Diffusible Signals in Associations of Rice with Rhizobia and Arbuscular Mycorrhizal Fungi.

Arbuscular mycorrhization and legume nodulation are two root endosybioses which are initiated by plant perception of microbe produced signaling molecules, referred to as Myc and Nod factors respectively. These signals are lipo-chito-oligosaccharides (LCOs), but recently chitin oligosaccharides (COs) have also been found to play a role in symbiosis. Mycorrhization occurs in approximately 90% of land plants, while nodulation by rhizobia is limited to legumes and the interaction between bacterium and host is species specific. It is known that LCO structures produced by rhizobia are determining factors in the host specificity of legume nodulation, but some unique strains are able to colonize the roots of cereals and the role of LCOs and COs in these interactions is still not well understood. One such strain, Rhizobium sp. IRBG74, colonizes rice and also has been shown to enhance growth. We have found that IRBG74 requires components of the mycorrhizal signaling pathway to colonize its cereal host suggesting that COs play a role in this symbiosis. Additionally, rice roots respond to LCOs, COs and IRBG74 Nod factors with enhanced lateral root development, similar to the effect of colonization by mycorrhizal fungi. Using Rhizobium sp. IRBG74, we hope to identify LCO or CO structures which are required for the colonization of rice roots. Our results indicate that rice can perceive and respond to COs, mycorrhizal LCOs and Nod factors by altering root morphology, and that this response is mediated by the mycorrhizal signaling pathway.