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作 者 ：Yan Hua and Yang Hong-yuan

Keywords:Helianthus annuus L., Ovule culture, Parthenogenesis, Ultrastructure,

Abstract:This is the first report of ultrastructural studies on the parthenogenetic processes induced by in vitro culture of unfertilized ovules. A high responsive sunflower cultivar “Dang Yang” was used as material. Ovules at one day before anthesis were inoculated on solid N6 medium supplemented with 12% sucrose, 100 mg/l inositol but free from exogenous hormone. The culture was maintained at 28℃ and in darkness. Samples were collected prior to inoculation and after 5 and 10 (lays of culture respectively. The chalazal part of the ovules was cut off and the micropylar part containing embryo sac was fixed with glutaraldehyde and osmium tetroxide. After dehydration through ethanol and propylene oxide series, the samples were infiltrated and embedded in Poly/Bed 812. Ultrathin sections were made with a Sorval MT-6000 ultratome, stained with uranyl acotate and lead citrate, and observed under a JEM-100 CX/II electron microscope. By careful selection of the sections, we were able to find several activated egg cells and parthenogenetic proembryos. The activated egg cells, as compared with the egg prior to inoculation, showed profound ultrastructural changes. These included locomotion of the nucleus from its former position near the chalazal pole toward the center of the cell, loss of polarity, increase and activation of organelles, wall formation at chalazal part of the cell and so on, indicating a triggering to parthenogenetic development in such cells. The consequent parthenogenetic proembryos showed some peculiar features other then in vivo zygotic proembryos: In some proembryos, two parts with different electronic density were found; the micropylar part consisting of dense cells was similar to an embryo, and the chalazal part consisting of light cells bore resemblance to a suspensor. This was likely an inversion of polarity. Another feature of the parthenogenetic proembryos was frequent occurrence of autophagic vacuoles in their cells, which might be a degeneration symptom, or alternatively a self-regulatory activity or a continuation of the dedifferentiation process. There were also some peculiar aspects concerning the cell wall of proembryos, including a free growth mode often resulting in incomplete wall formation. The authors tend to refer the above mentioned ultrastructural peculiarities to the effects of in vitro culture.

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