Analysis of the membrane binding mechanism of Remorins and their role in beneficial endosymbioses

The plasma membrane is highly organized and within the plasma membrane proteins cluster into so-called membrane domains. Remorins are well-established membrane domain marker proteins. However, the general plasma membrane anchoring mechanism of these proteins was so far unknown. Biochemical approaches and localization studies investigating different remorins from Medicago truncatula and Arabidopsis thaliana enabled us to demonstrate that S-acylation (palmitoylation) within a C-terminal plasma membrane anchoring motif mediates tight plasma membrane attachment of these proteins. However, we could show that S-acylation is not the sole driving force for remorin immobilization in membrane nanodomains. The focus of the second part of this thesis was on the beneficial interaction between plants and symbionts. More than 80% of today´s land plants can undergo an interaction with endosymbiotic fungi that is known as Arbuscular Mycorrhiza (AM). In addition, legume plants have gained the ability to establish a second type of endosymbiosis by interacting with nitrogen-fixing rhizobia: the Root Nodule Symbiosis (RNS). Both interactions are partly controlled by the same pathway, the so-called Common Symbiosis Pathway (CSP) that has evolved through recruitment of signaling components from the evolutionary older AM to the more recently evolved RNS signaling pathway. Depending on the recognition of either fungi or rhizobia downstream of this pathway two morphologically different symbiotic structures are formed within the inner root cortex, either arbuscules or root nodules, respectively. In parallel to the evolution of RNS a local negative regulatory circuit must have evolved to suppress root nodule organogenesis when both interacting symbionts are present and arbuscule formation takes place. In this study first evidence for such a postulated regulatory pathway is presented based on the characterization of the legume-specific remorin MYCREM, which co-evolved with RNS. Phenotypic studies of mutant plants revealed that in the presence of both symbionts MYCREM functions as a negative regulator with respect to root nodule organogenesis events in a CSP-dependent manner. Analyzing the effect of overexpression of auto-active CSP-signaling components, which are known to spontaneously induce root nodule organogenesis, demonstrated a negative regulatory function of MCYREM as well. In summary, this work could serve as basis for further studies to understand the tripartite interaction of legume plants, fungi and rhizobia, as it is found in nature