Short introduction to PhD thesis

Effect of the biological plant growth promotion agent Proradix® (Pseudomonas fluorescens) on the grwoth of barley (Hordeum vulgare L. cv. Barke) and the bacterial community in the rhizosphere

Over the last few decades, the ability of rhizosphere bacteria to promote plant growth has been considered to be of scientific, ecological and economic interest.
Such bacteria are highly relevant for modern agricultural applications, including biological plant protection and growth stimulation to increase the productivity of crop plants. The properties and mechanisms of interaction of these root-colonizing bacteria have been extensively investigated, and plant protection agents that are based on bacterial strains have been developed.
In the present study, the effect of the commercially available plant protection agent Proradix®, that contains Pseudomonas sp. DSMZ 13134, was examined. To analyze the plant growth promoting effect of Proradix® following its application to barley plants, experiments were carried out in the laboratory, in the greenhouse and in the field. The colonization of barley roots by the Proradix® (Pseudomonas sp. DSMZ 13134) strain was studied. Furthermore, the question arose to what extent the introduced strain influences the composition of the root-associated bacterial community.
Proradix® (Pseudomonas sp. DSMZ 13134) was able to produce siderophores and indole-3-acetic acid in vitro. In dual culture tests, the inhibition of the soil-borne fungal plant pathogens Gaeumannomyces graminis Ggt 13 und Pythium ultimum was demonstrated. However, the Proradix® strain was unable to synthesize the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase under the chosen experimental conditions. This enzyme also plays an important role in plant growth promotion.
Beneficial plant growth promoting effects could be shown in greenhouse experiments with lack of light and nutrient deficiency conditions. No differences between controls and inoculated plants could be observed in experiments under optimal growth conditions. This supports the conclusion that is also published by other authors, that pronounced PGPR-effects appear under limiting conditions. In field experiments that were carried out during two years at different plot locations, up to 19% higher crop yields but also no beneficial effects were found in barley plants that were treated with Proradix®. To discuss such varying results of field experiments, a huge multitude of complex factors has to be taken into consideration. In this study, the intensity of the plant growth stimulating effect was mainly influenced by different weather conditions and by the inoculation density of Pseudomonas sp. DSMZ 13134. Based on these results, the necessity of long time studies with time periods of at least five years becomes evident.
Using the fluorescence in situ hybridization (FISH) method with specific oligonucleotide probes and gfp-tagging in combination with confocal laser scanning microscopy, an effective colonization of barley roots was demonstrated. Pseudomonas sp. DSMZ 13134 could be detected in all parts of the root. Gfp-tagged cells were localized particularly in the root hair zone, and high cell densities were apparent on the root hair surface. An endophytic colonization could only be found in roots that were grown in the monoxenic system.
The application of Pseudomonas sp. DSMZ 13134 exerted a transient impact on the dominant bacterial community of barley roots. This effect was restricted to the very early stage of root development. The importance of the inoculation density rose, because the additional inoculation with Proradix® (Pseudomonas sp. DSMZ 13134)-cells (10⁸ cells/ml) showed a more pronounced community effect compared to the conventional way of inoculation using vacuum infiltration.

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