Submitted to: Federation of European Microbiological Societies Microbiology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2007
Publication Date: 3/28/2007
Citation: Krishnan, H.B., Kim, W., Sun-Hyung, J. 2007. Calcium regulates the production of nodulation outer proteins (Nops) and precludes pili formation by Sinorhizobium fredii USDA257, a soybean symbiont. Federation of European Microbiological Societies Microbiology Letters. 271:59-64. 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. Sinorhizobium fredii USDA257 secretes proteins into the rhizosphere when it comes into contact with soybean root exudates. Some of these proteins are involved in regulating nodulation on soybean plants. Currently, very little is known about how these proteins are exported to the rhizosphere. In this study we have demonstrated that calcium regulates the accumulation of nodulation outer proteins (Nops). 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 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: Sinorhizobium fredii USDA257 forms nitrogen-fixing nodules on primitive soybean cultivars such as “Peking” but is unable to establish efficient symbiosis with North American cultivars. USDA257 when grown in presence of genistein, a potent nodD inducing isoflavonoid, secretes at least six nodulation outer proteins (NopX, NopB, NopL, NopP, NopA and NopC) to the extracellular milieu through a type III secretion system. These proteins regulate legume nodulation in a host-specific manner. Here, we demonstrate that calcium prevents the accumulation of NopB and NopA, and drastically reduces that of NopX and NopL. The inhibitory effect on Nops accumulation appears to be mediated specifically by calcium since other divalent cations such as Mg2+ and Mn2+ had no detectable effect. Calcium does not appear to interfere with the secretion of these proteins since western blot analysis revealed that these Nops do not accumulate inside the cell. The inhibitory effect of calcium on Nops production is mediated at the post-transcriptional level. Our studies indicate that the production of Nops, which function as determinants of host-range, is regulated by calcium.