Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 29, 2001
Publication Date: February 1, 2002
Citation: KRISHNAN, H.B. NO1X OF SINORHIZOBIUM FREDII USDA257, A TYPE III-SECRETED PROTEIN INVOLVED IN HOST RANGE DETERMINATION, IS LOCALIZED IN THE INFECTION THREADS OF COWPEA AND SOYBEAN NODULES. JOURNAL OF BACTERIOLOGY. 2002. V. 184(3). P. 831-839. Interpretive Summary: Sinorhizobium fredii USDA257, a soil-bacterium, forms nodules on the roots of primitive soybean plants. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by soybean plants for growth and development. This process is termed biological nitrogen fixation and it enables soybean plants to grow in nitrogen-poor soils. S. fredii USDA257, however, does not form nodules on most of the North American cultivars. The inability of this strain to nodulate North American soybean cultivars seem to be controlled by the NolX protein. Currently, very little is known about the function or the cellular location of this protein. Such information is required in order to develop methods to overcome the incompatibility between USDA257 and advanced soybean cultivars. We have investigated the subcellular location of the NolX protein in soybean nodules by immunological methods. Information obtained from this basic study will help to better understand the factors that limit the formation of nitrogen-fixing nodules on North American soybean cultivars. Such a understanding should enable us to manipulate biological nitrogen fixation so that farmers can increase the yield of soybean with minimal use of nitrogen fertilizers.
Technical Abstract: 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 pv. vesicatoria. It is not known if NolX functions at the bacterial-plant interface or acts inside the host cell. Antibodies raised against S. fredii USDA257 NolX were used in immunocytochemical studies to investigate the sub-cellular 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 early stages of nodule development.