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...

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