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-ketoglutarate 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 nodules about 5 d later than the wild-type strain, and the nodules, although 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 showed large accumulations of starch, but by 32 d after inoculation the host cells infected with the mutant appeared normal. The onset of nitrogen fixation was delayed about 15 d for plants inoculated with LSG184, and 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 that the mutant, on an individual bacteroid basis, was fixing nitrogen at near wild-type rates. Therefore, the loss of alpha-ketoglutarate dehydrogenase in B. japonicum does not prevent the formation or the functioning of nitrogen-fixing bacteroids in soybean.