Kaup Marianne T, Froese Carol D, Thompson John E
Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
Plant Physiol. 2002 Aug;129(4):1616-26. doi: 10.1104/pp.003087.
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.
对拟南芥莲座叶的脂质分析显示，随着叶片衰老的推进，三酰甘油（TAG）会积累，同时质体小球的丰度和大小也会增加。拟南芥中TAG生物合成的最后一步由二酰甘油酰基转移酶1（DGAT1；EC 2.3.1.20）催化。当用已注释为DGAT1的拟南芥表达序列标签克隆E6B2T7对从不同发育阶段的莲座叶中分离的RNA进行凝胶印迹杂交时，衰老叶片中DGAT1转录本水平急剧增加，这与TAG的积累同时出现。DGAT1转录本的增加在时间上与通过免疫检测到的DGAT1蛋白水平的提高相关。两条证据表明，衰老叶片中的TAG是在叶绿体中合成的，并隔离了从类囊体半乳糖脂分解代谢中释放的脂肪酸。首先，从衰老叶片中分离出的TAG被证明富含十六碳三烯酸（16:3）和亚麻酸（18:3），这些通常存在于类囊体半乳糖脂中。其次，发现衰老叶片中的DGAT1蛋白与叶绿体膜相关。这些发现共同表明，二酰甘油酰基转移酶通过将从半乳糖脂中去酯化的脂肪酸隔离到TAG中，在衰老过程中发挥作用。这似乎是叶片衰老期间类囊体脂肪酸向韧皮部可移动蔗糖转化的中间步骤。
