PLANT PHYSIOLOGY , Vol 114, Issue 4 1359-1368, Copyright © 1997 by American Society of Plant Biologists
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PLANT-MICROBE AND PLANT-INSECT INTERACTIONS |
The Formation of Nitrogen-Fixing Bacteroids Is Delayed but Not Abolished in Soybean Infected by an [alpha]-Ketoglutarate Dehydrogenase-Deficient Mutant of Bradyrhizobium japonicum
L. S. Green and D. W. Emerich
Biochemistry Department, University of Missouri-Columbia, Columbia, Missouri 65211
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.
