Identification of Novel Components of the Unfolded Protein Response in Arabidopsis

Abstract

Unfavorable environmental and developmental conditions may cause disturbances in protein folding in the endoplasmic reticulum (ER) that are recognized and counteracted by components of the Unfolded Protein Response (UPR) signaling pathways. The early cellular responses include transcriptional changes to increase the folding and processing capacity of the ER. In this study, we systematically screened a collection of inducible transgenic Arabidopsis plants expressing a library of transcription factors for resistance toward UPR-inducing chemicals. We identified 23 candidate genes that may function as novel regulators of the UPR and of which only three genes (bZIP10, TBF1, and NF-YB3) were previously associated with the UPR. The putative role of identified candidate genes in the UPR signaling is supported by favorable expression patterns in both developmental and stress transcriptional analyses. We demonstrated that WRKY75 is a genuine regulator of the ER-stress cellular responses as its expression was found to be directly responding to ER stress-inducing chemicals. In addition, transgenic Arabidopsis plants expressing WRKY75 showed resistance toward salt stress, connecting abiotic and ER-stress responses.

A Tripartite Amplification Loop Involving the Transcription Factor WRKY75, Salicylic Acid, and Reactive Oxygen Species Accelerates Leaf Senescence

“…It is conceivable that, upon leaf senescence, agedependent WRKY75 expression progressively leads to the concomitant increase in SA and H 2 O 2 levels, two well-defined inducers of leaf senescence. In fact, WRKY75 has been reported to modulate diverse biological processes, particularly stress responses such as phosphate deficiency (Devaiah et al, 2007), root hair development (Rishmawi et al, 2014), oxalic acid stress resistance (Chen et al, 2013), and the unfolded protein response (Hossain et al, 2016). Given that the downstream regulatory networks dictated by WRKY75 in these processes are still unclear, our findings about the regulatory role of WRKY75 in SA biosynthesis and ROS scavenging in leaf senescence offers a potential mechanism for these processes as well.…”

Salicylic acid-independent role of NPR1 is required for protection from proteotoxic stress in the plant endoplasmic reticulum

“…TBF1 has been identified as a TF triggering the NPR1-dependent UPR genes to promote PR1 secretion during plant defense signaling (22). In addition, the compromised UPR phenotype of a tbf1 mutant was further confirmed by enhanced Tm resistance in transgenic plants overexpressing the TBF1 gene (22,55). In other reports, however, the tbf1 mutant displayed Tm sensitivity and induction profiles of UPR marker genes similar to those of Col-0 plants (23).…”

Calcium-Mediated Abiotic Stress Signaling in Roots

“…Salt exposure puts the plant’s ER under stress, leading to an accumulation of unfolded or misfolded proteins that could lead to cell death ( Liu et al, 2007 , 2011 ). Such ER stress triggers upregulation of a suite of responses termed the “Unfolded Protein Response” (UPR), in which folding capacity is upregulated (including by Ca 2+ -regulated chaperones), translation is curtailed and the ER-associated degradation pathway acts to lower the aberrant protein load ( Deng et al, 2013 ; Ruberti et al, 2015 ; Hossain et al, 2016 ; Wan and Jiang, 2016 ). Expression of the ER Ca 2+ -binding chaperones Calnexin and Calreticulin has been shown to upregulated in the Arabidopsis UPR ( Christensen et al, 2008 ; Liu et al, 2011 ) and it would now be interesting to test whether these are involved in regulating levels of Ca 2+ in the ER under stress.…”