Fig. 9.

Hypothetical model for the regulation of ER-UPR by HsfA1a in tomato under proteotoxic conditions. bZIP60 and bZIP28 are the major and essential regulators of ER-UPR. Under heat stress, HsfA1a can regulate directly the transcription of ER sHSPs and co-regulate with bZIP60/bZIP28 the expression of BIP3. Through the control of ATG genes, HsfA1a can also regulate the ER stress–induced autophagy, a process that has been shown to be IRE1-dependent in A. thaliana. An active HsfA1a can also enhance the accumulation of bZIP60s, either by accumulation of bZIP60u and/or the increased activity of IRE1. The signal that triggers the activation of HsfA1a in response to ER stress is currently unknown and is marked with a question mark