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PLANTS INTERACTING WITH OTHER ORGANISMS

Medicago LYK3, an Entry Receptor in Rhizobial Nodulation Factor Signaling^1,[W]

Laboratory of Molecular Biology, Graduate School of Experimental Plant Sciences, Wageningen University, Wageningen 6703 HA, The Netherlands (P.S., E.L., R.G., E.F., E.D., T.B.); and Laboratoire des Interactions Plantes-Microorganismes, Institut National de la Recherche Agronomique, Centre National de la Recherche Scientifique, 31326 Castanet-Tolosan cedex, France (C.G.)

Rhizobia secrete nodulation (Nod) factors, which set in motion the formation of nitrogen-fixing root nodules on legume host plants. Nod factors induce several cellular responses in root hair cells within minutes, but also are essential for the formation of infection threads by which rhizobia enter the root. Based on studies using bacterial mutants, a two-receptor model was proposed, a signaling receptor that induces early responses with low requirements toward Nod factor structure and an entry receptor that controls infection with more stringent demands. Recently, putative Nod factor receptors were shown to be LysM domain receptor kinases. However, mutants in these receptors, in both Lotus japonicus (nfr1 and nfr5) and Medicago truncatula (Medicago; nfp), do not support the two-receptor model because they lack all Nod factor-induced responses. LYK3, the putative Medicago ortholog of NFR1, has only been studied by RNA interference, showing a role in infection thread formation. Medicago hair curling (hcl) mutants are unable to form curled root hairs, a step preceding infection thread formation. We identified the weak hcl-4 allele that is blocked during infection thread growth. We show that HCL encodes LYK3 and, thus, that this receptor, besides infection, also controls root hair curling. By using rhizobial mutants, we also show that HCL controls infection thread formation in a Nod factor structure-dependent manner. Therefore, LYK3 functions as the proposed entry receptor, specifically controlling infection. Finally, we show that LYK3, which regulates a subset of Nod factor-induced genes, is not required for the induction of NODULE INCEPTION.

² Present address: All-Russia Research Institute for Agricultural Microbiology, Podbelsky Shousse, 3, St. Petersburg-Pushkin 8, 196608 Russia.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Ton Bisseling (ton.bisseling{at}wur.nl).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.100495

^* Corresponding author; e-mail ton.bisseling{at}wur.nl.

Received April 1, 2007; accepted June 15, 2007; published June 22, 2007.

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