BIP and the unfolded protein response are important for potyvirus and potexvirus infection

Abstract

Plant potexvirus and potyvirus infection can trigger endoplasmic reticulum (ER) stress. ER stress signaling increases the expression of cytoprotective ER-chaperones, especially the BiP chaperones which contribute to pro-survival functions when plants are subjected to infection. The inositol requiring enzyme (IRE1) is one ER stress sensor that is activated to splice the bZIP60 mRNA which produces a truncated transcription factor that activates gene expression in the nucleus. The IRE1/bZIP60 pathway is associated with restricting potyvirus and potexvirus infection. Recent data also identified the IRE1-independent UPR pathways led by bZIP28 and bZIP17 contribute to potexvirus and potyvirus infection. These three bZIP pathways recognize cis-regulatory elements in the BiP promoters to enhance gene expression. BiP is part of a negative feedback loop that regulates the activities of the ER stress transducers IRE1, bZIP28, and bZIP17 to block their activation. We discuss a model in which bZIP60 and bZIP17 synergistically induce BiP and other genes restricting Plantago asiatica mosaic virus (PlAMV; a potexvirus) infection while bZIP60 and bZIP28 independently induce genes supporting PlAMV infection. Regarding Turnip mosiac virus (TuMV, a potyvirus) infection, bZIP60 and bZIP28 serve to repress local and systemic infection. Finally, tauroursodeoxycholic acid treatments were used to demonstrate that the protein folding capacity significantly influences PlAMV accumulation.

Keywords: BIP; Unfolded protein response; endoplasmic reticulum stress; molecular chaperones; plant stress; potexvirus; potyvirus.

Figure 1.

UPR mediated suppression of plant virus infection. The IRE1a/IRE1b-dependent, bZIP28/bZIP17 dependent pathways are routes leading to transcriptional activation of genes that regulate pro-survival and pro-death events. BiP is an ER resident molecular chaperone that is a master regulator of these ER stress transducers to block their activation. BiP also functions in the ER lumen to facilitate protein folding in an ATPase dependent manner. The bZIP60, bZIP28, and bZIP17 bind to the BiP promoter and increasing its expression to enhance protein folding in the ER lumen and to serve as part of a negative feedback loop to block further activation of these stress transducers in the ER. IRE1a/IRE1b endonuclease activity splice the bZIP60 mRNA to produce a transcription factor that is mobilized to the nucleus. NAC089 and NAC103 are activated to regulate programmed cell death and enhance protein quality control. bZIP60 and bZIP17 form complexes that activate unknown genes that limit PlAMV infection. The bZIP60 and bZIP28 activate unknown genes that support PlAMV infection but limit TuMV infection. Overexpression of BiP or enhancing protein folding capacity through TUDCA limits virus infection and suppressed PCD