Rhizosphere microbes in metal hyperaccumulation

The aim of this project is to clarify the contribution of rhizosphere microorganisms on metal tolerance and accumulation of plants growing on serpentine soils. Serpentine soils are an extreme habitat for plants, because of high metal concentrations (nickel, chromium, in some cases cobalt) and of low nutrient levels. Plants growing on this site are extremly tolerant to the soil metals (hypertolerant), some of them have developed the strategy to accumulate large amounts of metals in plant tissue (hyperaccumulators). Little is known on the rhizosphere processes influencing metal mobility and plant availability in metalliferous soils. Especially the role of root exudates is largely unknown and previous data are somewhat contradictory. Due to the enhanced microbial activity in rhizosphere it is expected, that those microbes also play an important role in metal availability to plants and are therefore crucial for plant survival in this environment. Little research work has been conducted on the plant-microbe interactions in metal contaminated soils. In this project the following tasks should be invesigated on Thlaspi goesingense (hypertolerant) and Silene vulgaris (hypertolerant) growing on serpentine soil: - Biogeochemistry of metals under influence of two different plant strategies (tolerance and accumulation) - Root exudates and their influence on metal mobilisation and microbial turnover - Isolation and characterisation of rhizosphere bacteria and investigation of their contribution to hypertolerance and hyperaccumulation - Rolle of mycorrhizae on heavy metal tolerance This experiments will clarify the contribution of rhizosphere microbes on the metal tolerance and accumulation of plants on serpentine soils. This information may also help to improve phytoremediation technologies.

Hyperaccumulating plants are able to store unusual high amounts of toxic metals in their aboveground biomasse without symptoms of toxicty. On the other hand, excluders are able to avoid uptake of the elements present in excess in the soil. For both plant groups, rhizosphere microbes are considered to play a key role in this accumulation / avoidance processes. The aim of this project was to clarify the plant-soil-microbe relationships of the nickel hyperaccumulator Thlaspi goesingense and metal-excluding plants growing on nickel-rich serpentine soil. Organic acids are released by plant roots as exudates and might play a crucial role in the rhizosphere of both plant groups. In the rhizosphere of field-grown plants, large amounts of lactic acid was found for the exluder plants and high concentrations of citric acid for the hyperaccumulator. This indicates that lactic acid might be involved in detoxification and reducing the bioavailability of Ni, as is has been previously shown for the role of malic acid for Al tolerance of wheat. Citric acid on the other hand could be involved in efficient mobilisation of Ni from soil solid phase. T. goesingense has no mycorrhizal association (root-fungi symbiosis), but contains various bacterial species in its rhizosphere. A large number of strains were isolated from plant (shoot-associated) and rhizosphere samples obtained from the indigenous site and identified. The population structure in the rhizosphere was very different from plant-shoot, but some species were found to occur in both environments. To identify the influence of single bacterial strains on Ni accumulation in the plant, specific species were inoculated to plants grown on sterilized soils. For non of the treatments, a significant influence on either Ni concentration in shoots or biomass production was found. It was concluded that the experimental period might have been to short to find significant effects. On the indigenous serpentine site, two tree species are able to grow naturally (pine and oak). Since both of them have mycorrhizal associations, the root-associated fungi were isolated and identified. For all cultivated isolates, a high Ni tolerance was observed. Further it was found that mycorrhizal fungi accumulate less Ni than saprophytic fungi in their fruiting bodies. The exclusion of Ni by the mycorrhizal fungi is considered to be crucial for the Ni tolerance of the native tree species. The results of these studies might have significant implications for the development of plant-based soil-clean up (phytoremediation) and stabilization technologies.