Medicago LYK3, an Entry Receptor in Rhizobial Nodulation Factor Signaling

Abstract

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.Legume plants have the unique ability to establish root nodule symbiosis with nitrogen-fixing bacteria of the Rhizobiaceae family. These bacteria-collectively called rhizobia-secrete signal molecules, named nodulation (Nod) factors, which play an important role in the induction of early steps of root nodule formation, including the infection process. All Nod factors are lipochito-oligosaccharides composed of a b-1,4-linked N-acetyl-D-glucosamine backbone of which the nonreducing sugar moiety is substituted with an acyl chain.

The Small GTPase ROP10 of Medicago truncatula Is Required for Both Tip Growth of Root Hairs and Nod Factor-Induced Root Hair Deformation

“…NFP and LYK3 are LysMcontaining receptor-like kinases that are localized to the PM, with the LysM domain protruding to the extracellular space perceiving extracellular NF signals and the intracellular kinase domain facing the cytoplasm for the transduction of the signals (Limpens et al, 2003;Smit et al, 2007). Therefore, we tested a possible interaction of ROP10 with NF receptors using the GAL4-based yeast two-hybrid (Y2H) system.…”

“…NF-induced symbiotic signaling in the model legume Medicago truncatula depends on NF receptors, namely NFP and LYK3. These receptors are LysM domain receptor-like kinases with an extracellular domain and an intracellular kinase domain located in epidermal cells, which perceive sulfated NFs secreted by the symbiont Sinorhizobium meliloti (Limpens et al, 2003;Smit et al, 2007). NFP lacks protein kinase activity, which is required for all early NF-induced responses Radutoiu et al, 2003), whereas LYK3 has an active kinase activity allowing downstream signaling via phosphorylation, but its mutant is still sensitive to NFs and shows Ca 2+ spiking and root hair deformation responses.…”

“…NFP lacks protein kinase activity, which is required for all early NF-induced responses Radutoiu et al, 2003), whereas LYK3 has an active kinase activity allowing downstream signaling via phosphorylation, but its mutant is still sensitive to NFs and shows Ca 2+ spiking and root hair deformation responses. Therefore, LYK3 is thought to mediate bacterial entry into root hairs (Catoira et al, 2001;Limpens et al, 2003;Smit et al, 2007). Downstream components of the NF signal transduction pathway in M. truncatula include DMI2, a leucine-rich repeat receptor kinase (Esseling et al, 2004), DMI1, a potassium ion channel (Ané et al, 2004;Riely et al, 2007), NENA, a nuclear pore component (Groth et al, 2010), CCaMK/DMI3, a Ca 2+ and calmodulin-dependent protein kinase (Lévy et al, 2004;Mitra et al, 2004), the CCaMK-regulated transcriptional regulator CY-CLOPS/IPD3 (Messinese et al, 2007;Yano et al, 2008;Singh et al, 2014), and various transcriptional regulators such as NSP1, NSP2, ERN1, and NIN (Kaló et al, 2005;Smit et al, 2005;Marsh et al, 2007, Middleton et al, 2007.…”

LYR3, a high‐affinity LCO‐binding protein of Medicago truncatula, interacts with LYK3, a key symbiotic receptor

“…These two proteins, however, are both essential for establishing the symbiosis with rhizobial bacteria, where they play a role in Nod factor perception leading to the accommodation of the bacteria in plant roots and nodules [12][13][14].…”

Deep Sequencing of the Medicago truncatula Root Transcriptome Reveals a Massive and Early Interaction between Nodulation Factor and Ethylene Signals

“…Nod factor receptors, which include Lysine motif domain-containing receptorlike kinase3 (LYK3) and Nodulation factor perception (NFP) in Medicago truncatula and Nodulation factor receptor1 (NFR1) and NFR5 in Lotus japonicus, are plasma membrane-localized receptor-like kinase and kinase-like (RLK) proteins (Amor et al, 2003;Smit et al, 2007;Haney et al, 2011;Moling et al, 2014) consisting of an intracellular kinase domain and an extracellular region with two or three chitin-binding Lys motifs (LysM). LysM domains bind Nod factors through their chitin backbone (Petutschnig et al, 2010;Broghammer et al, 2012) and are implicated in Nod factor recognition specificity .…”