Abstract text: Heat stress promotes the accumulation of misfolded proteins during synthesis, overloading the endoplasmic reticulum and activating the unfolded protein response (UPR). As cell wall polysaccharides are synthesized and processed in the Golgi apparatus, this raises the question of whether UPR activation impacts cell wall assembly. To address this, we examined the role of key UPR components in the synthesis and modification of Arabidopsis seed mucilage polysaccharides. Heat treatment was used to induce the UPR transcription factors bZIP28 and bZIP17, and mutant lines deficient in these regulators were analyzed. Biochemical analyses combined with immunoassays provide the first evidence linking the UPR to seed mucilage architecture. The results show that the absence of bZIP28 and bZIP17 altered mucilage release, the degree of homogalacturonan methylesterification, monosaccharide composition, and RG-I distribution, as determined by immunostaining. In addition, changes in the type and abundance of mucilage-associated proteins were detected. These findings uncover a previously undescribed role for the UPR in regulating cell wall architecture, and highlight, for the first time, the integration of protein quality control with cell wall dynamics.