Population dynamics of endophytic bacteria

Endophytic bacteria may play a significant role in plant health as many bacterial isolates from plant tissues have shown antagonistic activities towards fungal and bacterial pathogens. In addition, plant nutrition may be affected by endophytes as nitrogen-fixing bacteria have been isolated from various, non-leguminous plants. However, very little is known on the interaction between plants and bacteria as well as on the microbial diversity within plants. The main objective of this project is to carry out a systematic approach in order to understand a complex ecological system in which micro-organisms, plant, soil and the environment interact. This project is aiming at analysing bacterial dynamics within the plant and investigating population shifts caused by the presence of a pathogen. Microbial populations responding to such a stress as well as populations within better growing individual plants will be studied in detail as they provide promising inoculant strains for biocontrol and biofertilization. Molecular techniques circumvent limitations related to bacterial isolation used in traditional approaches and will therefore be applied to follow community changes. The bacterial endophytes of potato, an agronomically important root and tuber crop, will be investigated. An additional task of this project is to develop simple molecular tools to detect bacterial contaminations in tissue cultures that are in many cases not found by conventional methods. Up to date, the molecular ecology of endophytic bacteria has been rarely addressed and this project will provide fundamental information for further research as well as results of practical importance.

Endophytic bacteria are ubiquituous in most plant species and colonize plants internally without exhibiting plant pathogenicity. Recent reports have shown that bacterial endophytes are able to promote plant growth and to act as plant pathogen antagonists, but in general little information is available on the diversity and stress response of endophytic populations. Most studies rely on the isolation of endophytic bacteria on common cultivation media. However, only a small percentage (0.1 to 1%) of naturally occurring bacteria can be cultured, and therefore in this project cultivation-independent methods relying on phylogenetic markers were applied to study bacterial endophytes of potato. The aim of this project was to analyse, which bacterial groups are able to colonize potato plants internally and whether they respond to stress conditions such as the presence of a plant pathogen. Experiments were carried out in order to compare plants that suffered from light deficiency with healthy and robust plants. In general, potato stems, roots and tubers were colonized by a high number of different bacterial species and strains belonging to highly different phylogenetic groups. Plants from stressed and robust plants possessed comparable endophytic bacteria, but healthy potato plants showed a significantly higher diversity of endophytes than stressed plants. In addition, some tissue and plant genotype specific strains were detected. Our results further indicated that species related to Streptomyces scabiei may be involved in the high tolerance of the cultivar Mehlige Mühlviertler against potato common scab disease. In order to analyse the response of endophytic bacteria to the invasion with a plant pathogen, two potato cultivars were infected with the blackleg pathogen Erwinia carotovora ssp. atroseptica. Infected plants were colonized by the phytopathogen, but showed only few disease symptoms. Endophytic populations colonizing infected and uninfected control plants were compared and our results clearly demonstrated an increased bacterial diversity in plants containing the pathogen. Furthermore, the activity of endophytes was affected by the presence of the pathogen as well as by the plant genotype. In parallel, endophytes were cultivated and tested for their biocontrol activity against the pathogen Erwinia carotovora ssp. atroseptica. A high percentage of isolates (38%) protected tissue culture plants from disease. As the genus Pseudomonas is frequently found in association with plants, endophytic pseudomonads were analysed. Potato plants were colonized by a particularly high diversity of Pseudomonas species that also responded to the presence of the blackleg pathogen. Our results indicated that bacteria colonizing plants internally are involved in the disease resistance of plants and that endophytes may represent a promising source of biocontrol bacteria.