SIGNAL EXCHANGE IN THE BRADYRHIZOBIUM-SOYBEAN SYMBIOSIS

Rhizobium, Bradyrhizobium and Azorhizobium are able to infect and establish an N-2-fixing symbiosis with a variety of leguminous plants. The result of this infection process is the formation of a novel plant organ, the nodule, in which the bacteria reside. This nodulation process is controlled, at least in part, by the exchange of diffusable signals between the bacterial symbiont and plant host. Understanding this signaling process in plant-microbe interactions may lead to agronomic benefit. The bacterial nodulation genes are essential for the infection of the host root and the establishment of the nodule. The expression of these genes requires the NodD regulatory protein and the presence of specific flavonoids released from the host plant roots. Our research on B. japonicum, symbiont of soybean, has shown that, in addition to these factors, nod gene expression requires NodVW, members of the large family of two-component regulatory proteins. In addition to positive regulators, nod gene expression is controlled by a repressor, NolA. The proteins encoded by the nod genes encode the biosynthesis of substituted lipo-chitin molecules (so-called nod factors). B. japonicum strains produce a large variety of nod factors but the common and most active factor is a pentamer of ca. 1-4 linked N-acetylglucosamine acylated at the non-reducing end with vaccenic acid and substituted at the reducing, terminal sugar with 2-O-methylfucose. This purified molecule induces root hair curling and cortical cell division in soybean roots when applied at nM concentrations or lower. The products of the nodABC genes apparently encode for the synthesis of the acylated lipo-chitin backbone while specific modifications to this molecule are made by proteins encoded by the host-specific nodulation genes. In B. japonicum, NodZ encodes for the fucosylation of the terminal, reducing sugar. The diversity of regulatory responses and signal molecules involved in legume nodulation is an important determinant of host species and genotype specificity. In the latter case, the specificity shown by plant genotypes for particular rhizobial strains may be of importance in bacterial interstrain competition, an important agronomic problem for the enhancement of symbiotic N-2 fixation. Understanding the signaling pathways between rhizobia and their host plants may allow modifications of this interaction to improve symbiotic performance.