Probing the plant growth-promoting and heavy metal tolerance characteristics of Bradyrhizobium japonicum CB1809

Bradyrhizobium japonicum strain CB1809 was recently identified as a plant growth-promoting rhizobacteria in high arsenic substrate. However, it is not known if B. japonicum has growth promoting properties in plant species other than its leguminous host or the bacterium's tolerance to arsenic and metals. Solution culture was used to test the response of sunflower (Helianthus annus L.) and wheat (Triticum aestivum L.) to B. japonicum inoculation under elevated arsenic. The resazurin assay was used to determine the tolerance of B. japonicum to bioavailable heavy metals in solution. Inoculated sunflower and wheat were more tolerant of arsenic than uninoculated treatments. Thus, the growth promoting attributes of B. japonicum are not limited by the legume-rhizobium symbiosis. The concentration of indolic compounds did not differ between inoculated and uninoculated treatments suggesting the growth-promoting mechanism is not mediated by auxin. The effective concentrations for a 50% decrease in activity were arsenic >50 μM, cadmium 5.8 μM, copper 0.86 μM, manganese 83 μM, nickel 7.4 μM and zinc 29 μM. These results suggest B. japonicum has potential as a plant growth-promoting rhizobacterium for a range of plant species in arsenic contaminated sites and possibly for sites contaminated with other heavy metals.