Occurrence and role of vanillin in root exudates of peanut (Arachis hypogaea)

The purpose of this work was to identify phenolic compounds released by young peanut roots growing hydroponically under low nitrogen supply and to investigate if these substances may be implicated in the establishment and functioning of an effective Bradyrhizobium-peanut symbiosis. Vanillin was found among the phenolics exuded by young peanut roots growing under these conditions. The effect of this compound on the growth rate of a Bradyrhizobium strain infective for peanut was further investigated. At concentrations ranging from 0.2 to 20 mug ml(-1) vanillin had detrimental effects on the multiplication rate of Bradyrhizobium sp. (Arachis) strain C145. Bacterial cultures exposed to 20 mug ml(-1) of this compound showed a two-fold increase in their lag phase and after 5 days of incubation optical densities were 48% decreased in respect to control cultures. In an inoculation pot experiment using this strain significant decreases in total dry weight, nodule biomass, acetylene reduction activity and nitrogen content were observed when 10 mug of vanillin were added to each germinating peanut seed prior to inoculation. However, no adverse effects were detected at a lower vanillin dose (2.5 mug ml(-1)) or when peanut seeds were inoculated using rhizobia cells previously exposed to 1 mug ml(-1) of vanillin.