2013
Volume 13, Number 3, pp. 90–95
Effect of magnetite nanoparticles on symbiotic nitrogen fixation and growth of soybean plants
Mohammad R. Ghalamboran1 and Kazem Khavazi2
1 School of Biological Sciences, Shahid Beheshti University, Tehran, Iran
2 Soil & Water Research Institute, Tehran, Iran
After photosynthesis, symbiotic nitrogen fixation is the second most important feature sustaining life on Earth. Symbiotic fixation inspired the first economically feasible artificial method, which synthesizes NH4 from H2 gas and atmospheric N2 directly. Symbiotic nitrogen fixation (SNF) occurs in swelling root hair cells, called nodules. One of the greatest problems facing the nodulation process is iron deficiency. The aim of this study was to investigate whether coating Bradyrhizobium japonicum with iron oxide (magnetite) nanoparticles affects symbiotic nitrogen fixation and soybean plant growth. Nanoparticles were suspended in liquid medium and mixed until they adhered to the B. japonicum, which were investigated as coated inocula at different concentrations of the nanoparticles. Statistical design was a complete randomized block (CRB). The observed variables were amount of ethylene generated from the nodules, nodule weight, number of nodules, root dry weight, stem dry weight, leaf dry weight, number of branches and number of nodes per plant. Results showed that coating Bradyrhizobium japonicum with the nanoparticles increased the amount of generated ethylene, nodule weight and number of nodules per plant, and root dry weight; the number of branches decreased at nanoparticle concentrations of 40 and 60 µg mL–1. However, the dry weight of shoots (stem and leaves) slightly increased. It is proposed that iron particles fabricated in the nanoscale bring four major benefits: (1) the free nanoparticles scavenge possibly deleteriously acting radical molecules; (2) nanoparticles residing on the surface of bacteria, seeds, and roots act as a protective layer against invading decomposers; (3) these nanoparticles compensate for iron deficiency and enhance oxygen availability to the bacteria under extreme conditions; (4) the nanoparticles, with their huge surface area, have a buffering rôle and catalyse reactions tending to keep the pH neutral. Due to the diminution of siderophore production the nanoparticles reduce bacterial energy expenditure under extreme conditions, hence in the presence of the nanoparticles trends such as diminished viability and generated ethylene (as an indicator of nitrogen fixation) were reversed. The use of nanoscale iron particles to coat the bacterial cells increases their survival under extreme conditions and causes enhanced SNF and the favourable development of legume cultivation under environmental stress.
Keywords: Bradyrhizobium japonicum, magnetite nanoparticles, soybean, symbiotic nitrogen fixation