栽培甜菜雌配子体发育中的超微结构

Ultrastructure of Megagametophyte during the Development Period in Sugar Beet(Beta vulgaris L.)

应用透射电镜技术研究栽培甜菜(Beta vulgaris L.)雌配子体发育过程的超微结构特征,丰富被子植物生殖生物学方面的资料,并为甜菜相关研究提供借鉴。观察结果表明,栽培甜菜雌配子体发育类型为蓼型。功能大孢子时期核糖体密集,线粒体和质体分裂以增加数目;单核胚囊时期,细胞体积增大,小液泡融合为2个大液泡,分别位于珠孔端和合点端,细胞核增大,核仁显著,至发育后期核膜呈微裂齿状,细胞器数量不断增加,珠孔端形成明显的壁内突;二核胚囊时期,胚囊迅速扩张,形成中央大液泡,胞质中细胞器增多;四核胚囊时期除细胞器的数量继续增加外,质体中开始出现淀粉粒;八核胚囊时期相当短暂,很快细胞化,初期的七细胞八核胚囊中各细胞均以初生壁相隔,壁上存在胞间连丝,沿细胞壁分布着众多伸展状粗面内质网和活跃分泌小泡的高尔基体;细胞化后期,卵、助细胞出现极性分化,胞质中的众多小泡与细胞膜融合,参与细胞壁及丝状器的建成;中央细胞在珠孔端与胚囊壁相邻的部位形成壁内突。栽培甜菜雌配子体发育进程中胚囊体积不断增大,细胞器种类与数量呈增加趋势,表明各时期代谢较活跃。在功能大孢子及单核胚囊早期是通过合点端的胞间连丝与珠心细胞以共质体形式相通;在单核胚囊发育后期至细胞化胚囊,是通过珠孔端所形成的壁内突来扩展质膜表面积,以加强非共质体之间物质的运输。

The experiment was conducted by using TEM in order to provide more information for reproductive biology of angiosperm and relative research on sugar beet. The results were as follows： the megagametophyte development type is Polygonum-type. Functional megaspore was rich in ribosomes, and mitochondrium and plastid were increased in number by division. Embryo sac elongated at 1-nucleate stage by fusion of small vacuoles. Nucleus with obvious nucleolus and carnassial nuclear membrane got larger at later stage. Organelle numbers increased and wall ingrowths occurred at the micropylar end. Embryo sac expanded rapidly at 2-nucleate stage due to the formation of central vacuole and the increase of organelles. Starch grains occurred and organelles increased continually at 4-nucleate stage. Eight-nucleate stage was so short that cellularization took place rapidly to form a 7 cell-8 nuclei embryo sac. At early stage of cellularization, all cells were separated by primary wall with many plasmodesmata in it. Abundant rough endoplasmic reticulum stretched along the wall with Golgi bodies and vesicles near it. At late stage of cellularization, egg and synergids were polarized, large amount of vesicles fused with membrane to join the formation of cell wall and filiform apparatus. Wall ingrowths occurred in central cell at micropylar region near embryo sac wall. In conclusion, embryo sac enlarges gradually with the increase of organelles during megagametophyte development in sugar beet. All the ultrastructural characteristics suggest there is a high level of metabolism in megagametophyte development. Plasmodesmata in the chalazal end wall as a site for symplastic transportation between nucellar cells and embryo sac only appear in functional megaspore and 1-nucleate embryo sac at early stage. Ingrowths at the micropylar end of embryo sac from late stage of 1-nucleate to cellularization enlarge the surface of membrane greatly so as to enhance non-symplastic transportation.