ULTRASTRUCTURAL STUDIES ON MEMBRANED INCLUSIONS IN THE CELLS OF MERISTEMATIC REGIONS OF STEVIA REBAUDIANA CALLUS
Ouyang Xuezhi; and Xie Shaoping(Zhongshau College, Zhongshan,Guangdong 528403)Chen Muchuan and Hong Weilian(Department of Biology, Xiamen University, Xiamen,Fujian 361005)
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.