Induction of lipid metabolic enzymes during the endoplasmic reticulum stress response in plants

The endoplasmic reticulum (ER) stress response is a signal transduction pat hway activated by the perturbation of normal ER metabolism. We used the mai ze (Zea mays) floury-2 (fl2) mutant and soybean (Glycine max) suspension cu ltures treated with tunicamycin (Tm) to investigate the ER stress response as it relates to phospholipid metabolism in plants. Four key phospholipid b iosynthetic Enzymes, including DG kinase and phosphatidylinositol (PI) 4-ph osphate 5-kinase were upregulated in the fl2 mutant, specifically in protei n body fractions where the mutation has its greatest effect. The third up-r egulated enzyme, choline-phosphate cytidylyltransferase, was regulated by f l2 gene dosage and developmental signals. Elevated accumulation of the four th enzyme, PI 4-kinase, was observed in the fl2 endosperm and soybean cells treated with Tm. The activation of these phospholipid biosynthetic enzymes was accompanied by alterations in membrane lipid synthesis and accumulatio n. The fl2! mutant exhibited increased PI content in protein body membranes at 18 d after pollination and more than S-fold higher triacylglycerol accu mulation in the endosperm by 36 d after pollination. Incorporation of radio -labeled acetate into phospholipids in soybean culture cells increased by a bout 30% with Tm treatment. The coordinated regulation of ER stress related proteins and multiple components of phospholipid biosynthesis is consisten t with signaling through a common pathway. We postulate that the plant ER s tress response has an important role in general plant metabolism, and more specifically in integrating the synthesis of protein and lipid reserves to allow proper seed formation.