Does MtN5 play a double role in the root response to symbiotic and pathogenic microorganisms?

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