PLANT BIOLOGY Rhizobial Receptors Revealed

Two groups report the identification of the long-sought receptor for the recognition of the Nod factor responsible for initiating communication between symbiotic bacteria rhizobia and plant roots. Establishment of this symbiotic relationship allows legumes to develop nitrogen-fixing root nodules (see Parniske and Downie). Madsen et al. cloned the gene responsible for the nrf5 Lotus japonicus mutant phenotype and determined that the gene encoded a LysM-type serine-threonine receptor kinase. LysM domains are found in other peptidoglycan-binding proteins and chitinases, consistent with NRF5 serving as a receptor for Nod factor, which is a lipochitin-oligosaccharide. Radutoiu et al. characterized the nrf1 and nrf5 mutants, which encode members of the LysM receptor kinase family. Neither nrf1 nor nrf5 mutants exhibited root hair ballooning, membrane depolarization, or extracellular alkalinization in response to application of purified Nod factor. Roots, not nodules, expressed both NRF1 and NRF5. Double-mutant studies indicated that NRF1 and NRF5 were upstream in the nod-signaling pathway from the receptor kinase SYMRK (symbiosis receptor-like kinase), which is involved in the response to both nitrogen-fixing bacteria and phosphate-fixing fungi (mycorrhizal fungi).

M. Parniske, J. A. Downie, Locks, keys, and symbioses. Nature 425, 569-570 (2003). [Online Journal]

S. Radutoiu, L. H. Madsen, E. B. Madsen, H. H. Felle, Y. Umehara, M. Grønlund, S. Sato, Y. Nakamura, S. Tabata, N. Sandal, J. Stougaard, Plant recognition of symbiotic bacteria requires two LysM receptor-like kinases. Nature 425, 585-592 (2003). [Online Journal]

E. B. Madsen, L. H. Madsen, S. Radutoiu, M. Olbryt, M. Rakwalska, K. Szczyglowski, S. Sato, T. Kaneko, S. Tabata, M. Sandal, J. Stougaard, A receptor kinase gene of the LysM type is involved in legume perception of rhizobial signals. Nature 425, 637-640 (2003). [Online Journal]

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