Molecular signals exchanged between host plants and rhizobia: Basic aspects and potential application in agriculture

Rhizobia have the ability to infect and establish a N2-fixing symbiosis with many leguminous and a few nonleguminous plants. The result of this interaction is the formation of a novel plant organ, the nodule, where N2 fixation occurs. Research has shown that the establishment of this symbiosis requires coordinate bacterial and plant gene expression that is regulated through the mutual exchange of diffusible signal molecules. For example, each legume host exudes signals, mostly flavonoids, that induce the transcription of bacterial genes (i.e. nod, nol or noe genes), whose protein products are.required for the infection process. It is now known that some of these bacterial nodulation genes encode proteins involved in the biosynthesis of novel lipo-chitin oligosaccharide (LCO) nodulation signals active on the roots of the plant host. Both the induction of bacterial nodulation gene expression and the activity of the LCO nodulation signals are host specific. These two communication steps are likely determine to a large extent the specificity of rhizobia-legume host range. At present, only limited efforts are being made to put this basic information concerning the mechanisms of rhizobia-legume communication to practical use. However, there are promising results that suggest that nodulation of economically important legume crops, such as soybean and bean, can be enhanced by the exogenous application of nodulation gene-inducing compounds. It may be possible to modify commercial inoculant preparations to include nodulation gene-inducing compounds. Alternatively, it may be possible to select for legume host varieties that produce large amounts of nodulation gene-inducing compounds. Additional results provide clues that additional complexities are likely to be discovered in the communication networks that control rhizobial-host interaction. These complexities are further compounded by the myriad of interactions that occur in the plant rhizosphere. Yet, the speed of research advances in this area leads one to be optimistic concerning the advance in our basic understanding of rhizobial-host interaction and the eventual application of this information for agronomic benefit.