Interplay between autophagy, the unfolded protein response and the cytoplasmic heat stress response during heat stress in maize

High temperatures can substantially reduce plant survival and reproduction, and therefore decrease crop productivity. Plants activate several pathways in response to high temperatures, including the endoplasmic reticulum (ER) unfolded protein response (UPR), the cytoplasmic heat stress response, and autophagy, which together aid in stress tolerance by refolding or degrading misfolded and unfolded proteins. The relationship between the UPR and autophagy is known, as ER stress activates the UPR, and a key activator of the UPR, Inositol-requiring enzyme type 1 (IRE1B), upregulates autophagy. To assess the relationship between the distinct responses to heat stress in maize, we analyzed the effect of disruption of the UPR, via a bzip60 mutant, on autophagy. We found that genes related to autophagy are upregulated in the bzip60 mutant, and that autophagy is activated in this mutant even in the absence of stress. We in turn analyzed the effect of loss of autophagy on the UPR. The bZIP60 mRNA spliced form, a marker for the UPR, was increased in mutants in ATG10, a core component of the autophagy machinery, during ER stress. By contrast, bZIP60 splicing was unaffected in atg10 mutants in response to heat, whereas cytoplasmic heat stress components were increased. Loss of autophagy therefore differentially affects other heat stress response pathways, depending on the specific stress conditions leading to the activation of the pathways.