Role of the Transcription Factor NbTFIISL in Enhancing Bamboo Mosaic Virus Accumulation via Mitochondria Localisation

The transcription factor NbTFIISL, a TFIIS-like protein in Nicotiana benthamiana, plays a critical role in facilitating bamboo mosaic virus (BaMV) infection. BaMV infections significantly downregulate NbTFIISL expression, with transcript levels reduced to 38% by 7 days post-inoculation. Virus-induced gene silencing of NbTFIISL impaired BaMV accumulation, reducing coat protein levels to ~45% and leading to smaller GFP-labelled infection foci. Protoplast-based assays further confirmed its involvement in viral replication, with BaMV RNA levels dropping to 12% in NbTFIISL-silenced cells. Subcellular localisation analysis revealed that NbTFIISL is primarily nuclear, directed by a classical nuclear localisation signal (NLS) and an LW motif. Mutant constructs lacking these signals-NbTFIISL/ΔNLS and NbTFIISL/ΔNLS/ΔLW-lost nuclear targeting and instead localised to mitochondria, with the double mutant forming distinct speckle-like aggregates. Overexpression experiments uncovered a dual role for NbTFIISL: nuclear-localised wild-type protein induced necrosis, whereas mitochondria-localised NbTFIISL/ΔNLS significantly enhanced BaMV accumulation (~148%). In contrast, protein aggregation in the NbTFIISL/ΔNLS/ΔLW mutant partially impaired this enhancement. Yeast two-hybrid assays revealed specific interactions between NbTFIISL and BaMV proteins, including the replicase RdRp domain and movement protein TGBp1. These findings suggest that NbTFIISL promotes BaMV replication through mitochondrial relocalisation and interaction with viral proteins, whereas its nuclear presence may trigger necrosis, potentially limiting viral spread. This study highlights the multifunctional roles of NbTFIISL and advances our understanding of host factors in plant-virus interactions and viral pathogenesis.