Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2004
Publication Date: 5/1/2005
Citation: Deakin, W.J., Marie, C., Saad, M.M., Krishnan, H.B., Broughton, W.J. 2005. Nopa is associated with cell surface appendages produced by the type iii secretion system of rhizobium sp. strain ngr234. Molecular Plant-Microbe Interactions. 18:499-507. 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. Rhizobium sp. strain NGR234 secretes 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 Rhizobium sp. strain NGR234 uses 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 legumes. 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: Rhizobium sp. strain NGR234, which is capable of interacting with a large number of legumes, utilizes a variety of signaling molecules to establish nitrogen-fixing symbioses. Among these are nodulation outer proteins (Nops) that transit through a type III secretion system (TTSS). Abolition of Nop secretion affects nodulation of certain legumes. Under free-living conditions, the secretion of Nops can be induced by the addition of flavonoids. Here, we show that an in-frame deletion of nopA abolishes secretion of all other Nops and has the same impact on nodule formation as mutations that lead to a nonfunctional TTSS. This secretion-minus phenotype of the nopA mutant, as well as bioinformatics analysis of NopA itself, suggests that NopA could be an external component of the TTSS. Electron microscopy showed that NGR234 synthesizes fibrillar structures on the cell surface in a flavonoid-inducible and NopA-dependent manner. Purification of the macromolecular surface appendages revealed that NopA is a major component of these structures.