Inactivation of nolC Conditions Developmental Abnormalities in Nodulation of Peking Soybean by Rhizobium fredii USDA257. Hari B. Krishnan. Department of Plant Pathology, University of Missouri, Columbia 65211 U.S.A. Steven G. Pueppke. Department of Plant Pathology, University of Missouri, Columbia 65211 U.S.A. MPMI 5:14-21. Accepted 21 October 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992.

Inactivation of nolC in Rhizobium fredii USDA257 is known to expand host range for nodulation to include the soybean cultivar McCall. We show here that nolC also influences symbiotic interactions with Peking, a primitive soybean cultivar that forms nitrogen-fixing nodules with USDA257. Peking nodules containing nolC mutants reduce less acetylene than controls; nodule ultrastructure is altered, and shoot dry weights are diminished. Rhizobia with homogenotized nolC::lacZ fusions produce enhanced levels of extracellular polysaccharides and express high constitutive levels of Beta-galactosidase activity. Using X-gal as a histochemical stain, we detected sustained activity of nolC during all stages of nodule development on Peking, including preinfection. Transcription of key plant genes is modulated in nodules induced by nolC mutants. Expression of leghemoglobin is reduced, and that of VuA, a member of the ENOD2 early nodulin family, is abolished. The number and levels of transcripts homologous to VuB, another early nodulin, also are influenced by the gene. We conclude that nolC may have multiple effects on R. fredii USDA257, only some of which regulate cultivar specificity in McCall.