Function of polyamines in growth regulation of rhizobia in host and rhizosphere

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1.Soybean plants accumulated sufficient putrescine and spermidine in their nodules to inhibit the growth of Bradyrhizobium japonicum.2.Another substance that inhibits the growth of B.japonicum strains was isolated from soybean roots, and was identified as 4,7-dihydroxyisoflavone, daizein. The growth inhibition of bacteroids by daizein was depressed by spermidine.3.The following results suggest that H_2O_2 derived from the oxidative deamination of spermidine is responsible for the inhibitory effect of spermidine.1) Bacterois were sensitive to H_2O_2.2) The addition of catalase depressed the growth inhibition of spermidine.3) There was a correlation between the tolerances to H_2O_2 and spermidine in many B.japonicum strains.4) Spermidine resistant mutants of B.japonicum gained resistance to the inhibitory efrect of H_2O_2.4.The results in this study suggest that spermidine is concerned in the inhibition of nitrogen fixation activity of nodules by nitrate.5.Amine oxidase was purified from soybean seedling roots. This enzyme was classified into the type I amine oxidase. The experiments using this purified enzyme suggest that amine oxidase located on the surface of bacteroids catalyzes the oxidative deamination or polyamines, and the produced H_2O_2 cases the growth inhibition of bacteroids.6.Bacterial density in the homogenate of nodules did not decrease during the incubation of the homogenate, although there was enough amine oxidase and polyamines in the homogenate to inhibit the growth of bacteroids.The results obtained in this study suggest a mechanism regulating amine oxidase activity to control the density of bacteroids in the host plant.