BRADYRHIZOBIUM ELKANII INDUCES OUTER CORTICAL ROOT SWELLING IN SOYBEAN

When soybean roots were inoculated with Bradyrhizobium elkanii USDA76, USDA94 or USDA31, a localized swelling of the root outer cortical cel ls was observed within 10 days after inoculation. This phenotype was d esignated outer cortical swelling (OCS). OCS was not observed in roots inoculated with B. japonicum USDA110 or USDA122. Therefore, the OCS r esponse appears to be specific for B. elkanii. To determine the involv ement of the nodulation genes in OCS formation, we constructed a nod(- ) mutant, strain USDA94 Delta NOD, of B. elkanii USDA94 in which the n odD(2)D(1)KABC genes were deleted. An OCS response was observed on roo ts inoculated with USDA94 Delta NOD, indicating that the lipo-chitin n odulation signals synthesized by the nodABC gene products are not requ ired for OCS formation.