Henkes, Gunnar Jakob (2008)
Plant-microbe interactions alter the allocation of carbon in barley (Hordeum vulgare).
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Plants fix atmospheric carbon via the photosynthetic pathway in above-ground plant parts and translocate the synthesized photoassimilates from places of carbon fixation (source) through the vascular system to organs with carbon demand like roots or fruits (sink). Besides the plants own carbon demand for growth and metabolism plant derived carbon also represents the primary source for microorganisms in soil. This PhD Thesis was performed to investigate how interactions between plants and soil borne microorganisms alter the carbon partitioning within the plant system. Effects of particular signal compounds as well as effects of pathogenic and mutualistic microorganisms on carbon allocation were investigated. In the first experiment (Chapter 2) a sterile hydroponic split root system allowing 11C measurement was established using barley (Hordeum vulgare) as model plant. Experiments with gfp and DSred labelled Pseudomonas fluorescens strains ensured that the established split root system was appropriate to restrict the inoculated bacteria to the inoculated root fraction throughout the experiment. Pulse labelling of plant leaves with 11CO2 demonstrated that the system allowed to follow the labelled carbon from the leave into the root system in vivo. The plants allocated about 50 % of the mobilized carbon fraction into the root system. From the carbon translocated into roots 2-4% were respired by roots 150 min after labelling. The soluble carbohydrates could be detected, however, the amount of exudates was too low for quantification. In the second experiment (Chapter 3) the effect of exogenous applied jasmonic acid (JA) to the roots of barley grown in the split root system on the partitioning of recently fixed carbon were investigated. JA applied to one root half inhibited root growth locally and reduced carbon partitioning to the JA-treated tissue within minutes, whereas the untreated side showed a delayed increase in carbon partitioning. In contrast, the reduction of the carbon sink strength of one root half by cooling did not cause an enhanced carbon partitioning to the other root half. The different effects of JA and cooling suggest that JA triggers a specific signal transduction from root to shoot and further induces an enhanced carbon export to the non-treated root with carbon sink capacity. This was also supported by results of the JA shoot treatment which resulted in a fast increase in carbon partitioning to the root system. In the third experiment (Chapter 4) the effect of the root pathogen Fusarium graminearum on carbon partitioning towards and within the root system of barley was investigated. It was hypothesized that the preincoluation with the plant growth promoing rhizobacterial wild type strain Pseudomonas fluorescens CHA0 attenuates the effect of the pathogen on carbon partitioning. To investigate the involvement of bacterial secondary metabolites the P. fluorescens mutant CHA19 lacking the ability to produce secondary metabolites was used. The application of F. graminearum to barley roots caused a reduction of the carbon allocation towards infected roots and an increase in carbon allocation towards the non-infected root part. Local or systemic preinoculation with the wild type P. fluorescence annihilated the effect of F. graminearum on carbon allocation, whereas the mutant CHA19 did not repress the pathogen effect. The results suggest that secondary metabolites of P. fluorescens induce a systemic resistance against F. graminearum in barley plants. Overall, the results indicate that plants are able to sense the emergence of particular soil microorganisms and to respond to these microorganisms by immediate changes in carbon partitioning to optimize carbon allocation and investment.

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