Bacteria and heavy metal accumulating plants

Phytoremediation is an emerging cleanup technology for the restoration of soils and is based on the use of plants. Phytoextraction is the application of higher plants to remove inorganic contaminants, primarily metals, from polluted soil. Willow (Salix caprea L.) is a suitable candidate for phytoremediation applications due to its rapid growth and high biomass production and highly accumulating clones have been identified in a previous project. In addition, a high number of bacterial strains (346) have been isolated from the rhizo- and endosphere of Salix caprea L. growing in heavy metal contaminated sites and characterized. Some studies and results obtained by the project consortium indicate that the plant-associated microflora plays an essential role in the metal accumulating capacity of the plant. Bacteria are known to interact synergistically with roots to enhance the potential for metal uptake by diverse mechanisms such as secreting organic compounds which increase the bio-availability and facilitate root absorption of metals by influencing metal solubility. They may further alter root exudation as well as plant gene expression. Plant growth promoting bacteria with a highly beneficial effect on phytoextraction efficiency have been identified by the project consortium. Within this project isolated strains will be tested for their effect on plant gene expression as well as their colonization efficiency. Compounds responsible for mobilizing effects will be identified. Subsequently, selected strains will be thoroughly tested in plant experiments regarding their application potential. A detailed population analysis of bacteria associated with highly and poorly accumulating Salix caprea L. clones will reveal, which bacterial groups correlate with a good accumulation potential. The results of this project will identify promising inoculant strains and will as well provide the basis for the further development of improved phytoextraction technologies.

Plant-associated bacteria may mobilize or stabilize heavy metals in soil. The latter is a desirable for reducing heavy metal uptake in plants however, for phytoextraction applications pollutant uptake by the plant has to be maximized to warrant rapid removal from the soil environment. In a previous project we identified several bacterial strains, which were isolated from Zn / Cd accumulating willows (Salix caprea) grown in a contaminated site and which showed heavy metal mobilization activities. In this project we investigated, which mechanisms are involved in this process and how these bacteria interact with the plant. We found that the production of organic acids may play a role in mobilization activity. In addition, some bacterial strains influenced the expression of plant metal transporters and / or redox enzymes and thereby they may influence metal uptake as well as the plant stress resilience. As it is for any soil application not only important that bacteria are able to mobilize but also that they are competitive in a natural environment and colonize the plant or rhizosphere, we particularly addressed competitive ability. Strains were selected, which proved to be competitive and which showed beneficial activities in lab assays, however, in a plant experiment with willows no effects on plant growth or heavy metal uptake were observed. This indicates the need to better understand the mechanisms involved to achieve reliable beneficial effects, also under real-life conditions. One strain isolated from Salix caprea was assigned to a novel species, which was named Spirosoma endophyticum. We further hypothesized that plant genotypes with different accumulation potential host distinct microbial communities in the rhizosphere. To test this hypothesis we analyzed the rhizosphere microflora of eight Salix genotypes with differing Zn / Cd accumulation capabilities by 454-sequencing of 16S rRNA genes. Our results showed that the heavy metal uptake as well as the plant line shaped the rhizosphere microbiome indicating a strong interaction between plants and microbes influencing the capacity of a plant to accumulate heavy metals.