GNPS - Transition metal (W, Mo) stress response strategies of leguminous plants (Glycine max) and their symbiotic partners (B. japonicum)

Tungsten (W) and molybdenum (Mo) are economically important transition elements residing in the chromium group of the periodic table. Due to a myriad of industrial applications, ranging from household appliances to war-ammunition and high-tech products, they are increasingly discharged into the environment. However, little is known about their eco-toxicology. While molybdenum is an important micronutrient and serves as co-factor in various key enzymes of microbes, plants and other higher lifeforms, W-enzymes are only found in some archaea and bacteria. Due to their chemical similarities regarding structure, atomic radii and electron configuration, it is proposed that molybdenum ions bound to co-factors of enzymes can be substituted by tungsten. In plants it has been repeatedly demonstrated that tungsten renders the four molybdoenzymes (nitrate reductase, aldehyde oxidase, xanthine dehydrogenase and sulfite oxidase), functionless if incorporated instead of molybednum. In addition, the rhizobial molybdoenzyme, nitrogenase, was found to retain its function even if tungsten is incorporated into the enzyme.

Although it has been shown that both transition metals can be phytotoxic when occurring in excess concentrations, knowledge about their effect on plant metabolic processes is still limited. Due to preliminary experimental data obtained for W, we believe that the two transition metals interfere with integral metabolic pathways, especially phosphorous and nitrogen dependent processes. Additionally, it has already been shown that symbiotically grown leguminous plants respond differently to tungsten toxicity than their non-symbiotically grown counterparts. Still, our understanding of the mechanisms behind the molecular response to tungsten toxicity and similarities to molybdenum metabolism remains limited. We aim to comprehensively understand the molecular mechanisms behind phytotoxicity and stress response induced by excess tungsten and molybdenum as well as possible stress alleviation through Rhizobium symbiosis.

In order to uncover the mechanisms that govern toxicity of Mo and W in plants as well as to identify possible benefits of bacterial symbiosis, soybean plants inoculated with Bradyrhizobium japonicum and a surface sterilized non-symbiotic control supplied with nitrate (10mM KNO3) were grown in a semi hydroponic setup for three weeks, until nodules were formed and symbiosis was fully established. After these three weeks, three different metal treatments (control, 0.5 mM tungsten and 0.5 mM molybdenum) for were applied for two weeks and subsequently harvested for metabolomic and proteomic analysis.