Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 1/20/2009
Publication Date: 9/21/2009
Citation: Gotthert, M., Krishnan, H.B. 2009. Nodulation genes and type III secretion systems in rhizobia. Book Chapter. 65-94.
Technical Abstract: For establishment of symbiosis, rhizobia and legumes have to communicate. Specific signaling starts with the release of flavonoids by the plant. All rhizobia encode at least one NodD protein, which responds to the presence of specific flavonoids by activation of nodulation genes. In Bradyrhizobium japonicum, the nodVW genes encode a second flavonoid-responsive activation system. Non-flavonoid inducers, e.g., jasmonates, may play an important role in specific interactions. Nod genes might be also under negative control. Several NodD2 proteins and NolR have been described as repressors of nod genes. Nod proteins produce modified N-acetylglucosaminyl oligosaccharides (Nod factors) that serve as rhizobial signals to the plant. Rhizobia differ in the sets of Nod factors they produce. Environmental signals, e.g., temperature, can influence quantity and profile. With exceptions, nod genes do not seem to be expressed in fully developed nodules. Some rhizobial strains encode a type III secretion system (T3SS). The corresponding genes are regulated by ttsI, which seems to be under the control of a nod box promoter. Therefore, flavonoids that induce nod genes also induce the T3SS, linking Nod factor production with protein export. The T3SS influences symbiosis in a host-dependent manner. It may support or impede symbiosis or have no obvious effect. Detailed knowledge about genes involved in symbiosis is the basis for the selection of efficient symbiont-host partners. To increase competitiveness for agricultural purposes, treatment with plant signal substances is the most promising approach established so far.