A class of non-specific lipid transfer proteins of Medicago truncatula modulates the host response to rhizobia infection.

The establishment of the symbiotic interaction between leguminous plants and nitrogen fixing bacteria is a complex process that requires a molecular dialogue between the host and the bacteria during the root infection and the colonization of the nodule, where the bacteria fix nitrogen. The formation of a functional root nodule needs a spatial and temporal coordination of all these events. Rhizobia enter root hair via a tubular plant-derived structure, named infection thread (IT), and then they are released into the cortical layers, where the bacteria enclosed in a plant-derived membrane called symbiosome, enter nodule primordia cells. The formation of the ITs as well as the release of rhizobia in the nodule cells require the synthesis of new membranes and modifications of their lipid composition. Lipid transfer proteins (LTPs) are small basic secreted proteins, characterized by lipid-binding capacity and putatively involved in lipid trafficking. In plants, the non specific LTPs (nsLTPs) are a large group of proteins implicated in various processes; most of them possess antimicrobial activity and are likely involved in pathogen defence. Recently, two LTPs, AsE246 from Astragalus sinicus and MtN5 from Medicago truncatula, have been implicated in the host regulation of N-fixing symbiosis suggesting their participation in the de-novo formation and rearrangement of infection threads and symbiosome membranes (Lei et al. 2014; Santi et al., 2017). We have identified in M.truncatula genome two novel nsLTPs that cluster with MtN5 in the type III nsLTP group (Wang et al., 2012), an intermediate group between the two major nsLTP1 and nsLTP2 families. We have demonstrated that these LTPs are transcriptionally induced by rhizobia infection and they can modulate the host nodulation capacity.