Impact of the Plant Growth-promoting Rhizobacterium Raoultella Terrigena TFi08N on Plant Growth and Root Architecture
The input of mineral fertilizers in agriculture increased drastically in the last 50 years. The use of mineral fertilizers, in particular nitrogen fertilizers requires a high input of energy and can cause nitrogen leakage out of agro-ecosystems. An opportunity to solve this problem is the use of biofertilizers, which contain plant growth promoting rhizobacteria (PGPR). The use of associative rhizobacteria is a possibility to make plants profiting from growth promotion and to substitute mineral fertilizers. However, this requires a predictable growth stimulation and in-vitro test system allowing to investigate the mechanisms underlying plant growth promotion. It was therefore the aim of this thesis to establish an in-vitro plant culture system that allows investigating newly isolated rhizobacteria for their plant growth promotion effects and characterizing the mechanisms responsible for growth stimulation. Besides several rhizobacteria, isolated in frame of the EU-RHIBAC project, Raoultella terrigena TFi08N, a soil bacterium isolated in the frame of the former EU-Micro-N-Fix Project (2001-2006) is until now not characterized as a PGPR. In the first part of this thesis, different bacterial strains were investigated for their ability to promote plant growth. A considerable improvement of plant growth, in terms of enhanced dry matter production was found for Raoultella terrigena TFi08N, Azospirillum brasilense SP245, Bacillus megaterium M3 and Bacillus subtilis OSU142 but not for Pseudomonas fluorescens C139. As inoculation with Raoultella terrigena resulted in strong plant stimulation and as it represents a so far uncharacterized PGPR strain, it was decided to study the mechanisms responsible for plant growth promotion conferred by this bacterium in more detail. Evidence for compensatory pH changes in the rhizosphere, a nitrification of supplied ammonium or the release of growth-promoting volatile substances by Raoultella were not found. Regarding the genetic constitution of Arabidopsis, it was observed that the four investigated accession lines responded differently to Raoultella inoculation and that expression of the major root plasma membrane H+-ATPase, AHA2, is required to confer plant growth stimulation. Further considering that Raoultella is able to produce auxin, its growth stimulatory effect may rely on an auxin-induced stimulation of lateral root growth and auxin-stimulated proton extrusion at the root plasma membrane that improves nutrient uptake and plant growth. Although this thesis could not yet fully elucidate the mechanism of plant growth stimulation by Raoultella terrigena TFi08N, it contributes to a better understanding of the possible modes of action of PGPR by defining growth conditions and plant factors required for growth stimulation by Raoultella terrigena TFi08N.