The Medicago truncatula Lysine Motif-Receptor-Like Kinase Gene Family Includes NFP and New Nodule-Expressed Genes

Abstract

Rhizobial Nod factors are key symbiotic signals responsible for starting the nodulation process in host legume plants. Of the six Medicago truncatula genes controlling a Nod factor signaling pathway, Nod Factor Perception (NFP) was reported as a candidate Nod factor receptor gene. Here, we provide further evidence for this by showing that NFP is a lysine motif (LysM)-receptorlike kinase (RLK). NFP was shown both to be expressed in association with infection thread development and to be involved in the infection process. Consistent with deviations from conserved kinase domain sequences, NFP did not show autophosphorylation activity, suggesting that NFP needs to associate with an active kinase or has unusual functional characteristics different from classical kinases. Identification of nine new M. truncatula LysM-RLK genes revealed a larger family than in the nonlegumes Arabidopsis (Arabidopsis thaliana) or rice (Oryza sativa) of at least 17 members that can be divided into three subfamilies. Three LysM domains could be structurally predicted for all M. truncatula LysM-RLK proteins, whereas one subfamily, which includes NFP, was characterized by deviations from conserved kinase sequences. Most of the newly identified genes were found to be expressed in roots and nodules, suggesting this class of receptors may be more extensively involved in nodulation than was previously known.

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…MtNFP is a member of the LysM family of receptors, and is required for root hair curling and the induction of a subset of transcriptional changes in response to Nod factor 17,18 . Recently, RNA interference (RNAi) partial depletion experiments demonstrated an additional role for MtNFP during a later step in bacterial penetration of the root hair 18 (infection thread formation; see below).…”

“…MtNFP is a member of the LysM family of receptors, and is required for root hair curling and the induction of a subset of transcriptional changes in response to Nod factor 17,18 . Recently, RNA interference (RNAi) partial depletion experiments demonstrated an additional role for MtNFP during a later step in bacterial penetration of the root hair 18 (infection thread formation; see below). Another LysM family receptor, encoded by MtLYK3/HCL, is known to be required for CCRH formation and finetuning of the plant responses to Nod factor during infection thread formation (P. Smit and T. Bisseling, unpublished data) 19 .…”

“…Aberrant, abortive infection threads are formed when Nod factor is supplied in a structurally incomplete form, or if Nod factor perception is disrupted 18,19,42 . An S. meliloti nodF nodL double mutant that produces Nod factor that lacks the acetyl substituent and has an altered acyl chain cannot effectively invade plant cells, and induces approximately 20% of colonized root hairs to form infection threads 19,42 .…”

“…RNAi knockdown has been used to assess the role of Nod factor receptors in infection thread formation. M. truncatula plants that are partially depleted by RNAi for the primary Nod factor receptor MtNFP, although capable of root hair curling and initiation of cortical cell divisions, form aberrant infection threads 18 (FIG. 4 and TABLE 1).…”

Cell autonomous and non-cell autonomous control of rhizobial and mycorrhizal infection inMedicago truncatula

“…NFP (Nod Factor Perception) encodes a receptor-like kinase protein possessing three extracellular LysM domains, presumed to perceive LCOs. [12][13][14] Like dmi3 mutants, M. truncatula nfp mutants are blocked for root hair curling and IT formation but, in contrast to dmi3 mutants, they are not impaired for mycorrhization. 15 When NFP was expressed under the control of the epidermal promoter in nfp roots, rhizobia were entrapped in curled of roots.…”