<italic>Bradyrhizobium elkanii</italic> Induces Outer Cortical Root Swelling in Soybean

Abstract

When soybean roots were inoculated with Bradyrhizobium elkanii USDA76, USDA94 or USDA31, a localized swelling of the root outer cortical cells was observed within 10 days after inoculation. This phenotype was designated outer cortical swelling (OCS). OCS was not observed in roots inoculated with B.japonicum USDA110 or USDA122. Therefore, the OCS response appears to be specific for B. elkanii. To determine the involvement of the nodulation genes in OCS formation, we constructed a nod- mutant, strain USDA94δNOD, of B. elkanii USDA94 in which the nodD2D1KABC genes were deleted. An OCS response was observed on roots inoculated with USDA94δNOD, indicating that the lipo-chitin nodulation signals synthesized by the nodABC gene products are not required for OCS formation.