Symbiotic function of defensin-like peptides in root nodules of legumes

In the root nodules of Medicago truncatula, bacteroids undergo a remarkable differentiation process involving increased membrane permeability and inability for cell division. NCRs (Nodule specific Cystein Rich peptides) were identified as the candidate of plant factors that induce the bacteroidal differentiation. The NCR consists of more than 400 members that have conserved signal peptides and variable mature peptides except for the conserved 4 or 6 Cysteine residues. Some of the NCRs induced bacteroidal features on cultured bacterial cells. A mutant of M. truncatula, dnf1-1, forms non-effective nodules with infected cells. The rhizobial cells inside the nodules did not show the features of the bacteroids. The NCRs were localized in the endocytoplasmic reticulum of dnf1-1 nodules whereas in the symbiosomes of the wild type. Considering that Dnf1-1 encodes a signalpeptidase, NCRs will be delivered to the symbiosomes by the secretion transport system and function as plant factors that induce bacteroidal differentiation. Leguminous plants exploited the potential of innate immunity and evolved an unprecedented diversity of nodule specific antimicrobial peptides to establish symbiosis.