Cellular dynamics during rhizobia infection

Abstract: When Medicago truncatula is infected by Sinorhizobium meliloti, it will cause obvious root hair morphology changes which require cellular cytoskeleton reorganization and re-arrangement of the plasma membrane. However, although microtubule reorganization upon rhizobial inoculation and infection has been reported, our knowledge about the genes specifically regulating microtubule dynamics remained unclear. Here, we identified the DEVELOPMENTALLY REGULATED PLASMA MEMBRANE POLYPEPTIDE (DREPP) protein in M. truncatula and investigated its functions during rhizobial infections. We show that in standard conditions DREPP associates with the plasma membrane via both myristoylation and positively charged amino acids at the N-terminus. While the protein shows a relative homogenous distribution in the absence of the symbiont, rhizobial inoculation will trigger a relocalization of the protein to symbiosis-specific membrane nanodomains, which strictly depends on the scaffold proteins SYMREM1 and FLOT4. Moreover, this membrane nanodomain pattern of DREPP coincided with the MT fragmentation in root hairs and our genetic data indicate that this process is essential for MT remodeling during root hair deformation. Upon completion of the root hair curl, rhizobia are entrapped in the ‘infection chamber’ (IC). Subsequently, rhizobial divisions inside the IC lead to the formation of a tunnel-like structure inside root hair called ‘infection thread’ (IT). While the signaling cascade required to initiate IT growth has been unravelled, the molecular mechanism of this very first membrane invagination remains elusive. In a second project, we identified a legume-specific transmembrane-containing lectin-domain protein (LDP1) and its closely related member (LDP2) by detailed phylogenetic analyses. Stable M. truncatula CRISPR-CAS-mediated LDP1/2 knockout alleles display a severe defect in IT formation, while root hair curling remained fully operational indicating the specific function of LDP1 in IT initiation. Focusing on the molecular characterization of LDP1, we show that the LDP1 promoter remains exclusively activated in those root hairs that will be subsequently infected. The LDP1 protein localizes to the membrane of the IC with its lectin domain reaching into the bacteria-filled 6extracellular space. Furthermore, we demonstrate that LDP1 physically binds to polarly secreted rhizobial exopolysaccharides, enabling an immobilization of the bacteria at the plant cell surface and leading to LDP1 clustering. To test whether such clustering has an impact in membrane topology, we generated a recombinant and His-tagged lectin domain and applied this to artificial membrane bilayer liposomes reconstituted from 18:1 DGS-NTA(Ni) lipids. Indeed, this resulted in LDP1 clustering and membrane invaginations. As a third project, we unraveled the mechanism used by plant cells to maintain these membrane invaginations and ITs in turgoid cells. Here, we report on a new feature of the symbiosis-induced remorin protein SYMREM1 that stabilizes symbiotic membrane curvatures in the absence of a cell wall. We show that ectopic expression of SYMREM1 stabilizes membrane-enclosed nanotubes in protoplasts while it equally maintains membrane invaginations obtained by nano-indentation. Genetically, SYMREM1 stabilizes controlled tubular pre-release membrane reservoirs around infection threads and droplets while these structures are lost in symrem1 mutants explaining the bacterial release phenotypes in these lines. This function is, most likely, conferred by a curved anti-parallel SYMREM1 trimer that can further oligomerize into higher-order structures including liquid-liquid-phase condensates

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2021

Classification and Topics

• Classification: Biowissenschaften, Biologie

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2021
• Creator: Su, Chao
• Contributor: Ott, Thomas ; Albert-Ludwigs-Universität Freiburg. Fakultät für Biologie

Further information

• DOI: 10.6094/UNIFR/219675
• URN: urn:nbn:de:bsz:25-freidok-2196754
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:42 PM CET