Which role for Medicago truncatula non-specific lipid transfer proteins in rhizobial infection?

Lipid transfer proteins (LTPs) are secreted cysteine-rich proteins highly distributed among the plant kingdom, characterized by lipid-binding capacity and possessing antimicrobial activity. Several biological roles have been proposed for LTPs in plants, including response to biotic stress. In Medicago truncatula, the N5 LTP has been proved to be necessary for the establishment of a successful symbiosis with Sinorhizobium meliloti by limiting bacterial spread at the epidermal level and promoting nodule primordia invasion. We identified two novel MtLTPs, named LTP3 and LTP7 highly similar to MtN5 for the amino acid sequence and the cysteine pattern. Their putative promoter regulatory regions contain response elements related to rhizobial symbiosis. We demonstrated that the expression of these LTP genes is induced after infection, particularly in nodule primordia and mature nodules. LTP7-silencing impaired nodule formation: the number of nodules in MtLTP7—silenced roots was reduced by ~60% as compared with control roots. When roots were inoculated with mycorrhiza, we did not observe any change in LTP3 and LTP7 expression, suggesting that these LTPs specifically respond to the rhizobial symbiont. These data suggest that LTPs might represent a novel group of proteins involved in the regulation of the symbiosis. In this chapter, we propose several hypotheses about the mechanisms of action of these LTPs.