A Thioredoxin Domain-Containing Protein Interacts with Triple Gene Block Protein 1

(PepMV) is a mechanically-transmitted tomato pathogen of importance worldwide. Interactions between the PepMV coat protein and triple gene block protein (TGBp1) with the host heat shock cognate protein 70 and catalase 1 (CAT1), respectively, have been previously reported by our lab. In this study, a novel tomato interactor (TXND9) was shown to bind the PepMV TGBp1 in yeast-two-hybrid screening, in vitro pull-down and bimolecular fluorescent complementation (BiFC) assays. TXND9 possesses part of the conserved thioredoxin (TRX) active site sequence (W__PC vs. WCXPC), and TXND9 orthologues cluster within the TRX phylogenetic superfamily closest to phosducin-like protein-3. In PepMV-infected and healthy plants, TXND9 mRNA levels were comparable, and expression levels remained stable in both local and systemic leaves for 10 days post inoculation (dpi), as was also the case for catalase 1 (CAT1). To localize the TXND9 in plant cells, a polyclonal antiserum was produced. Purified α-TXND9 immunoglobulin (IgG) consistently detected a set of three protein bands in the range of 27⁻35 kDa, in the 1000 and 30,000 pellets, and the soluble fraction of extracts of healthy and PepMV-infected leaves, but not in the cell wall. These bands likely consist of the homologous protein TXND9 and its post-translationally modified derivatives. On electron microscopy, immuno-gold labelling of ultrathin sections of PepMV-infected leaves using α-TXND9 IgG revealed particle accumulation close to plasmodesmata, suggesting a role in virus movement. Taken together, this study highlights a novel tomato-PepMV protein interaction and provides data on its localization in planta. Currently, studies focusing on the biological function of this interaction during PepMV infection are in progress.