Phytoalexins synthesis is enhanced in groundnut plants inoculated with Bradyrhizobium sp (Arachis)

Bradyrhizobium spp. colonize groundnut (Arachis hypogaea) through cracks in roots and spreads intercellularly. Phytoalexin biosynthesis may be one mechanism by which groundnut plants respond to Bradyrhizobium inoculation; but this idea has not been sufficiently investigated to date. Therefore, phytoalexin production by roots and leaves of uninoculated and inoculated groundnut plants was studied by thin layer chromatography and standard bioassays using two fungal species: Aspergillus niger and Cladosporium cucurnerintan. Phytoalexin production was significantly enhanced in leaf extracts of Bradyi-hizobiturt-inoculated plants 50 days after planting. These phytoalexins were identified as medicarpin and formononetin. Formononetin was active only against Cladosporiurn cucumerinum and its concentration, estimated as total phenolic substances recovered from the inhibition halos obtained, was 1.5-fold increased in inoculated plants compared to the uninoculated controls. Medicarpin was active against both C cucumerinum and A. niger. The extent of the inhibition zones detected for medicarpin in leaf extracts obtained from inoculated plants was twice that observed for uninoculated plants, corresponding to more than three-fold increase in concentration of medicarpin compared to formononetin. These results suggest that symbiotic interactions between Bradyrhizobium spp. (Arachis) and its host trigger biochemical changes, which tend to reinforce natural groundnut defenses against pathogens. (C) 2004 Elsevier Ltd. All rights reserved.