The involvement of Medicago truncatula non-specific lipid transfer protein N5 in the control of rhizobial infection.

Cysteine-rich proteins seem to play important regulatory roles in Medicago truncatula/Sinorhizobium meliloti symbiosis. In particular, a large family of nodule-specific cysteine-rich (NCR) peptides is crucial for the differentiation of nitrogen-fixing bacteroids.The Medicago truncatula N5 protein (MtN5) is currently the only reported non-specific lipid transfer protein necessaryfor successful rhizobial symbiosis; in addition, MtN5 shares several characteristics with NCR peptides: a smallsize, a conserved cysteine-rich motif, an N-terminal signal peptide for secretion and antimicrobial activity. UnlikeNCR peptides, MtN5 expression is not restricted to the root nodules and is induced during the early phases of symbiosisin root hairs and nodule primordia.Recently, MtN5 was determined to be involved in the regulation of root tissue invasion; while, it was dispensablefor nodule primordia formation. Here, we discuss the hypothesis that MtN5 participates in linking the progressionof bacterial invasion with restricting the competence of root hairs for infection.