (Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria) based on 16S rRNA gene analysis; heterotrophic isolates were primarily psychrotolerant, salt-tolerant, facultative anaerobes. Some of the isolates contained genes for thiosulfate oxidation (soxB) and anoxygenic photosynthesis (pufM), possibly enabling the strains to better compete in these sulfur-rich environments subject to long periods of illumination in the Arctic summer. Although leucine uptake by the spring water microbial community was low, CO2 uptake was relatively high under dark incubation, reinforcing the idea that primary production by chemoautotrophs is an important process in the springs. The small amounts of hydrocarbons in gases exsolving from the springs (0.38 to 0.51% CH4) were compositionally and isotopically consistent with microbial methanogenesis and possible methanotrophy. Anaerobic heterotrophic sulfur oxidation and aerobic autotrophic sulfur oxidation activities were demonstrated in sediment slurries. Overall, our results describe an active microbial community capable of sustainability in an extreme environment that experiences prolonged periods of continuous light or darkness, low temperatures, and moderate salinity, where life seems to rely on chemolithoautotrophy