Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/2/2010
Publication Date: 6/1/2010
Citation: Schechter, L.M., Guenther, J., Olcay, E.A., Jang, S., Krishnan, H.B. 2010. Sinorhizobium fredii USDA257 Translocates NopP into Vigna unguiculata Root Nodules. Applied and Environmental Microbiology. 76(11):3758-3761. Interpretive Summary: Rhizobia are soil-dwelling bacteria that form nodules on the roots of legume plants. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by legumes for growth and development. This process is termed biological nitrogen fixation and it enables legumes to grow in nitrogen-poor soils. Sinorhizobium fredii USDA257 secrete proteins into the rhizosphere when they come into contact with legume root exudates. Some of these proteins are involved in regulating nodulation on soybean and other legumes. Currently, very little is known about how these proteins interact with legume roots. An important first step in determining the function of these secreted proteins during symbiosis is the demonstration that these proteins are delivered into legume roots. Such information is required for designing strategies targeted toward improving biological nitrogen fixation. We have demonstrated for the first time that Sinorhizobium fredii USDA257 injects NopP into legume roots by a type III transport system. Information obtained from this basic study will help scientists to better understand the factors that limit the formation of nitrogen-fixing nodules on legumes. Such an understanding should enable scientists to manipulate biological nitrogen fixation so that farmers can increase yields with minimal use of nitrogen fertilizers.
Technical Abstract: Type III secretion systems (T3SSs), which are found in many Gram-negative bacterial pathogens, inject virulence proteins directly into host cells during infection. T3SSs are also present in some strains of rhizobia, bacteria that form symbiotic associations with legumes and fix nitrogen in specialized root organs called nodules. Proteins secreted by rhizobial T3SSs, termed Nops, can influence the ability of bacteria to nodulate certain plant species. Although studies have shown that rhizobia secrete Nops into culture medium, demonstration of Nop delivery into plant root cells during symbiosis has remained elusive. Here, we show for the first time that Sinorhizobium fredii injects NopP into Vigna unguiculata root cells in a T3SS-dependent manner. Protein translocation occurred at an intermediate step during symbiosis, in young root nodules.