A Role for Diacylglycerol Acyltransferase during Leaf Senescence

Abstract

Lipid analysis of rosette leaves from Arabidopsis has revealed an accumulation of triacylglycerol (TAG) with advancing leaf senescence coincident with an increase in the abundance and size of plastoglobuli. The terminal step in the biosynthesis of TAG in Arabidopsis is catalyzed by diacylglycerol acyltransferase 1 (DGAT1; EC 2.3.1.20). When gel blots of RNA isolated from rosette leaves at various stages of development were probed with the Arabidopsis expressed sequence tag clone, E6B2T7, which has been annotated as DGAT1, a steep increase in DGAT1 transcript levels was evident in the senescing leaves coincident with the accumulation of TAG. The increase in DGAT1 transcript correlated temporally with enhanced levels of DGAT1 protein detected immunologically. Two lines of evidence indicated that the TAG of senescing leaves is synthesized in chloroplasts and sequesters fatty acids released from the catabolism of thylakoid galactolipids. First, TAG isolated from senescing leaves proved to be enriched in hexadecatrienoic acid (16:3) and linolenic acid (18:3), which are normally present in thylakoid galactolipids. Second, DGAT1 protein in senescing leaves was found to be associated with chloroplast membranes. These findings collectively indicate that diacylglycerol acyltransferase plays a role in senescence by sequestering fatty acids de-esterified from galactolipids into TAG. This would appear to be an intermediate step in the conversion of thylakoid fatty acids to phloem-mobile sucrose during leaf senescence.Diacylglycerol (DAG) acyltransferase (DGAT; EC 2.3.1.20) mediates the final acylation step in the synthesis of triacylglycerol (TAG). It is present in most plant organs, including leaves, petals, fruits, anthers, and developing seeds (Hobbs et al., 1999). In seeds, TAG is thought to be synthesized within the membranes of the endoplasmic reticulum and subsequently released into the cytosol in the form of oil bodies, which bleb from the cytoplasmic surface of the endoplasmic reticulum (Huang, 1992). The stored TAG is localized in the interior of the oil body, and the surfaces of oil bodies are coated with a monolayer of phospholipid associated with oleosin, the major protein of oil bodies. The acyl chains of the phospholipid monolayer are embedded in the TAG interior of the oil body. Oleosin is a structural protein that is thought to prevent coalescence of oil bodies during seed dehydration (Huang, 1996). That oil bodies originate from the endoplasmic reticulum is consistent with the finding that enzymes of TAG synthesis, including DGAT, are present in microsomal membrane fractions, which are known to contain vesicles of endoplasmic reticulum (Kwanyuen and Wilson, 1986). In addition, TAG can be synthesized in vitro in the presence of microsomes isolated from developing seeds (Lacey et al., 1999).Although TAG formation in seeds is believed to occur in the ER, there have been several reports indicating that purified chloroplast envelope membranes from leaves are also capable of synthesizing this storage lipid (Siebe...

A mechanism implicating plastoglobules in thylakoid disassembly during senescence and nitrogen starvation

“…The implication of PG in the storage of catabolic molecules produced during senescence and nitrogen Planta (2013) 237:463-470 465 starvation has been described previously (Kaup et al 2002;Gaude et al 2007). However, the characterization of two PG acyl transferases has led to an advance in mechanistic understanding of the accumulation of fatty acid phytyl esters and triacylglycerols (Fig.…”

“…In contrary, the amount of TAGs in the mutant leaves is only 30 % lower, certainly due to the implication of other diacylglycerol acyltransferase enzymes such as DGAT1 (Zou et al 1999;Dahlqvist et al 2000;Lardizabal et al 2001;Kaup et al 2002). Moreover, in the updated and quantitative plastoglobule proteome study published recently by Lundquist and colleagues, in addition to PES1 and 2, a third esterase (At5g41120) was identified that may also participate in the synthesis of FAPEs and TAGs in plastoglobules (Lundquist et al 2012b).…”

“…These hydrolytic products are considered toxic and can be metabolized to form fatty acid phytyl esters, tocopherols and triacylglycerols. All three of these accumulate in plastoglobules during senescence and nitrogen starvation (Kaup et al 2002;Ischebeck et al 2006;Vidi et al 2006;Gaude et al 2007). …”

“…Conditions provoking oxidative stress such as high light, nitrate starvation, drought, high saline concentration, viral infection, chilling and ozone (Nordby and Yelenosky 1985;Locy et al 1996;Rey et al 2000;Oksanen et al 2001;Gaude et al 2007;Lichtenthaler 2007) or developmental stages such as senescence and fruit development (Kaup et al 2002) result in the dismantling of thylakoid membrane. In parallel, the number and the size of lipid droplets increase, due to the accumulation of lipids in their hydrophobic core.…”

Influence of damaging and wilting red clover on lipid metabolism during ensiling and in vitro rumen incubation

“…Second, diacylglycerol acyltransferase (DGAT) activity can result in an increase in TAG. The DGAT is responsible for the incorporation of FFA in mono-and digalactosyl glycerides resulting in TAG (Kaup et al, 2002;Lung and Weselake, 2006). The activity of DGAT has been reported before in silages Van Ranst et al, 2009a).…”