Abstract
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 differ-entiation of nitrogen-fixing bacteroids. The Medicago truncatula N5 protein (MtN5) is currently the only reported non-specific lipid transfer protein necessary for successful rhizobial symbiosis; in addition, MtN5 shares several char-acteristics with NCR peptides: a small size, a conserved cysteinerich motif, an N-terminal signal peptide for secretion and antimicrobial activity. Unlike NCR peptides, MtN5 expression is not restricted to the root nodules and is induced during the early phases of sym-biosis in root hairs and nodule primor-dia. Recently, MtN5 was determined to be involved in the regulation of root tissue invasion; while, it was dispensable for nodule primordia formation. Here, we discuss the hypothesis that MtN5 participates in linking the progression of bacterial invasion with restricting the competence of root hairs for infection.