Membrane glycerolipid equilibrium under endoplasmic reticulum stress in Arabidopsis thaliana

Non-specific phospholipase C3 is involved in endoplasmic reticulum stress tolerance in Arabidopsis

Non-specific phospholipase C (NPC) is an emerging family of lipolytic enzymes unique to plants and bacteria that play crucial roles in growth and stress responses. Among six copies of NPC isoforms found in Arabidopsis, the role of NPC3 remains elusive to date. Here, we show that NPC3 is a functional non-specific phospholipase C involved in tolerance to tunicamycin (TM)-induced endoplasmic reticulum (ER) stress through the synthesis of phosphocholine (PCho), a reaction product of NPC3. The npc3 mutant exhibited reduced sensitivity to TM treatment. Recombinant NPC3 possessed pronounced phospholipase C activity that hydrolyses phosphatidylcholine (PC). The hyposensitivity of npc3 to TM treatment was complemented by exogenous PCho, suggesting that NPC3-catalysed PCho production is involved in TM-induced ER stress tolerance. NPC3 was localized at the ER and was predominantly expressed in the roots, and it was further induced by TM-induced ER stress. Intriguingly, npc3 mutants showed a markedly reduced PCho content in shoots under ER stress. Our results indicate that ER stress induces NPC3 to produce PCho, which is involved in TM-induced ER stress tolerance.

Sterol Biosynthesis Contributes to Brefeldin-A-Induced Endoplasmic Reticulum Stress Resistance in Chlamydomonas reinhardtii

The endoplasmic reticulum (ER) stress response is an evolutionarily conserved mechanism in most eukaryotes. In this response, sterols in the phospholipid bilayer play a crucial role in controlling membrane fluidity and homeostasis. Despite the significance of both the ER stress response and sterols in maintaining ER homeostasis, their relationship remains poorly explored. Our investigation focused on Chlamydomonas strain CC-4533 and revealed that free sterol biosynthesis increased in response to ER stress, except in mutants of the ER stress sensor Inositol-requiring enzyme 1 (IRE1). Transcript analysis of Chlamydomonas experiencing ER stress unveiled the regulatory role of the IRE1/basic leucine zipper 1 pathway in inducing the expression of ERG5, which encodes C-22 sterol desaturase. Through the isolation of three erg5 mutant alleles, we observed a defect in the synthesis of Chlamydomonas' sterol end products, ergosterol and 7-dehydroporiferasterol. Furthermore, these erg5 mutants also exhibited increased sensitivity to ER stress induced by brefeldin A (BFA, an inhibitor of ER-Golgi trafficking), whereas tunicamycin (an inhibitor of N-glycosylation) and dithiothreitol (an inhibitor of disulfide-bond formation) had no such effect. Intriguingly, the sterol biosynthesis inhibitors fenpropimorph and fenhexamid, which impede steps upstream of the ERG5 enzyme in sterol biosynthesis, rescued BFA hypersensitivity in CC-4533 cells. Collectively, our findings support the conclusion that the accumulation of intermediates in the sterol biosynthetic pathway influences ER stress in a complex manner. This study highlights the significance and complexity of regulating sterol biosynthesis during the ER stress response in microalgae.

Uptake and Metabolic Conversion of Exogenous Phosphatidylcholines Depending on Their Acyl Chain Structure in Arabidopsis thaliana

Fungi and plants are not only capable of synthesizing the entire spectrum of lipids de novo but also possess a well-developed system that allows them to assimilate exogenous lipids. However, the role of structure in the ability of lipids to be absorbed and metabolized has not yet been characterized in detail. In the present work, targeted lipidomics of phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs), in parallel with morphological phenotyping, allowed for the identification of differences in the effects of PC molecular species introduced into the growth medium, in particular, typical bacterial saturated (14:0/14:0, 16:0/16:0), monounsaturated (16:0/18:1), and typical for fungi and plants polyunsaturated (16:0/18:2, 18:2/18:2) species, on Arabidopsis thaliana. For comparison, the influence of an artificially synthesized (1,2-di-(3-(3-hexylcyclopentyl)-propanoate)-sn-glycero-3-phosphatidylcholine, which is close in structure to archaeal lipids, was studied. The phenotype deviations stimulated by exogenous lipids included changes in the length and morphology of both the roots and leaves of seedlings. According to lipidomics data, the main trends in response to exogenous lipid exposure were an increase in the proportion of endogenic 18:1/18:1 PC and 18:1_18:2 PC molecular species and a decrease in the relative content of species with C18:3, such as 18:3/18:3 PC and/or 16:0_18:3 PC, 16:1_18:3 PE. The obtained data indicate that exogenous lipid molecules affect plant morphology not only due to their physical properties, which are manifested during incorporation into the membrane, but also due to the participation of exogenous lipid molecules in the metabolism of plant cells. The results obtained open the way to the use of PCs of different structures as cellular regulators.

