Gamma-zeins, but not phaseolin, trigger the endoplasmic reticulum unfolded protein response

Cereal and legume seeds are the primary source of food protein for humans and farm animals. Both major classes of seed proteins, globulins and prolamins, are multimeric proteins that start their life in the ER, where they are co-translationally folded and assembled. Biotic and abiotic stresses alter endoplasmic reticulum (ER) proteostasis and may therefore disturb proper seed protein accumulation, inducing the unfolded protein response (UPR) an as adaptive reaction to restore proper cellular homeostasis. Climate change, which obviously affects plant growth and crop production, may therefore also influence the nutritional quality and technological properties of seed proteins. After folding and assembly in the ER, legume globulins are then deposited in storage vacuoles (PSV) via the secretory pathway, whether cereal prolamins accumulate directly in the ER lumen, forming protein bodies. Thus, during seed development, the functions of the ER are challenged by the massive synthesis of storage proteins, but it is unknown if this physiological process engages stress response mechanisms. In order to assess the physiological extent of the ER stress response during the accumulation of storage proteins, we compared by RT-PCR the expression of UPR/autophagy genes in wild type A. thaliana plants and in transgenic Arabidopsis constitutively expressing 27kD gamma-zein, 16kD gamma-zein, 27kD 1C (a mutated version of 27kd gamma-zein that is not retained in the ER lumen) or phaseolin (PHSL, the 7S vacuolar globulin of P. vulgaris). The analysis shows that gamma-zein polypeptides, but not PHSL or 27kD 1C, trigger the UPR. This indicates that the engagement of UPR is physiological during prolamin deposition in protein bodies, but is not necessary to sustain the synthesis of an oligomeric storage protein that traffic out of the ER. Supported by the FILAGRO Project of CNR-Regione Lombardia