Does MtN5 play a double role in root responses to symbiontic and pathogenic microorganisms?

MtN5, a new Lipid Transfer Protein, has been identified in nodulated roots of Medicago truncatula andpreliminarily classified as early nodulin, which is expressed in response to rhizobial symbiosis. Wehave shown that the recombinant MtN5 exerts antifungal and antimicrobial activity in vitro againstFusarium semitectum and Rhizobium leguminosarum, respectively. In vivo, the fungal infection leadsto the expression of MtN5 in the whole root apparatus of M. truncatula plants, whereas the inoculationwith rhizobia induces an early and nodule-specific expression of the protein, that is also maintained inmature nodules. These two different expression patterns suggest a putative double role for MtN5, whichcould be involved both in a general response mechanism against fungi and in sensing or controlling theinfection of the symbiont. This last hypothesis is supported by the observation that M.truncatula rootstransformed with an hairpin construct aiming to silence endogenous MtN5, are impaired in noduleformation respect to control roots. Therefore, MtN5 is hereby proposed as a novel, multifunctionalprotein taking part in the symbiotic process.