A Central Role for Triacylglycerol in Membrane Lipid Breakdown, Fatty Acid β-Oxidation, and Plant Survival under Extended Darkness

Abstract

ORCID IDs: 0000-0002-6821-6583 (J.F.); 0000-0002-0595-081X (L.Y.); 000-0002-0179-9462 (C.X.).Neutral lipid metabolism is a key aspect of intracellular homeostasis and energy balance and plays a vital role in cell survival under adverse conditions, including nutrient deprivation in yeast and mammals, but the role of triacylglycerol (TAG) metabolism in plant stress response remains largely unknown. By thoroughly characterizing mutants defective in SUGAR-DEPENDENT1 (SDP1) triacylglycerol lipase or PEROXISOMAL ABC TRANSPORTER 1 (PXA1), here we show that TAG is a key intermediate in the mobilization of fatty acids from membrane lipids for peroxisomal b-oxidation under prolonged dark treatment. Disruption of SDP1 increased TAG accumulation in cytosolic lipid droplets and markedly enhanced plant tolerance to extended darkness. We demonstrate that blocking TAG hydrolysis enhances plant tolerance to dark treatment via two distinct mechanisms. In pxa1 mutants, in which free fatty acids accumulated rapidly under extended darkness, SDP1 disruption resulted in a marked decrease in levels of cytotoxic lipid intermediates such as free fatty acids and phosphatidic acid, suggesting a buffer function of TAG accumulation against lipotoxicity under fatty acid overload. In the wild type, in which free fatty acids remained low and unchanged under dark treatment, disruption of SDP1 caused a decrease in reactive oxygen species production and hence the level of lipid peroxidation, indicating a role of TAG in protection against oxidative damage. Overall, our findings reveal a crucial role for TAG metabolism in membrane lipid breakdown, fatty acid turnover, and plant survival under extended darkness.Photosynthesis provides the energy and reduced carbon for metabolism, growth, storage, and maintenance throughout the daily cycle. During the day, light energy is used to fuel photosynthetic carbon assimilation to produce organic compounds. In many plants including Arabidopsis (Arabidopsis thaliana), the majority of the immediate stable products of photosynthesis (up to 80%) are used for the synthesis of sugars and starch (Smith and Stitt, 2007;Stitt and Zeeman, 2012). At night when photosynthesis is not possible, starch accumulated during the day is hydrolyzed to provide a steady sugar and energy supply. A small fraction (approximately 10%) of photosynthetic carbon is used for the synthesis of fatty acids in the chloroplast in the light (Murphy and Leech, 1981). The end products of fatty acid synthesis can be used to acylate glycerol-3-phosphate (G3P) by acyltransferases to produce phosphatidic acid (PA) in the chloroplast, or in the endoplasmic reticulum (ER) following their export from the chloroplast (Bates et al., 2013). Dephosphorylation of PA yields diacylglycerol (DAG) in the ER and the chloroplast. Because the substrate specificity of enzymes responsible for PA assembly in the two compartments differs, DAG formed in the chloroplast or the ER is characterized by the presence of 16-or 18-carbon fatty acids at the sn-2 position of glycer...

Dual Role for Autophagy in Lipid Metabolism in Arabidopsis

“…Lipids in membranous organelles are used as alternative substrates for energy production via b-oxidation of fatty acids in mitochondria in mammals (Shatz et al, 2016) and peroxisomes in yeast (Kohlwein, 2010) and plants (Graham, 2008) during times of nutrient scarcity. Emerging evidence suggests that rather than directly being used for b-oxidation, fatty acids released from cellular membranes are first stored in the form of triacylglycerol (TAG) in lipid droplets (LDs), and TAG in LDs is then hydrolyzed by a process named lipolysis to supply the cell with fatty acids for the generation of energy (Cabodevilla et al, 2013;Fan et al, 2014Fan et al, , 2017Rambold et al, 2015). In mammalian cells, autophagic digestion of membranous organelles is the major source of fatty acids for TAG synthesis and LD biogenesis under starvation conditions (Rambold et al, 2015;Nguyen et al, 2017).…”

“…Disruption of SDP1 blocks TAG hydrolysis in germinating seeds (Eastmond, 2006) and in vegetative tissues such as mature leaves and roots (Kelly et al, 2013;Fan et al, 2014), suggesting that cytosolic lipolysis plays a dominant role in TAG breakdown in both seed and nonseed tissues under normal growth conditions. Under extended darkness, TAG levels in leaves of sugar dependent1 (sdp1) mutants increased rapidly and then declined, suggesting an activation of unknown, alternative pathways for TAG hydrolysis under starvation conditions (Fan et al, 2017).…”

Single cell‐type analysis of cellular lipid remodelling in response to salinity in the epidermal bladder cells of the model halophyte Mesembryanthemum crystallinum

“…It has been suggested that these neutral glycerolipids are more than a carbon and energy reserve in plant storage tissues, being involved in vegetative tissues in cell division and expansion, stomatal opening, and membrane lipid remodelling (Yang & Benning, ). In plants, certain stress conditions have most commonly been linked with TG accumulation, as shown for heat stress, oxidative stress, and prolonged darkness (Higashi, Okazaki, Myouga, Shinozaki, & Saito, ; Fan, Yu, & Xu, ). Physiological functions of stress‐accumulated TG indicate their importance in storage of chemical energy and materials for membrane lipid biosynthesis and remodelling, transient sequestration of free fatty acid (FA), as well as for production of FA‐derived defensive compounds (Shimada, Hayashi, & Hara‐Nishimura, ; Yang & Benning, ), however, a role of TG in salt tolerance has remained understudied.…”

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“…The former is a more likely scenario given that dic2-1 phenotypes are observed mostly during darkness when fatty acid biosynthesis is ineligible (71), and the transcripts of two peroxisomal citrate synthase isoforms are more abundant at night (72, 73) and during prolonged darkness (74). Lowering fatty acid accumulation by limiting triacylglycerol depletion helps plants survive prolonged darkness by dampening the degree of lipid peroxidation (75). Thus, the increased susceptibility to dark-induced senescence by dic2-1 (Fig.…”