| Summary: | The early stages of drought affect plant hormonal homeostasis, including root to shoot signalling. This thesis focuses on manipulating this signalling (to enhance plant drought tolerance) by adding bacterial inocula (Variovorax paradoxus 5C-2 or Bacillus subtilis IR15) to the rhizosphere of pea and lettuce plants. These bacteria can decrease plant ACC (ethylene) or increase cytokinin status, respectively. Firstly, characterising V. paradoxus 5C-2. indicated that it tolerates drought conditions, and colonises the roots of different plant species with different specificity. In the highly specific V. paradoxus 5C-2 / Pisum sativum interaction, bacterial genes of attraction (chemotaxis) are switched on. Secondly, V. paradoxus 5C-2 was applied to pea plants under field conditions and soil drying imposed. V. paradoxus 5C-2 did not alter rhizospheric microorganism community structure and activity, but pea biomass and nodule numbers increased, even in drying soil. Thirdly, root samples from the field were further studied using molecular biology techniques to find rhizobium candidate strains interacting with V. paradoxus 5C-2. The detection of specific RFLP patterns lead to the identification of a specific Rhizobium leguminosarum strain, which physically interacts with V. paradoxus 5C-2 as seen in LAMP-FISH (loop mediated isothermal amplification - fluorescence in situ hydridisation) by confocal microscopy. V. paradoxus 5C-2 increased rhizobium recruitment on root hairs by stimulating rhizobium expression of nodC genes needed for attachment to the roots. Finally, B. subtilis IR15 applied to the rhizosphere of lettuce plants increased foliar cytokinin concentration, and shoot biomass of plants grown in drying soil. Since cytokinins can inhibit root elongation, B. subtilis IR15 was transformed to express the ACC deaminase gene of V. paradoxus 5C-2: ~This increased root elongaljon of lettuce seedlings in in vitro experiments. Further studies to allow the practical transfer of these inocula to farming systems were discussed.
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