大葱卵器及受精后助细胞的超微结构

Ultrastructure of the Egg Apparatus and the Synergids after Fertilization in Allium fistulosum

章丘大葱(Allium fistulosum L. cv. Zhangqiu )的卵器由1个卵细胞及2个助细胞组成。观察到不少卵器没有卵细胞,只有2个助细胞。卵细胞的核及大部分细胞质位于细胞的合点端,1个大液泡占据了细胞其它部位。卵细胞含有很多的核糖体及多聚核糖体、嵴明显的线粒体、粗面内质网,高尔基体具小泡。卵细胞似是一个活跃的细胞。细胞外被细胞壁,其合点端及侧方与助细胞共同壁不连续。助细胞有一较大的核,位于细胞膨大的部位,众多的小液泡遍布细胞质中。核糖体及聚合核糖体、线粒体、粗面内质网及同心圆环状粗面内质网丰富,高尔基体及小泡常见,反映了其活跃的代谢作用。助细胞合点端及侧方与卵细胞、中央细胞的共同壁不连续。与卵细胞共同壁含胞间连丝,壁不连续处,有环状多层膜结构伸入卵细胞质,显示助细胞可能对卵细胞提供营养。传粉后,一个助细胞退化,宿存助细胞至椭形胚期尚存在。它经历了一个缓慢的退化过程 ,出现质壁分离,细胞质变稀,液泡扩大,细胞器逐渐减少。在椭形胚期,宿存助细胞核内的染色质及核仁消失,有细胞质侵入核内。因宿存助细胞壁变厚,细胞质内出现脂滴,宿存助细胞可能仍有合成功能。宿存助细胞壁出现若干无壁部位,细胞内的营养物质可能通过无壁部位向胚乳转运,供游离核胚乳及胚乳细胞化初期的发?

The egg apparatus in Allium fistulosum cv. Zhangqiu consists of an eg g and two synergids. A few apparatus observed lacked egg cells but, two had syn ergids. The egg cell had a nucleus with much of the cytoplasm at the chalazal en ds and a large vacuole occupying the other area of the cell. The egg cell conta ined many ribosomes , polysomes, mitochondria with distinct cristae, RER. Dictyo somes had vesicles. The egg cell appeared to be an active cell. The cell wall su rrounds the cell, but its chalzal ends and the lateral wall with synergid is dis continuous. The synergid possesses a larger nucleus located in the expanded port ion of the cell and many small vacuoles throughout the cytoplasm. Ribosomes, pol ysomes, mitochondria, RER and concentric ring- shaped RER are abundant and dict yosomes with vesicles are frequently seen, reflecting its active metabolism. Its wall has discontinuous sites at the chalazal end and side wall sharing with the egg cell and the central cell. Plasmodesmata are present in the wall between th e egg and synergid and the circulate multimembranes at discontinuous sites exten d to the egg cytoplasm. The synergid may contribute to egg cell nutrition. After pollination one synergid degenerates and one synergid still exists during the ellipsoidal embryo stage and undergoes a slow degeneration. Then plasmolysis occ urs, cytoplsm becomes thin, vacuoles expand and the amount of organelles decre ases gradually. At ellipsoidal embryo stage, the chromatin and nucleoli disappea r and some cytoplasm enters the nucleus. The persistent synergid may still have a synthetic function as its cell wall becomes thick and lipids occur. Several s ites without cell wall appear in persistent synergid, suggesting that the nutrit ion in the persistent synergid may transport through these no wall sites to the endosperm supplying the development of free nuclear endosperm and early endosper m cellularization.