Tungsten (W) leads to a disruption of C/N-metabolic processes in Glycine max–modified stress response through rhizobia symbiosis?


Julian Preiner, Wolfram Weckwerth, Stefanie Wienkoop, Eva Oburger


The transition metal tungsten (W) finds increasing application in military, aviation and household appliance industry, opening new paths for the heavy metal into the environment. Since tungsten shares certain chemical properties with the essential plant micro nutrient molybdenum, it is proposed to inhibit enzymatic activity of molybdoenzymes by replacing the Mo-ion bound to the co-factor. However, recent studies suggest the inhibition of enzymatic activity might not be the only effect W has on plants and that, much like other heavy metals, tungsten exerts toxicity on its own. Still, our understanding of the mechanisms behind the apparent phytotoxicity remains limited.


This study investigates the effects of W on growth, nutrient levels (ionome, ICP-MS), starch levels and nitrogen nutrition (IRMS, enzyme activity assays) as well as root and nodule proteome (LC-MS/MS) of Glycine max cv. Primus. Plants were inoculated with Bradyrhizobium japonicum and grown in a semi hydroponic set up using three different tungsten concentrations (zero, 0.1 mM and 0.5 mM). To identify possible benefits of a functional bacterial symbiosis on W induced stress response, two different environmental growth conditions, one with suppressed N-fixation, supplied with Nitrate (10 mM KNO3) and one solely relying on symbiotic N-fixation (zero KNO3), were applied.

Results and Discussion

Glycine max accumulated up to 703 ± 136 mg kg-1 at 0.5 mM W resulting in a strong reduction of shoot (N fix & N fed) and root biomass (N fed only) as well as alterations in nutrient and starch contents. Irrespective of N regime, nitrate reductase activity and nodulation decreased with increasing W concentrations; however, nodule fixation activity (mg N g-1 nodule dry weight) remained comparable to the respective control (N fed – low; N fix – high). Proteomic data revealed that relevant nitrogenase precursor proteins (NifD, NifU) as well as MoFe cofactor biosynthesis were strongly decreased indicating that synthesis rather than functionality of nitrogenase was affected at high W concentrations. Furthermore, we showed that, unlike in N fed plants, symbiotically active nodules of N fix soybean mirrored root stress response to a large extend, with general stress proteins (PR-proteins, chitinases, thaumatin-like and germin-like proteins), peptidase inhibitors and peroxidases being induced by high external W concentrations. Interestingly, proteins involved in secondary metabolism (flavonoid biosynthesis) and hormone biosynthesis were increased in abundance in nodules but not in roots, irrespective of N regime.

Innovative aspects

  •     Integration of proteomic and ionomics to unravel the effects of tungsten toxicity
  •     W differentially affects N2 fixation (nitrogenase) and N assimilation (nitrate reductase)
  •     Novel putative marker proteins (of flavonoid and hormone biosynthesis) involved in W stress response