A membrane-tethered transcription factor defines a branch of the heat stress response in Arabidopsis thaliana

In plants, heat stress responses are controlled by heat stress transcription factors that are conserved among all eukaryotes and can be constitutively expressed or induced by heat. Heat-inducible transcription factors that are distinct from the "classical" heat stress transcription factors have also been reported to contribute to heat tolerance. Here, we show that bZIP28, a gene encoding a putative membrane-tethered transcription factor, is up-regulated in response to heat and that a bZIP28 null mutant has a striking heat-sensitive phenotype. The heat-inducible expression of genes that encode BiP2, an endoplasmic reticulum (ER) chaperone, and HSP26.5-P, a small heat shock protein, is attenuated in the bZIP28 null mutant. An estradiol-inducible bZIP28 transgene induces a variety of heat and ER stress-inducible genes. Moreover, heat stress appears to induce the proteolytic release of the predicted transcription factor domain of bZIP28 from the ER membrane, thereby causing its redistribution to the nucleus. These findings indicate that bZIP28 is an essential component of a membrane-tethered transcription factor-based signaling pathway that contributes to heat tolerance.