Effect of Common ER Stress-Inducing Drugs on the Growth and Lipid Phenotypes of Chlamydomonas and Arabidopsis

Endoplasmic reticulum (ER) stress is caused by the stress-induced accumulation of unfolded proteins in the ER. Several compounds are used to induce the unfolded protein response (UPR) in animals, with different modes of action, but which ER stress-inducing drugs induce ER stress in microalgae or land plants is unclear. In this study, we examined the effects of seven chemicals that were reported to induce ER stress in animals on the growth, UPR gene expression and fatty acid profiles of Chlamydomonas reinhardtii (Chlamydomonas) and Arabidopsis thaliana (Arabidopsis): 2-deoxyglucose, dithiothreitol (DTT), tunicamycin (TM), thapsigargin, brefeldin A (BFA), monensin (MON) and eeyarestatin I. In both model photosynthetic organisms, DTT, TM, BFA and MON treatment induced ER stress, as indicated by the induction of spliced bZIP1 and bZIP60, respectively. In Chlamydomonas, DTT, TM and BFA treatment induced the production of transcripts related to lipid biosynthesis, but MON treatment did not. In Arabidopsis, DTT, TM, BFA and MON inhibited seed germination and seedling growth with the activation of bZIP60. These findings lay the foundation for using four types of ER stress-inducing drugs in photosynthetic organisms, and they help uncover the mode of action of each compound.

A lipid viewpoint on the plant endoplasmic reticulum stress response

Organisms, including humans, seem to be constantly exposed to various changes, which often have undesirable effects, referred to as stress. To keep up with these changes, eukaryotic cells may have evolved a number of relevant cellular processes, such as the endoplasmic reticulum (ER) stress response. Owing to presumably intimate links between human diseases and the ER function, the ER stress response has been extensively investigated in various organisms for a few decades. Based on these studies, we now have a picture of the molecular mechanisms of the ER stress response, one of which, the unfolded protein response (UPR), is highly conserved among yeasts, mammals, higher plants, and green algae. In this review, we attempt to highlight the plant UPR from the perspective of lipids, especially membrane phospholipids. Phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) are the most abundant membrane phospholipids in eukaryotic cells. The ratio of PtdCho to PtdEtn and the unsaturation of fatty acyl tails in both phospholipids may be critical factors for the UPR, but the pathways responsible for PtdCho and PtdEtn biosynthesis are distinct in animals and plants. We discuss the plant UPR in comparison with the system in yeasts and animals in the context of membrane phospholipids.

Phospholipids in Salt Stress Response

High salinity threatens crop production by harming plants and interfering with their development. Plant cells respond to salt stress in various ways, all of which involve multiple components such as proteins, peptides, lipids, sugars, and phytohormones. Phospholipids, important components of bio-membranes, are small amphoteric molecular compounds. These have attracted significant attention in recent years due to the regulatory effect they have on cellular activity. Over the past few decades, genetic and biochemical analyses have partly revealed that phospholipids regulate salt stress response by participating in salt stress signal transduction. In this review, we summarize the generation and metabolism of phospholipid phosphatidic acid (PA), phosphoinositides (PIs), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), as well as the regulatory role each phospholipid plays in the salt stress response. We also discuss the possible regulatory role based on how they act during other cellular activities.

How Lipids Contribute to Autophagosome Biogenesis, a Critical Process in Plant Responses to Stresses

Throughout their life cycle, plants face a tremendous number of environmental and developmental stresses. To respond to these different constraints, they have developed a set of refined intracellular systems including autophagy. This pathway, highly conserved among eukaryotes, is induced by a wide range of biotic and abiotic stresses upon which it mediates the degradation and recycling of cytoplasmic material. Central to autophagy is the formation of highly specialized double membrane vesicles called autophagosomes which select, engulf, and traffic cargo to the lytic vacuole for degradation. The biogenesis of these structures requires a series of membrane remodeling events during which both the quantity and quality of lipids are critical to sustain autophagy activity. This review highlights our knowledge, and raises current questions, regarding the mechanism of autophagy, and its induction and regulation upon environmental stresses with a particular focus on the fundamental contribution of lipids. How autophagy regulates metabolism and the recycling of resources, including lipids, to promote plant acclimation and resistance to stresses is further discussed.

An Overview of Biomembrane Functions in Plant Responses to High-Temperature Stress

Biological membranes are highly ordered structures consisting of mosaics of lipids and proteins. Elevated temperatures can directly and effectively change the properties of these membranes, including their fluidity and permeability, through a holistic effect that involves changes in the lipid composition and/or interactions between lipids and specific membrane proteins. Ultimately, high temperatures can alter microdomain remodeling and instantaneously relay ambient cues to downstream signaling pathways. Thus, dynamic membrane regulation not only helps cells perceive temperature changes but also participates in intracellular responses and determines a cell's fate. Moreover, due to the specific distribution of extra- and endomembrane elements, the plasma membrane (PM) and membranous organelles are individually responsible for distinct developmental events during plant adaptation to heat stress. This review describes recent studies that focused on the roles of various components that can alter the physical state of the plasma and thylakoid membranes as well as the crucial signaling pathways initiated through the membrane system, encompassing both endomembranes and membranous organelles in the context of heat stress responses.