| Most land plants form a symbiotic association with arbuscular mycorrhizal fungi. In addition, plants of the legume family also associate with nitrogen-fixing rhizobia which results in the formation of root nodules. In both symbioses, the microbes produce lipochitooligosaccharidic (LCO) signals which are recognized by the host plant. Genetic approaches used to characterize the LCO signaling pathway have most likely reached saturation since a similar set of genes was identified in Medicago truncatula, pea and Lotus japonicus. This indicated the need for complementary approaches in order to find additional components in this signaling pathway. Protein-protein interaction-based studies were conducted to identify interactors of two essential symbiotic proteins: NORK (NOdulation Receptor Kinase) and DMI3 (Does not Make Infections 3). A 3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGR1) was identified by yeast two-hybrid as interacting with NORK. HMGR1 is a key regulator of the isoprenoid biosynthetic pathway. It was also found to interact with two other symbiotic receptor-like kinases, LysM domain-containing receptor-like kinase 3 (LYK3) and Nod factor perception (NFP). HMGR1 is phosphorylated by NORK. Its enzymatic activity is regulated not only by protein interaction with NORK but also by mutations mimicking phosphorylation. Using reverse genetics, HMGR1 was found to be required for rhizobial and mycorrhizal infection as well as nodule organogenesis. HMGR1 also regulates the expression of ENOD11, a gene induced rapidly and specifically in response to rhizobia and mycorrhizal LCOs. Therefore, HMGR1 probably plays a key role in the transduction of symbiotic signals from the plasma membrane to the nucleus in both symbiotic associations. In the nucleus. DMI3 was found to interact with IPD3 ( Interacting protein of DMI3) by yeast two-hybrid and Bimolecular Fluorescence Complementation. IPD3 is required for proper infection in both nodulation and arbuscular mycorrhization. Therefore, protein interaction techniques coupled with reverse genetics have been extremely successful for the identification of new players in symbiotic signaling. |
