Title : Effect of copper on the plant-bacteria interaction between Arabidopsis thaliana and the metal resistant bacterium Cupriavidus metallidurans CH34
Abstract:
Chilean soils, as many other in the world, are rich in metals, most notably copper. As a result of the contamination leached from copper tailings, freshwater sources destined for irrigation of agricultural soils are compromised by excess of metallic pollutants. Although copper is a micronutrient for plants, at high concentrations it produces toxic effects, inducing chlorosis, growth inhibiting, and potentially altering plant root exudation thus affecting the signaling and nutrient network between microorganisms and plants, specially in the rhizosphere environment. Among other effects, the rhizomicrobiota can modify the metal stress plant response, increasing or decreasing metal mobility and availability. The soil borne bacterium Cupriavidus metallidurans CH34 is a multi-metal resistant microorganism that carries in its genome two mega-plasmids, pMOL28 and pMOL30, which encode several metal resistance genes, including at least 30 export and import metal pumps, chelating agents, and siderophores. These elements could result in changes in the availability of metals present in soil, and therefore, modify their effects on plants. The interaction of metal resistant strains such as C. metallidurans CH34 with plants has been scarcely addressed. The aim of this study was to characterize the interaction between the model plant Arabidopsis thaliana and C. metallidurans, when subjected to copper stress, to answer questions such as: is C. metallidurans capable of affecting plant growth, respond to plant exudates, or to differentially express metal resistance related genes? Plant and bacteria co-cultivation assays where performed to measure plant growth parameters and metal accumulation in whole plant tissue as well as shoot and root metal translocation and accumulation. The results showed that the presence of strain CH34 positively affected plant growth parameters at relatively low copper concentrations, but was detrimental for the plant at higher copper. This observation may be explained because this bacterium not only mobilizes Cu but also Zn, Co, Mo and B. In addition, root exudates from plants exposed to copper showed higher levels of sugars and aromatic compounds compared with those from non-copper stressed plants, thus significantly changing the rhizosphere environment, and therefore the C. metallidurans activity. Finally, the bacterium gene expression was tested by qRT-PCR in hydroponic plant-bacteria cultures. As expected, genes related to metal resistance (copK, copC y copC (CHR2)) were up regulated in the presence of the plant and the metal. Interestingly, plant colonization and rhizospheric adaptation functions (phaC1 and tonB genes) were also up regulated in the presence of the plant, and the plant and the metal. Taken together, these results show that the plant – metal resistant bacteria interaction is clearly modified by the presence of metals, thus affecting the rhizosphere environment and plant´s performance.
