Arabidopsis Lipins, PDAT1 Acyltransferase, and SDP1 Triacylglycerol Lipase Synergistically Direct Fatty Acids toward β-Oxidation, Thereby Maintaining Membrane Lipid Homeostasis

Abstract

Triacylglycerol (TAG) metabolism is a key aspect of intracellular lipid homeostasis in yeast and mammals, but its role in vegetative tissues of plants remains poorly defined. We previously reported that PHOSPHOLIPID:DIACYLGLYCEROL ACYLTRANSFERASE1 (PDAT1) is crucial for diverting fatty acids (FAs) from membrane lipid synthesis to TAG and thereby protecting against FA-induced cell death in leaves. Here, we show that overexpression of PDAT1 enhances the turnover of FAs in leaf lipids. Using the trigalactosyldiacylglycerol1-1 (tgd1-1) mutant, which displays substantially enhanced PDAT1-mediated TAG synthesis, we demonstrate that disruption of SUGAR-DEPENDENT1 (SDP1) TAG lipase or PEROXISOMAL TRANSPORTER1 (PXA1) severely decreases FA turnover, leading to increases in leaf TAG accumulation, to 9% of dry weight, and in total leaf lipid, by 3-fold. The membrane lipid composition of tgd1-1 sdp1-4 and tgd1-1 pxa1-2 double mutants is altered, and their growth and development are compromised. We also show that two Arabidopsis thaliana lipin homologs provide most of the diacylglycerol for TAG synthesis and that loss of their functions markedly reduces TAG content, but with only minor impact on eukaryotic galactolipid synthesis. Collectively, these results show that Arabidopsis lipins, along with PDAT1 and SDP1, function synergistically in directing FAs toward peroxisomal b-oxidation via TAG intermediates, thereby maintaining membrane lipid homeostasis in leaves.

Arabidopsis TRIGALACTOSYLDIACYLGLYCEROL5 Interacts with TGD1, TGD2, and TGD4 to Facilitate Lipid Transfer from the Endoplasmic Reticulum to Plastids

“…Since 18:1 is the major fatty acid exported from the plastid (Ohlrogge and Jaworski, 1997), the majority of newly exported 18:1 is initially incorporated into PC through acyl editing (Bates et al, 2007(Bates et al, , 2009, and PC is the major site of 18:1 desaturation in the ER (Sperling and Heinz, 1993), a marked increase in fatty acid synthesis in tgd mutants, and thus in 18:1 flux through PC, may overwhelm the ER fatty acid desaturation machinery, leading to an increase in 18:1 accumulation in PC. Similarly, the increased TAG accumulation is at least in part due to the increased rate of fatty acid synthesis, which outpaces the rate of TAG turnover through the peroxisomal b-oxidation pathway that is limited by SDP1 as shown in the tgd1-1 mutant (Fan et al, 2014). Therefore, the majority of lipid phenotypes observed in tgd mutants likely result from the markedly increased fatty acid synthesis.…”

“…We recently demonstrated that TAG is a key intermediate in fatty acid turnover in leaves of tgd1-1 (Fan et al, 2014). To determine whether this is also the case in tgd5 mutants, we generated a double mutant between tgd5-3 and sdp1-4, a T-DNA insertion mutant disrupted in SUGAR-DEPENDENT1 (SDP1) TAG lipase responsible for initiating TAG breakdown during early seedling establishment (Eastmond, 2006).…”

“…The rationale behind this approach is based on the observations that (1) the tgd1 mutant shows a 4-fold increase in the rate of fatty acid synthesis in leaves (Fan et al, 2013a); (2) the increased fatty acid synthesis largely compensates for the defect in the eukaryotic lipid pathway so that the overall membrane lipid composition and plant growth remain largely unchanged (Xu et al, 2003); and (3) TAG is a key intermediate in fatty acid breakdown (Fan et al, 2014). Here, we show that knockout of TGD5 also results in marked increases in the rates of fatty acid synthesis and turnover and blocking TAG turnover by disrupting SDP1 in tgd5-3 boosts leaf TAG accumulation to a level comparable to that observed in tgd1-1 sdp1-4 (Fan et al, 2014). Importantly, unlike TGD1, which is essential for embryo development (Xu et al, 2005), knockout of TGD5 does not substantially affect seed development and has a less pronounced impact on plant vegetative growth than disruption of TGD1.…”

“…One approach to enhancing TAG accumulation in vegetative tissues is the combined disruption of TGD1 and SDP1 (Fan et al, 2014). The rationale behind this approach is based on the observations that (1) the tgd1 mutant shows a 4-fold increase in the rate of fatty acid synthesis in leaves (Fan et al, 2013a); (2) the increased fatty acid synthesis largely compensates for the defect in the eukaryotic lipid pathway so that the overall membrane lipid composition and plant growth remain largely unchanged (Xu et al, 2003); and (3) TAG is a key intermediate in fatty acid breakdown (Fan et al, 2014). Here, we show that knockout of TGD5 also results in marked increases in the rates of fatty acid synthesis and turnover and blocking TAG turnover by disrupting SDP1 in tgd5-3 boosts leaf TAG accumulation to a level comparable to that observed in tgd1-1 sdp1-4 (Fan et al, 2014).…”

Lipids in Leaves: Fatty Acid β-Oxidation Affects Lipid Homeostasis

“…To examine this, Fan et al (2014) used trigalactosyl diacylglycerol1-1 (tgd1) mutants, in which a defect in thylakoid lipid synthesis diverts fatty acids, causing accumulation of triacylglycerol in leaves. They found that overexpression of PHOSPHOLIPID: DIACYLGLYCEROL ACYLTRANSFERASE1 (PDAT1) causes increased fatty acid synthesis and turnover, and this requires the lipase SUGAR-DEPENDENT1 (SDP1), as PDAT1-overexpressing sdp1 plants produce high levels of triacylglycerol.…”