Autoregulation of soybean–Bradyrhizobium nodule symbiosis is controlled by shoot or/and root factors

Abstract

Two strains of Bradyrhizobium japonicum were evaluated with five commercial cultivars of soybean (Clark, Crauford, Davis, Centaur, and Nessen) and one hypernodulating mutant NOD1-3. The hypernodulating NOD1-3 produced 30–50 times the number of nodules of commercial cultivars either inoculated with B. japonicum strain USDA 123 or RCR 3409. Grafting of NOD1-3 shoots to Clark and Davis roots induced hypernodulation on roots of Clark and Davis but did not enhance nodulation when grafted onto the roots of Crauford, Centaur, and Nessen. In contrast, the shoots of Clark, Davis, Centaur and Nessen significantly inhibited nodule formation on the root of NOD1-3. However, Crauford shoots did not alter nodule formation on the roots of NOD1-3 as compared with self-grafts of NOD1-3. It appears that the shoot of NOD1-3 has the ability to alter autoregulatory control of nodulation of Clark and Davis cultivars, but not of Crauford, Centaur and Nessen. The results suggest that the regulation of nodulation in soybean cultivars Clark and Davis is controlled by the shoot factors, while the Crauford was root controlled. Reciprocal grafts between NOD1-3 and Centaur or Nessen indicate that both shoot and root factors are involved in regulation of nodulation. The results suggested that the regulation of nodulation did not depend on bradyrhizobial strains. The shoot control of hypernodulation may be causally related to differential root isoflavonoid levels, which are also controlled by shoot. Application of daidzein significantly enhanced the nodulation and nitrogenase activity of soybean cv. Clark. Root control of restricted nodulation of soybean cv. Centaur did not respond to the addition of daidzein in nutrient solution indicating that this character is not related to isoflavonoids. Therefore, autoregulation in Clark and Centaur plants may be separate events in legume–rhizobia symbiosis and regulated by different kinds of signals.