Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2003
Publication Date: 9/1/2003
Citation: MARIE, C., DEAKIN, W.J., VIPREY, V., KOPCINSKA, J., GOLINOWSKI, W., KRISHNAN, H.B., PERRET, X., BROUGHTON, W.J. CHARACTERIZATION OF NOPS, NODULATION OUTER PROTEINS, SECRETED VIA THE TYPE III SECRETION SYSTEM OF NGR234. MOLECULAR PLANT-MICROBE INTERACTIONS. 2003. V. 16(9). P. 743-751. 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 soybean plants for growth and development. This process is termed biological nitrogen fixation and it enables soybean plants to grow in nitrogen-poor soils. Sinorhizobium fredii USDA257 and its closely related sibling NGR234 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 are exported to the rhizosphere. Such information is required for designing strategies targeted toward improving biological nitrogen fixation. We have demonstrated that Sinorhizobium fredii USDA257 and NGR234 use a type III transport system to secrete proteins into the rhizosphere. Some of these proteins either promote nodulation or reduce nodulation in host-dependent manner. Information obtained from this basic study will help scientists to better understand the factors that limit the formation of nitrogen-fixing nodules on soybeans. Such an understanding should enable scientists to manipulate biological nitrogen fixation so that farmers can increase the soybean yields with minimal use of nitrogen fertilizers.
Technical Abstract: The nitrogen-fixing symbiotic bacterium Rhizobium species NGR234 secretes, via a type III secretion system (TTSS), proteins called Nops (nodulation outer proteins). Removal of TTSS-dependent protein secretion has either no effect, or leads to a change in the number of nodules on selected plants. More dramatically, Nops impair nodule development on Crotalaria juncea roots, resulting in the formation of non-fixing pseudonodules. Concomitant mutation of nopX and nopL (formerly called nolX and y4xL, respectively), which are codes for two previously identified secreted proteins, leads to a phenotype that differs from that of a strain in which functional TTSS is absent. Use of antibodies and modification of the purification protocol revealed that NGR234 secretes additional proteins in a TTSS-dependent manner. One of them was identified as NopA, a small 7 kDa protein. Single mutations in nopX and nopL were also generated to assess the involvement of each Nop in protein secretion and nodule formation. Mutation of nopX had little effect on NopL and NopA secretion, but greatly affected the interaction of NGR234 with all plants tested. NopL was not necessary for the secretion of any Nops, but was required for efficient nodulation of some plant species. NopL may thus act as effector protein whose recognition is dependent upon the host's genetic background.