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Journal of Bacteriology, February 2002, p. 831-839, Vol. 184, No. 3
0021-9193/01/$04.00+0     DOI: 10.1128/JB.184.3.831-839.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

NolX of Sinorhizobium fredii USDA257, a Type III-Secreted Protein Involved in Host Range Determination, Is Localized in the Infection Threads of Cowpea (Vigna unguiculata [L.] Walp) and Soybean (Glycine max [L.] Merr.) Nodules

Hari B. Krishnan*

Plant Genetics Research Unit, USDA Agricultural Research Service, and Department of Agronomy, University of Missouri, Columbia, Missouri 65211

Received 19 July 2001/ Accepted 29 October 2001

Sinorhizobium fredii USDA257 forms nitrogen-fixing nodules on soybean (Glycine max [L.] Merr.) in a cultivar-specific manner. This strain forms nodules on primitive soybean cultivars but fails to nodulate agronomically improved North American cultivars. Soybean cultivar specificity is regulated by the nolXWBTUV locus, which encodes part of a type III secretion system (TTSS). NolX, a soybean cultivar specificity protein, is secreted by TTSS and shows homology to HrpF of the plant pathogen Xanthomonas campestris pv. vesicatoria. It is not known whether NolX functions at the bacterium-plant interface or acts inside the host cell. Antibodies raised against S. fredii USDA257 NolX were used in immunocytochemical studies to investigate the subcellular localization of this protein. Immunostaining of paraffin-embedded sections of developing soybean and cowpea (Vigna unguiculata [L.] Walp) nodules revealed localization of NolX in the infection threads. Protein A-gold immunocytochemical localization studies utilizing affinity-purified NolX antibodies revealed specific deposition of gold particles in the fibrillar material inside infection threads. Similar immunogold localization studies failed to detect NolX in thin sections of mature soybean and cowpea nodules. The results from this study indicate that NolX is expressed in planta only during the early stages of nodule development.

* Mailing address: USDA-ARS, 108W Curtis Hall, University of Missouri, Columbia, MO 65211. Phone: (573) 882-8151. Fax: (573) 884-7850. E-mail: KrishnanH{at}missouri.edu..

Journal of Bacteriology, February 2002, p. 831-839, Vol. 184, No. 3
0021-9193/01/$04.00+0     DOI: 10.1128/JB.184.3.831-839.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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