Induction of Lipid Metabolic Enzymes during the Endoplasmic Reticulum Stress Response in Plants

Abstract

The endoplasmic reticulum (ER) stress response is a signal transduction pathway activated by the perturbation of normal ER metabolism. We used the maize (Zea mays) floury-2 (fl2) mutant and soybean (Glycine max) suspension cultures treated with tunicamycin (Tm) to investigate the ER stress response as it relates to phospholipid metabolism in plants. Four key phospholipid biosynthetic enzymes, including DG kinase and phosphatidylinositol (PI) 4-phosphate 5-kinase were upregulated in the fl2 mutant, specifically in protein body fractions where the mutation has its greatest effect. The third up-regulated enzyme, choline-phosphate cytidylyltransferase, was regulated by fl2 gene dosage and developmental signals. Elevated accumulation of the fourth 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 accumulation. The fl2 mutant exhibited increased PI content in protein body membranes at 18 d after pollination and more than 3-fold higher triacylglycerol accumulation in the endosperm by 36 d after pollination. Incorporation of radiolabeled acetate into phospholipids in soybean culture cells increased by about 30% with Tm treatment. The coordinated regulation of ER stress related proteins and multiple components of phospholipid biosynthesis is consistent with signaling through a common pathway. We postulate that the plant ER stress 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.Seed development requires coordination between formation of a pre-emergent embryo and synthesis of the major starch, lipid, and protein storage reserves. Regulation of storage reserve synthesis is complex because of a need to keep the individual components in their proper ratios for the development of mature, viable seeds that will desiccate and subsequently germinate properly. Little is known about how these processes are regulated or how seeds adapt to perturbations in the metabolic pathways essential for their growth and maturation. Because the endoplasmic reticulum (ER) is the site for processing of secretory proteins and formation of membrane and storage lipids, it is likely to play a key role in coordinating the metabolism of proteins and lipids during seed development.

The physiological role of the unfolded protein response in plants

“…Mouse embryo fi broblast culture cells transfected with the active form of XBP1 showed an enhanced activity of CCT (Sriburi et al, 2004). This result is in agreement with the fact that the CCT activity in protein bodies of fl oury-2 maize also increases when UPR is activated (Shank et al, 2001). Furthermore, Arabidopsis thaliana culture cells treated with salycilic acid leads to the upregulation of the ER stress markers BiP3 and PDI as well as the PI 4-kinase (Krinke et al, 2007), enzyme that is also activated in fl oury-2.…”

“…In fact, the synthesis of this mutated form of α-zein triggers ER stress since a high expression of the BiP gene can be observed both in fl oury-2 and transgenic maize that harbor the mutated form of this α-zein. But in addition to these early observations, an interesting phenomena was described by Shank et al (2001), who observed a modifi cation in the synthesis of lipids in the fl oury-2 maize due to the upregulation of four enzymes involved in lipids synthesis -diacylglycerol (DG) kinase, phosphatidylinositol (PI) 4-phosphate 5-kinase, cholinephosphate cytidylyltransferase (CCT) and PI 4-kinase-. In addition, they found a signifi cant increase in the incorporation of radiolabeled acetate into phospholipids of soybean culture cells treated with tunicamycin, suggesting that activation of the UPR is linked to an increase in phospholipid biosynthesis.…”

“…Houston et al (2005) showed that BiP and ZmPDIL5-1 proteins accumulate differentially in fl oury-2 maize following the onset of zein protein accumulation 15 days after pollination (DAP) and Shank et al (2001) reported an increase in CCT enzyme activity 16 days after pollination in this mutant. Unfortunately, wild-type maize shows no differential accumulation of BiP or ZmPDIL5-1 and CCT activity appears to drop off 10 to 16 days after pollination.…”

Evidence for and Characterization of Ca2+ Binding to the Catalytic Region of Arabidopsis thaliana Phospholipase Dβ

“…2 suggests that PLD␤ is active at resting Ca 2ϩ levels, this is unlikely to be the case in vivo, where PIP 2 levels are lower than those present in the substrate vesicles used for our in vitro activity assay (36). Our previous studies clearly demonstrate the concentration-dependent activation of PLD␤ by PIP or PIP 2 (3,4,16) and, like Ca 2ϩ , cellular levels of these signaling molecules increase in response to external stimuli and stress (37). In addition, although increased PIP 2 levels increase the binding of PLD␤cat to PC, PLD␤-C2 domain binding to PC is not stimulated by PIP 2 at any concentration (16).…”

Coordination of Endoplasmic Reticulum (ER) Signaling During Maize Seed Development

“…Lipid metabolism and ER stress We used both maize and soybean systems to investigate the ER stress response as it relates to phospholipid metabolism in plants (Shank et al, 2001). We made the key discovery that inducing a strong unfolded protein response (UPR) in the ER leads to an increase in triacylglycerol and phospholipid accumulation in endosperm, and a change in signaling kinases.…”