TGBp3 triggers the unfolded protein response and SKP1‐dependent programmed cell death

Abstract

The Potato virus X (PVX) triple gene block protein 3 (TGBp3), an 8-kDa membrane binding protein, aids virus movement and induces the unfolded protein response (UPR) during PVX infection. TGBp3 was expressed from the Tobacco mosaic virus (TMV) genome (TMV-p3), and we noted the up-regulation of SKP1 and several endoplasmic reticulum (ER)-resident chaperones, including the ER luminal binding protein (BiP), protein disulphide isomerase (PDI), calreticulin (CRT) and calmodulin (CAM). Local lesions were seen on leaves inoculated with TMV-p3, but not TMV or PVX. Such lesions were the result of TGBp3-elicited programmed cell death (PCD), as shown by an increase in reactive oxygen species, DNA fragmentation and induction of SKP1 expression. UPR-related gene expression occurred within 8 h of TMV-p3 inoculation and declined before the onset of PCD. TGBp3-mediated cell death was suppressed in plants that overexpressed BiP, indicating that UPR induction by TGBp3 is a pro-survival mechanism. Anti-apoptotic genes Bcl-xl, CED-9 and Op-IAP were expressed in transgenic plants and suppressed N gene-mediated resistance to TMV, but failed to alleviate TGBp3-induced PCD. However, TGBp3-mediated cell death was reduced in SKP1-silenced Nicotiana benthamiana plants. The combined data suggest that TGBp3 triggers the UPR and elicits PCD in plants.

The Potyviral P3 Protein Targets Eukaryotic Elongation Factor 1A to Promote the Unfolded Protein Response and Viral Pathogenesis

“…The plant UPR comprises two branches, one mediated by IRE1 and the other by ATF6-like receptors, which involves the transcription factors bZIP17 and 28 (Howell, 2013). Viral infection in plants is known to induce the IRE1 arm of UPR, which involves IRE1-mediated splicing of bZIP60 mRNA, resulting in the synthesis of a functional bZIP protein that can activate ER stress-inducible promoters in the nucleus (Ye et al, , 2013Verchot 2014). Examples include induction of bZIP60 and UPR chaperone gene expression by the ER associated movement protein of potato virus X and the turnip mosaic virus 6K2 protein-induced splicing of bZIP60 mRNA .…”

Cell death triggered by the P25 protein in Potato virus X‐associated synergisms results from endoplasmic reticulum stress in Nicotiana benthamiana

“…, ; Ye et al. , ). Similarly, we propose that co‐infections of PVX with other viruses expressing strong VSRs could mimic HC‐Pro action on P25, allowing P25 to accumulate to very high levels with a detrimental effect on ER homeostasis, subsequently leading to necrosis in systemic leaves.…”

“…Previous investigations have shown that the TGB3 protein of PVX triggers UPR and elicits PCD when expressed from TMV or under the control of the CaMV 35S promoter and delivered by agroinfiltration into N. benthamiana (Ye et al. , ). These findings argue that PVX encodes at least two protein products, TGB3 and P25, capable of inducing a UPR and cell death when overexpressed by either a viral vector or in the presence of a strong VSR.…”

“…In addition, the TGB3 protein elicited PCD when expressed either from the Tobacco mosaic virus (TMV) genome or from the Cauliflower mosaic virus (CaMV) 35S promoter (Ye et al. , ).…”

Plant Virus Infection and the Ubiquitin Proteasome Machinery: Arms Race along the Endoplasmic Reticulum

“…Silencing SKP1 directly alters the level of virus accumulation and systemic necrosis in tobacco plants. While the role of ubiquitin in this process is not established, the role of the proteasome in viral protein turnover is inferred [48,50]. These examples of TYMV, TMV and PVX demonstrate that the UPS can be subverted by some viruses to ensure cellular homeostasis and cell survival needed by both the plant and the virus to co-exist.…”