Fig. 2.

Heat stress sensing and response mechanism during male gametophyte development. Elevated temperature stress is perceived by the pollen vegetative cell, which triggers Ca²⁺ flux, ROS accumulation in the cytosol and activation of the unfolded protein response (UPR) in the endoplasmic reticulum (ER). The UPR pathway has two arms: (a) the ER membrane‐localized RNA splicing factor IRE1 is involved in the unconventional splicing of bZIP60 pre‐mRNA, resulting in the expression of functional transcription factor; (b) the pair of ER membrane‐anchored TFs, bZIP17 and bZIP28, is released and relocated to the Golgi apparatus, cleaved by S1P and S2P proteases, and transported to the nucleus. In the nucleus, bZIP17/60 and bZIP28/60 dimers activate stress‐responsive gene expression on the ERSE/UPRE. Abbreviations: bZIP, basic leucine zipper TF; Ca²⁺, calcium cation; ER, endoplasmic reticulum; ERSE, ER stress‐response element; GA, Golgi apparatus; IRE1, inositol requiring enzyme 1α; PM, plasma membrane; ROS, reactive oxygen species; S1P, site‐1 protease; S2P, site‐2 protease; TFs, transcription factors; UPR, unfolded protein response; UPRE, unfolded protein response element.