Contribution of NFP LysM Domains to the Recognition of Nod Factors during the Medicago truncatula/Sinorhizobium meliloti Symbiosis

Sandra Bensmihen,Françoise De Billy,Clare Gough

doi:10.1371/journal.pone.0026114

Abstract

The root nodule nitrogen fixing symbiosis between legume plants and soil bacteria called rhizobia is of great agronomical and ecological interest since it provides the plant with fixed atmospheric nitrogen. The establishment of this symbiosis is mediated by the recognition by the host plant of lipo-chitooligosaccharides called Nod Factors (NFs), produced by the rhizobia. This recognition is highly specific, as precise NF structures are required depending on the host plant. Here, we study the importance of different LysM domains of a LysM-Receptor Like Kinase (LysM-RLK) from Medicago truncatula called Nod factor perception (NFP) in the recognition of different substitutions of NFs produced by its symbiont Sinorhizobium meliloti. These substitutions are a sulphate group at the reducing end, which is essential for host specificity, and a specific acyl chain at the non-reducing end, that is critical for the infection process. The NFP extracellular domain (ECD) contains 3 LysM domains that are predicted to bind NFs. By swapping the whole ECD or individual LysM domains of NFP for those of its orthologous gene from pea, SYM10 (a legume plant that interacts with another strain of rhizobium producing NFs with different substitutions), we showed that NFP is not directly responsible for specific recognition of the sulphate substitution of S. meliloti NFs, but probably interacts with the acyl substitution. Moreover, we have demonstrated the importance of the NFP LysM2 domain for rhizobial infection and we have pinpointed the importance of a single leucine residue of LysM2 in that step of the symbiosis. Together, our data put into new perspective the recognition of NFs in the different steps of symbiosis in M. truncatula, emphasising the probable existence of a missing component for early NF recognition and reinforcing the important role of NFP for NF recognition during rhizobial infection.

Highlights

In plants, many receptor like kinases (RLKs) are involved in developmental responses, as well as in biotic and abiotic stress responses ([1] for review)
To test the implication of the Nod factor perception (NFP) extracellular domain (ECD) in specific recognition of S. meliloti sulphated Nod Factors (NFs), we swapped it for the ECD of SYM10, which is the ortholog of NFP from pea, a legume that interacts with rhizobia producing non sulphated NF
We have shown that the SYM10 ECD, but not the LYK3 ECD, can replace that of NFP to trigger early NF signalling in response to sulphated NFs in M. truncatula

Summary

Introduction

Many receptor like kinases (RLKs) are involved in developmental responses, as well as in biotic and abiotic stress responses ([1] for review). Among all the possible extracellular domains, Lysin Motif (LysM) domains have recently emerged to be involved in symbiotic and pathogenic interactions [3]. These LysM domains, originally found in bacteria [4], are well represented in plant genomes [5] and some of them were recently shown to bind directly to chitin or chito-oligosaccharides [6,7,8,9]. What is striking in LysM-RLKs (which are only present in the plant kingdom) is that they usually have 3 LysM domains (LysM1, LysM2, LysM3) in their extracellular region and that each of these LysM domains is quite different in sequence. In cases of other repeated extracellular motifs such as Leucine Rich Repeats (LRRs), it has been shown that motifs are not functionally equivalent ([13,14] and references )

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