This dissertation was only published as a printed edition. Polymyxa graminis-transmitted cereal viruses cause substantial crop losses and therefore represent an extensive threat for agriculture in Europe, Asia, and America. The coat protein-readthrough protein (CP-RT) or homologous proteins of these viruses are believed to play a role in virus transmission, as many studies indicated the importance of the RT-domain of CP-RT for successful virus infection of host plants by plasmodiophorid vectors. To study the role of CP-RT and the RT-domain in planta, with a specific focus on the subcellular localization and transport of the protein, a Japanese soil-borne wheat mosaic virus (JSBWMV) cDNA clone was modified to allow expression of a fluorescent CP-RT fusion protein (CP-RT:FP). Moreover, to study the function of the RT-domain, experiments using the JSBWMV-Bornum isolate from Germany and barley as host were performed. From the literature, it was known that deletions within the CP-RT protein occur under greenhouse conditions and that these mutations correlate with the loss of vector-mediated virus infection of the host plants. Therefore, here, the onset of deletions in the RT-domain of the JSBWMV-Bornum CP-RT gene, caused by different cultivation or infection techniques, was analyzed via RT-PCR analysis of virus infected plants. The obtained results show that the JSBWMV-Bornum CP-RT gene was not affected by repeated mechanical virus transmission, but the RT-domain was deleted after prolonged cultivation of infected plants. To directly test the role of CP-RT and the RT-domain in virus transmission, a hydroponic culture system was established, and the vector-mediated transmission of JSBWMV-Bornum was investigated with wild-type (WT) CP-RT and deletion mutants of CP-RT in barley. Using RT-PCR, WT and mutant CP-RT could be detected in RNA extracts of roots of donor and acceptor plants, but only WT CP-RT was detected in the leaves of acceptor plants. Based on this observation, WT CP:RT might be required for the transfer of the virus from P. graminis into the plant host root cell and maybe also for systemic movement of the virus. On the subcellular level, the JSBWMV CP-RT localizes to and labels the endoplasmic reticulum (ER) and forms tightly ER-associated dot-like particles during infection and when overexpressed, as shown by in-vivo confocal laser-scanning microscopy in N. benthamiana as host. The ER-associated particles correspond to sub-ER sites, as they colocalize with GTP-locked Sar1 and the ER import site (ERIS) marker AtSYP72. Additionally, CP-RT interacts with itself at the ER and in the dot-like particles. The studies involving the subcellular localization of CP-RT mutants demonstrated that accumulation within the ER network correlates with virus movement, as mutants lacking parts of the RT-domain were not found in systemic tissues of acceptor plants in the hydroponic culture system. However, dot-like structures were still visible in CP-RT mutants and for the CP protein. The dot-like structures at sub-ER sites may thus represent anchor sites for virus replication or sites of interference with host secretory pathway. The obtained results in this thesis were used to propose a model for a role of CP-RT in virus transmission and intercellular transport: Figure 29: Proposed model and summary of possible functions for JSBWMV CP-RT during virus transmission and infection. At the beginning of the infection process, CP-RT may facilitate the transmission of the virus from the vector plasmodium to the host cell cytoplasm or movement of virus out of the initially infected host cell. The host and vector ER and/or secretory pathway might be involved during this process. In the host cell, the CP-RT-labeled sub ER-sites may represent hubs for ER-associated virus replication or virus transport. Virus movement may occur along the ER-network to and through PD. CP-RT may have a role in virus transport along the ER network and between cells, as WT CP-RT is required for systemic movement of the virus and accumulation within the ER network. Movement proteins alter the PD structure during infection to overcome the size exclusion limit for protein transport, which is necessary for successful virus transport through PD. Strauch 2023 showed that the JSBWMV MP localizes to PD (Figure 32).
Nico Sprotte (Thu,) studied this question.