HIRSCH, A.M.: What are the Conserved Elements in Symbiotic Associations between Plants and Microbes?

The nitrogen-fixing symbiosis between Rhizobiaceae and legumes is one of the best-studied interactions established between prokaryotes and eukaryotes. Dinitrogen is fixed into ammonia by rhizobia and made available to theplant in exchange for carbon. The plant also develops root nodules that house the bacteria. It has been hypothesized that this symbiosis evolved from the more ancient vesicular-arbuscular mycorrhizal symbiosis. Support comes from legume mutants where Nod^– and Myc^– co-segregate. We also found that two early nodulin (ENOD) genes, ENOD40 and ENOD2, are expressed in alfalfa VA-mycorrhizae, supporting the idea that the signal transduction pathways between the two symbioses are conserved. Now, we have analyzed the responses of 5 classes of non-nodulating Melilotus alba (white sweetclover) mutants to both Sinorhizobium meliloti (the nitrogen-fixing symbiont) and Glomus intraradices (the mycorrhizal symbiont) to investigate how Nod^– mutations affect the establishment of each symbiosis and the induction of downstream genes. Of the different classes, only the sym3 mutant is both Nod^– and Myc^–. In addition, we found no up-regulation of ENOD40 gene expression in response to inoculation with either symbiont. Other responses downstream of the initial perception of the symbiont are also not activated compared to those of wild-type plants indicating that sym3 is blocked in a very early stage of the interaction. Moreover, there appears to be no elicitation of a defense response indicating that the host, whether wild type or mutant, recognizes the microbe as a “friend” and not as a “foe” even if the interaction is ultimately ineffectual. These data and their implications to symbiosis overall will be discussed.