Tulokset
Bacterial presence and diversity (Yrjälä et al. 2004), and even spatial distribution (Kurola et al. 2004) in soil and rhizosphere can be demonstrated using culture independent methods based on molecular biology. Cultivation of microbes still, however, serves as the only avenue to isolation, pure-culturing, and characterizaton of a microbe. Remarkable adaptability of the microbial flora in the rhizosphere of pine and in humus soil was demonstrated by the rapid appearance of a microbes capable of mineralizing pyrene in humus soil with no history of contamination (Koivula et al. 2004), and the formation of mutants capable degradation of various forms of methylated phenols (Sarand et al. 2001). A wide variety of both molecular and cultivation based methods are used in the project to detemine the microbial composition in a healthy and a stressed rhizosphere. Based on analysis of the pool of 16S rDNA using PCR and denaturing gradient gel-electrophoresis (DGGE) four dominant bacterial g roups were shown to be present in the rhizosphere of Scots pine growing in pristine soil. The identity of these groups has not yet been confirmed, but plating followed by sequencing of a number of pure cultures suggest that at least Paenibacillus –strains, Rahnellas, enterobacteria and mycobacteria were present. When the trees were felled, the diversity increased significantly. Also the bacterial biomass, as seen (semi-quantitatively) by phospholipid fatty acid (PLFA) inceased immediately upon felling. In contrast, the fungal DGGE banding pattern became less diverse in the rhizosphere of the newly cut stump, whereas also for the fungi the biomass increased. These results demonstrate the very rapid response that the rhizosphere microbes are capable of. As soon as the nature and abundance of root exudates change upon felling, this is reflected in the root microflora (Hernesmaa et al., 2004a, b). Particularly relevant in the context of the SOIL BIOLOGY project is the observation that tha t the microbial composition of the rhizosphere reflects the current situation (within days) of the host plant and not events that have occurred earlier. The research described above was funded by other sources. It is still continuing within the consortium, and it serves as a scientific and methodological basis for the work performed with HERC funding. In accordance with the original HERC research plan, ozone stress was studied with Arabidopsis thaliana Col 0 as a model plant. Stress reaction caused visible damage to the leaves, which was also detectable on molecular level. Plant experiments were performed in order to investigate the influence of bacteria, that previously had been characterized as plant growth promoting rhizobacteria (PGPR), on the growth of Arabidopsis thaliana Col 0. Inoculation with PGPR strains did not result detectable enhancement of growth. In similar experiments using germinated and cloned birch seedlings exposed to lead, PGPR strains did not produce detecta ble enhancement of growth neither in clean nor in lead contaminated environments. In order to characterize the bacterial flora in lead contaminated conditions, soil samples were collected and cultured from the Hälvälä shooting range area. Preliminary plate counts indicate that there is no significant difference in the number of bacteria between different contamination levels. About 30 fluorescent Pseudomonas strains were isolated from these samples. Some of these strains have shown properties characteristic for PGPRs. Tests are presently being performed to characterize these strains in more detail (Tolvanen et al. 2004). At the moment the emphasis is on comparing bacterial diversity between the different contamination levels with phospholipid fatty acid (PLFA) profiling and PCR based denaturing gradient gel-electrophoresis (DGGE) methods. This will give more information about the possible effect of lead to bacterial populations. Experiments with biodiversity may also reveal dif ferences in bacterial populations between the contamination levels, which cannot be detected with plate count method.
Vastaava tutkija
Romantschuk Martin, Helsingin yliopisto, Ympäristöekologian laitos Yhteistyötahot
Biotieteiden laitos, yleisen mikrobiologian laitos, Soveltavan kemian ja mikrobiologian laitos
Hankkeen kesto 2002 - 2004
Asiasanat
maaperä, mikrobit, juuristo, PGPR bakteerit
Hankkeen vaihe: päättynyt
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