Department of Botany, Graduate School of Science, Kyoto University, Japan.
Biol Pharm Bull. 2010;33(3):360-3. doi: 10.1248/bpb.33.360.
Oilseeds accumulate a large amount of storage lipids, which are used as sources of carbon and energy for seed germination and seedling growth. The storage lipids are accumulated in oil bodies during seed maturation. Oil bodies in seeds are surrounded with three oil-body-membrane protein families, oleosins, caleosins and steroleosins. These proteins are plant-specific and much abundant in seeds. Here we show a unique function of oleosins in preventing fusion of oil bodies and maintaining seed germination. Reverse genetic analysis using oleosin-deficient mutants shows the inverse proportion of oil-body sizes to total oleosin contents. The double mutant ole1 ole2 with the lowest levels of oleosins has irregularly-enlarged oil bodies throughout the seed cells, and hardly germinates. Germination rates are positively associated with oleosin contents, suggesting that the defects of germination are related to the expansion of oil bodies due to oleosin deficiency. Interestingly, freezing treatment followed by imbibition at 4 degrees C inhibits seed germination of single mutants (ole1 and ole2), which germinate normally without freezing treatment. The freezing treatment accelerates the fusion of oil bodies and generates eccentric nuclei in ole1 seeds, which caused seed mortality. Taken together, our findings suggest that oleosins increase the viability of oilseeds by preventing abnormal fusion of oil bodies for overwintering. Knowledge of oleosin contributes a great deal to not only an insight into freezing tolerance of oilseeds, but also creating genetically modified plants for developing a bioenergy and biomass resource.
油籽积累了大量的储存脂质，这些脂质可作为种子萌发和幼苗生长的碳和能量来源。储存脂质在种子成熟过程中积累在油体中。种子中的油体被三种油体膜蛋白家族包围，即油体蛋白、钙调蛋白和甾醇蛋白。这些蛋白质是植物特有的，在种子中含量丰富。在这里，我们展示了油体蛋白在防止油体融合和维持种子萌发方面的独特功能。利用油体蛋白缺失突变体的反向遗传学分析表明，油体大小与总油体蛋白含量呈反比。具有最低油体蛋白含量的 ole1 ole2 双突变体，其油体在整个种子细胞中大小不规则且增大，几乎不萌发。萌发率与油体蛋白含量呈正相关，表明由于油体蛋白缺失导致油体扩张，萌发缺陷与此有关。有趣的是，在 4°C 下进行冷冻处理然后再吸胀可抑制单突变体（ole1 和 ole2）的种子萌发，而这些突变体在没有冷冻处理的情况下可正常萌发。冷冻处理加速了油体的融合，并在 ole1 种子中产生偏心核，导致种子死亡。总之，我们的研究结果表明，油体蛋白通过防止油体异常融合来提高油籽的活力，以适应越冬。对油体蛋白的了解不仅有助于深入了解油籽的抗冻性，还有助于培育用于开发生物能源和生物质资源的转基因植物。
