bZIP17 and bZIP60 Regulate the Expression of BiP3 and Other Salt Stress Responsive Genes in an UPR‐Independent Manner in Arabidopsis thaliana

Abstract

Plants can be severely affected by salt stress. Since these are sessile organisms, they have developed different cellular responses to cope with this problem. Recently, it has been described that bZIP17 and bZIP60, two ER-located transcription factors, are involved in the cellular response to salt stress. On the other hand, bZIP60 is also involved in the unfolded protein response (UPR), a signaling pathway that up-regulates the expression of ER-chaperones. Coincidentally, salt stress produces the up-regulation of BiP, one of the main chaperones located in this organelle. Then, it has been proposed that UPR is associated to salt stress. Here, by using insertional mutant plants on bZIP17 and bZIP60, we show that bZIP17 regulate the accumulation of the transcript for the chaperone BiP3 under salt stress conditions, but does not lead to the accumulation of UPR-responding genes such as the chaperones Calnexin, Calreticulin, and PDIL under salt treatments. In contrast, DTT, a known inducer of UPR, leads to the up-regulation of all these chaperones. On the other hand, we found that bZIP60 regulates the expression of some bZIP17 target genes under conditions were splicing of bZIP60 does not occur, suggesting that the spliced and unspliced forms of bZIP60 play different roles in the physiological response of the plant. Our results indicate that the ER-located transcription factors bZIP17 and bZIP60 play a role in salt stress but this response goes through a signaling pathway that is different to that triggered by the unfolded protein response.

Unfolded protein response in plants: one master, many questions

“…The diagram illustrates that in addition to IRE1, bZIP28 and bZIP60, also bZIP17 plays a role in the control of UPR target genes in Arabidopsis, as recently proposed [27]. A role for the beta subunit of the heterotrimeric G-protein complex, AGB1, in modulating the UPR has also been reported [17••,53].…”

“…The evidence that plant-specific NAC TFs have a role in stress responses support that in addition to common routes plants have evolved unique strategies for ER stress defense that, as in the case of NAC062, may also depend on the activity of TFs that are localized to the membrane organelles other than the ER. Interestingly also, it has been recently suggested that unspliced AtbZIP60 may have a role in modulating the expression of BiP3 in conditions of ER stress [27]. Phosphorylation of the elf2α may occur in a hypothetical PERK-like pathway for ER stress responses.…”

“…It is also known however that activation of bZIP-TFs may lead to responses that are qualitatively distinct. For example, in conditions of salt stress, AtbZIP17 modulates only the expression of BiP3 among the known ER stress markers [27], which, on the other hand, is controlled by both AtbZIP28 and AtbZIP60 in conditions of ER stress induced by heat or by drugs such as tunicamycin and DTT. Furthermore, heat, tunicamycin and DTT but not salt stress can induce activation of AtbZIP60 and AtbZIP28, and a role of AtIRE1-AtbZIP60 but not AtbZIP28 has been demonstrated in virus-plant interactions [28].…”

Osmotic Stress Induced Cell Death in Wheat Is Alleviated by Tauroursodeoxycholic Acid and Involves Endoplasmic Reticulum Stress–Related Gene Expression

“…It is well known that bZIP60s are important plant transcription factors that participate in the expression and regulation of ER stress-related and other genes in Arabidopsis (Sabrina et al, 2012; Henriquez-Valencia et al, 2015). The results of this study showed that both TabZIP60B and TabZIP60D respond to osmotic stress very quickly.…”

In Brachypodium a complex signaling is actuated to protect cells from proteotoxic stress and facilitate seed filling

“…One possible explanation for the accumulation of unspliced BdbZIP60 in ER stress conditions in Brachypodium is that ER stress may induce BdbZIP60 transcription to levels that overcome the RNase activity of IRE1 to lead to a translated form of BdbZIP60 with specific roles in stress responses. For example, it has been suggested that the unspliced form of AtbZIP60 in Arabidopsis may have a role in salt-stress response (Henriquez-Valencia et al 2015; Parra-Rojas et al 2015). Analogously it is possible that the Brachypodium UPR also relies on the translated unspliced BdbZIP60.…”