甜菊愈伤组织分生区细胞中膜内含物的超微结构研究

ULTRASTRUCTURAL STUDIES ON MEMBRANED INCLUSIONS IN THE CELLS OF MERISTEMATIC REGIONS OF STEVIA REBAUDIANA CALLUS

生长在分化培养基上的甜菊（Stevttirebaudiana）愈伤组织分生区细胞中存在双膜和多膜内含物。电镜观察表明，这些膜内含物是由一圈或多圈呈同心圆或卷绕状排列的内质网包围部分细胞质而形成的。双膜内含物内外层膜的靠细胞质表面有核糖体附着，而多膜内含物仅在其最外层储油的外膜上偶有少量核糖体附着。随着细胞液泡化程度的提高，多膜内含物通过液泡膜内陷而转移到液池中或通过消化其中被包围的细胞质及内膜而转化为小液泡。此外，还观察到内质网潴泡膨大直接形成小池，有时在其外表面附有核糖体，且通过界膜的局部内陷吞噬部分细胞质。结果表明，某些内质网参与了对部分细胞质的分隔及继后的降解，而这对于甜菊愈伤组织器官分化前的细胞改建中细胞质组分的正常更新可能是必需的。

The presence of both double and multiple membraned inclusions were observed inthe cells from the meristematic regions formed in callus cultures of Stevia rebaudiana ondifferentiating medium.It was found that these membraned inclusions appear to be de-rived from the endoplasmic reticulum elements which arrange themselves in a multiple-concentric or convoluted manner around portions of cytop1asm or form a ring enclosingareas of cytoplasm. The doublemembraned inclusions have ribosomes attached to the cy-toplasmic surfaces of both the outer and the inner membranes, while the multimem-braned inclusions show a few ribosomes attached occasionally to the outer membrane ofthe outermost cisterna. As cellular vacuolation progresses,the multi-membraned inclu-sions are transferred to the enlarging vacuoles by tonoplast invagination or transformed into small vacuoles by the digestion of the sequestered cytoplasm and inner membranes.In addition,vesicles can be formed directly by the dilation of the endoplasmic reticulumcisternae,which,in some cases,bear ribosomes on the outer surface and engulf a portionof cytoplasm by local invagination of the limiting membrane. The results indicate thatcertain endoplasmic reticulum cisternae are engaged in the sequestration and subsequentdigestion of cytop1asm which may be required for the normal turnover of cytoplasmiccomponents in the cellular remodeling which precede organ differentiation in the cul-tured callus of Stevia rebaudiana.