Shimada Takashi L, Shimada Tomoo, Takahashi Hideyuki, Fukao Yoichiro, Hara-Nishimura Ikuko
Department of Botany, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Plant J. 2008 Sep;55(5):798-809. doi: 10.1111/j.1365-313X.2008.03553.x. Epub 2008 May 14.
Oil bodies in seeds of higher plants are surrounded with oleosins. Here we demonstrate a novel role for oleosins in protecting oilseeds against freeze/thaw-induced damage of their cells. We detected four oleosins in oil bodies isolated from seeds of Arabidopsis thaliana, and designated them OLE1, OLE2, OLE3 and OLE4 in decreasing order of abundance in the seeds. For reverse genetics, we isolated oleosin-deficient mutants (ole1, ole2, ole3 and ole4) and generated three double mutants (ole1 ole2, ole1 ole3 and ole2 ole3). Electron microscopy showed an inverse relationship between oil body sizes and total oleosin levels. The double mutant ole1 ole2, which had the lowest levels of oleosins, had irregular enlarged oil-containing structures throughout the seed cells. Germination rates were positively associated with oleosin levels, suggesting that defects in germination are related to the expansion of oil bodies due to oleosin deficiency. We found that freezing followed by imbibition at 4 degrees C abolished seed germination of single mutants (ole1, ole2 and ole3), which germinated normally without freezing treatment. The treatment accelerated the fusion of oil bodies and the abnormal-positioning and deformation of nuclei in ole1 seeds, which caused seed mortality. In contrast, ole1 seeds that had undergone freezing treatment germinated normally when incubated at 22 degrees C instead of 4 degrees C, because degradation of oils abolished the acceleration of fusion of oil bodies during imbibition. Taken together, our findings suggest that oleosins increase the viability of over-wintering oilseeds by preventing abnormal fusion of oil bodies during imbibition in the spring.
高等植物种子中的油体被油质蛋白所包围。在此，我们展示了油质蛋白在保护油籽免受冻融诱导的细胞损伤方面的新作用。我们从拟南芥种子中分离出的油体中检测到四种油质蛋白，并按照它们在种子中的丰度从高到低依次将其命名为OLE1、OLE2、OLE3和OLE4。为了进行反向遗传学研究，我们分离出了油质蛋白缺陷型突变体（ole1、ole2、ole3和ole4），并构建了三个双突变体（ole1 ole2、ole1 ole3和ole2 ole3）。电子显微镜观察显示油体大小与总油质蛋白水平呈负相关。油质蛋白水平最低的双突变体ole1 ole2在整个种子细胞中具有不规则增大的含油结构。发芽率与油质蛋白水平呈正相关，这表明发芽缺陷与由于油质蛋白缺乏导致的油体膨胀有关。我们发现，在4摄氏度下冷冻后再进行吸胀处理会使单突变体（ole1、ole2和ole3）的种子萌发受阻，而这些单突变体在未经冷冻处理时能够正常萌发。这种处理加速了ole1种子中油体的融合以及细胞核的异位和变形，从而导致种子死亡。相比之下，经过冷冻处理的ole1种子在22摄氏度而非4摄氏度下培养时能够正常萌发，因为油脂的降解消除了吸胀过程中油体融合的加速现象。综上所述，我们的研究结果表明，油质蛋白通过防止春季吸胀过程中油体的异常融合来提高越冬油籽的活力。
