香根草体细胞胚胎发生的细胞学特点与形成条件

香根草是一种优良的生态环境治理植物，但也存在着一些局限性。为了对香根草进行遗传改良，选育出性状更优、抗性更强的新品种，开展了香根草离体培养研究。离体培养采用了两种外植体，一是带腋芽的节，二是由器官发生方式所产生的无菌不定芽。基本培养基为MS，根据不同的目的附加不同种类或配比的生长素与细胞分裂素。观察到香根草的外植体的离体发育途径，有器官发生和体细胞胚胎发生两种，依培养基中所含细胞分裂素或生长素的种类和用量不同而异。结果表明，香根草的这两种离体发育途径的植株再生能力均可以长期保持。细胞学的研究显示，香根草离体发育的启动可在外植体的表皮细胞或薄壁细胞中进行，这些细胞逐渐发育成为胚性细胞。胚性细胞分裂活跃，经二细胞、四细胞而发育成为多细胞的胚性细胞团。由显微观察可知，香根草的体细胞胚胎发生是单细胞起源的，成熟的体细胞胚具有单子叶植物典型的胚胎结构。在分化培养基的作用下，体细胞胚组织上所有的胚状体可以出芽而形成再生植株。所建立的香根草体细胞胚胎发生的植株再生体系，完全适用于遗传转化等生物工程方法对离体培养要求。此外，还观察到一些一般只有在双子叶植物才出现的鱼雷形体细胞胚，这是体细胞胚胎发生中的异常现象。认为这种异常胚是离体培养所引起的。

Cytology observation and formation conditions of somatic embryog-enesis in Vetiveria zizaniodes

Vetiveria zizanioides, a perennial belonging to the grass family, is an excellent plant in the aspects of erosion control, polluted environment mitigation, and ecological restoration. However, the species itself has some shortcomings, such as too high height, and weak resistance to cold. Two kinds of explants were used in this experiment for in vitro culture of V. zizanioides. One was the node with axillary buds of the plant from the field and the other was the aseptic adventitious buds from organogenesis of cultured materials in test tube. We found that high quality embryogenic calli (E-calli) could be induced from small pieces of initiating adventitious buds from organogenesis because they possessed a lot of meristem using the basic MS medium supplemented with different kind or/and ratio of auxin (2,4-D, NAA, IBA and so on) or cytokinin (6BA, kinetin and so on), according to different culture purposes. The two kinds of in vitro developing ways in V. zizanioides were observed in this experiment. One was organogenesis and the other, somatic embryogenesis that was dependent upon the kind and dosage of auxin or cytokinin in the medium. When the medium contained 6BA 5 mg L -1 and IBA 0.2 mg L -1, the adventitious-buds clusters were directly induced from the explants without going through the callus stage. When the medium contained 2,4-D 2 mg L -1 without or with very low concentration cytokinin, E-callus was formed from the explants and than the E-callus differentiated to a large numbers of plantlets in the regenerated medium. The results showed that the differentiated ability of the two ways of regeneration in V. zizanioides could be kept for a long duration. The results of cytology observation proved that the initiation action of in vitro development in V. zizanioides started in epidermal and parenchyma cells of the explants. With auxin/cytokinin in the medium, the explants became swollen at the beginning stage of inoculation. Under the microscope, various embryonic cells from single cell to multiple cell clusters could be clearly observed. Each embryogenic cell possessed a thicken cytoplasm, large and big darkly stained nucleolus. The division of the embryogenic cells was very active and in a same single scope under microscope, different stages of embryogenic cells, from single cell, two-four-to- multiple-cells could be observed. From these results, it could be concluded that embryonic callus in V. zizanioides was single cell origin. A mature embryoid consisted of scutellum, coleoptile, and coleorhiza which were the typical structure of embryo in monocotyledon. In the regenerated medium, the embryoids on the E-calli could germinate into abundant buds and regenerated plantlets could be obtained by the somatic embryogenesis in V. zizanioides. The regeneration system established in this experiment was appropriate transformation method for genetic-improvement of V. zizanioides. Besides, we observed that some abnormalities in torpedo embryoids, which is a embryo-development stage in dicotyledon and without in monocotyledon, derived from in vitro culture.