The Unfolded Protein Response Is Triggered by a Plant Viral Movement Protein    

Abstract

Infection with Potato virus X (PVX) in Nicotiana benthamiana plants leads to increased transcript levels of several stress-related host genes, including basic-region leucine zipper 60 (bZIP60), SKP1, ER luminal binding protein (BiP), protein disulfide isomerase (PDI), calreticulin (CRT), and calmodulin (CAM). bZIP60 is a key transcription factor that responds to endoplasmic reticulum (ER) stress and induces the expression of ER-resident chaperones (BiP, PDI, CRT, and CAM). SKP1 is a component of SCF (for SKP1-Cullin-F box protein) ubiquitin ligase complexes that target proteins for proteasomal degradation. Expression of PVX TGBp3 from a heterologous vector induces the same set of genes in N. benthamiana and Arabidopsis (Arabidopsis thaliana) leaves. Virus-induced gene silencing was employed to knock down the expression of bZIP60 and SKP1, and the number of infection foci on inoculated leaves was reduced and systemic PVX accumulation was altered. Silencing bZIP60 led to the suppression of BiP and SKP1 transcript levels, suggesting that bZIP60 might be an upstream signal transducer. Overexpression of TGBp3 led to localized necrosis, but coexpression of TGBp3 with BiP abrogated necrosis, demonstrating that the unfolded protein response alleviates ER stress-related cell death. Steady-state levels of PVX replicase and TGBp2 (which reside in the ER) proteins were unaltered by the presence of TGBp3, suggesting that TGBp3 does not contribute to their turnover. Taken together, PVX TGBp3-induced ER stress leads to up-regulation of bZIP60 and unfolded protein response-related gene expression, which may be important to regulate cellular cytotoxicity that could otherwise lead to cell death if viral proteins reach high levels in the ER.

Role of unfolded protein response in plant virus infection

“…The prior analysis was of a large population, and while the median values were lower, the values obtained in this smaller experiment, fall within the range of values obtained from the larger population. 19 These data argue that PVX infection did not require synthesis of molecular markers occurring throughout the ER and secretory pathway. Thus, if membrane synthesis and expansion is occurring during PVX infection, we cannot state that there is a concomitant change in ER-Golgi network markers.…”

“…[16][17][18] Real time RT-PCR analyses revealed an increase in ER resident chaperones which we initially hypothesized could due to expansion of the ER network or that increased levels of chaperones are needed in response to PVX infection. 15,16,19 Thus, (e.g., IRE1, PERK and ATF6). 1,6 BIP is known to be upregulated by bZIP60 and is also reported to regulate cytotoxic damage to cells.…”

“…mock inoculated (treated with agro-infiltration buffer) was used as a control and calibrator sample. total rnA was isolated from samples and first strand cdnA synthesis was carried out as described in Ye et al (2011). 19 qPcr was carried out using 25 μl reactions and 100 to 900 nm primers designed using the coding sequences for known N. benthamiana genes.…”

“…total rnA was isolated from samples and first strand cdnA synthesis was carried out as described in Ye et al (2011). 19 qPcr was carried out using 25 μl reactions and 100 to 900 nm primers designed using the coding sequences for known N. benthamiana genes. Initial rt-Pcr tests of the gene specific primers confirmed their ability to amplify single bands of the predicted sizes from N. benthamiana cdnA.…”

Inositol-requiring enzyme 1 affects meristematic division in roots under moderate salt stress in Arabidopsis

“…Some external stimuli such as heat stress, salicylic acid treatment, and viral infection have been reported to activate bZIP60 and bZIP28 (Deng et al 2011;Gao et al 2008;Moreno et al 2012;Nagashima et al 2014;Ye et al 2011;Zhang et al 2015). Furthermore, it was reported that an Arabidopsis mutant defective in both IRE1A and IRE1B genes grown under high temperature contains less viable pollen grains, indicating the importance of the UPR in pollen development under high temperature (Deng et al 2016).…”