Legumes have the unique ability to undergo symbiotic associations with bacteria belonging to the Rhizobiaceaefamily. These rhizobacteria secret signaling molecules (Nod Factors) that trigger physiological and morphological plant responses. In the course of this interaction, rhizobia invade the host root leading to the formation of a novel symbiotic plant organ: the root nodule. Rhizobia remain surrounded by a plant-derived plasma membrane from the very first moment of host invasion until age-dependent degradation of the symbiotically active bacteroids. This membrane serves as an essential interface for plant-microbe signal transduction and is certainly one of the key determinants for the success of the association.
Using DNA microarrays, a remorin gene (SYMREM1) that is strongly induced in root nodules of Lotus japonicus and Medicago truncatula (Colebatch et al., 2004; El Yahyaoui et al., 2004) was identified several years ago. This isoform belongs to the subgroup II of the remorin family that is restricted to legumes (Raffaele et al., 2007). While all other members of this multi-gene family are ubiquitiously expressed in all plant organs, only SYMREM1 genes exhibit this unique expression signature. The protein itself localizes to infection thread membranes surrounding the bacteria inside of the root nodule and to symbiosome membranes encapsulating the differentiated bacteroids throughout the symbiotic interaction. Interestingly SYMREM1 is able to interact with at least the three receptor-like kinases (RLKs) NFP, LYK3 and DMI2 that are essential for root nodule symbiosis. Analogies to molecular scaffolds from mammalian cells such as the ability to interact with multiple signalling proteins, the formation of high-order oligomers and the strict localization to membrane rafts led to the hypothesis that SYMREM1 acts as a signalling mediator during rhizobial infection (Lefebvre et al., 2010).
The RLK-SYMREM1 interaction is mainly mediated by the conserved C-terminal domain. However, results from FLIM-FRET and in vitro kinase assay experiments clearly indicate that the intrinsically disordered N-terminal region contributes to this interaction and can be phosphorylated by the kinase domains (Tóth et al., 2012).
For functional analysis, we focused our attention on SYMREM1. We showed that it co-localizes with flotillin 4 (FLOT4), which actually recruits SYMREM1 into a specific nanodomain. Here, SYMREM1 interacts with LYK3 and stabilizes the receptor. This process is required for successful infection (Liang et al., 2018). Furthermore, we demonstrated that SYMREM1 not only scaffolds membrane-resident receptors but also membrane topologies (Su et al., 2023). This is a entirely new mechanism to maintain large-scale and cell wall-free membrane structures in living plant cells.