THE FORMATION OF NITROGEN-FIXING BACTEROIDS IS DELAYED BUT NOT ABOLISHED IN SOYBEAN INFECTED BY AN ALPHA-KETOGLUTARATE DEHYDROGENASE-DEFICIENT MUTANT OF BRADYRHIZOBIUM-JAPONICUM

A mutant strain of Bradyrhizobium japonicum USDA 110 devoid of alpha-k etoglutarate dehydrogenase activity (LSG184) was used to test whether this tricarboxylic acid cycle enzyme is necessary to support nitrogen fixation during symbiosis with soybean (Glycine max). LSG184 formed no dules about 5 d later than the wild-type strain, and the nodules, alth ough otherwise normal in structure, contained many fewer infected host cells than is typical. At 19 d after inoculation cells infected with the mutant strain were only partially filled with bacteroids and showe d large accumulations of starch, but by 32 d after inoculation the hos t cells infected with the mutant appeared normal. The onset of nitroge n fixation was delayed about 15 d for plants inoculated with LSG184, a nd the rate, on a per nodule fresh weight basis, reached only about 20 % of normal. However, because nodules formed by LSG184 contained only about 20% of the normal number of bacteroids, it could be inferred tha t the mutant, on an individual bacteroid basis, was fixing nitrogen at near wild-type rates. Therefore, the loss of alpha-ketoglutarate dehy drogenase in B. japonicum does not prevent the formation or the functi oning of nitrogen-fixing bacteroids in soybean.