A comparative structural analysis of direct and indirect shoot ...

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CELL SURFACE STRUCTURE AND MORPHOGENESIS IN VITRO celIular spaces. We have not observed and it has not been confirmed elsewhere, the presence of external connecting network on the surface of peripheral meristemoid cells corresponding to the fibrillar network around somatic proembryos. The mucilage surrounding the meristemoid surface was involved in direct contact with outer walls of peripheral cells which are regularly thicker compared to internal cell walls (Fig. 2b, Ove~ka et al. 1997). The mucilage layer together with thick outer cell wall could serve as a mechanical separation of meristemoids. There was clearly demonstrated the correlation between mucilage accumulation and starch depletion under unfavourable conditions which indicate mechanical and metabolic functions of external mucilage within the meristemoid polysaccharide pool. In shoot-promoting conditions accumulation of starch is a prerequisite of the shoot regeneration (Thorpe and Murashige 1968) and metabolic reserves are subsequently utilised during shoot primordia formation without extent mucilage production (Ove6ka et al. 1997). Plant mucilages are complex acidic or neutral polysaccharide polymers of high molecular weight (Fahn 1979) in some cases having a microfibrillar network texture (Mariani et al. 1988, Sawidis 1991). Material observed within the intercellular spaces of parenchyma tissues in redifferentiated leaves indicating the polysaccharide nature after PAS reaction (Figs. 4c, d) occurred as a fibrillar and reticular network mediating intercellular contacts of the differentiated cells. Communication within intercellular space containing tissue is developmentally regulated requiring specialised modifications of the wall while consequenced by the loss of cell adhesion (Knox 1992b) and though probably distinctly regulated from communication of tightly adherent proembryo cells. Mucilageous meristemoid structures as well as reticular filling of intercellular spaces are truly extracellular matrix (Connolly and Berlyn 1996), however the function of these materials is probably not comparable with the function of fibfillar network in early somatic embryogenesis. On the other hand, co-ordinated behaviour of the meristemoid cells have been documented. Meristemoid individuality visible from early developmental stages to shoot primordia establishment remaining throughout in the synchrony of meristemoid cell death indicates the existence of an effective communication system within the meristemoid although the character and the basic principle of such communication is still not fully understood. The morphological description of fine plant cell wall structure shows a greatimportance of cell adhesion in early stages of plant regeneration in vitro. However, more detailed description of fine plant cell wall structure together with molecular characterisation of cell surface epitopes is necessary for the understanding of the role of cell surfaces in the regulation of cell-to-cell communication and cell behaviour in asexual plant propagation. Acknowledgement This work was partly supported by Grant Academy VEGA, Grant No. 1/6107/99 from the Slovak Academy of Sciences. References Brownlee C, Berger F. 1995. Extracellular matrix and pattern in plant embryos: on the look out for developmental information. Trends in Genetics 11: 344-348. Connoly JH, Berlyn G. 1996. The plant extracellular matrix. Canadian Journal of Botany 74: 1545-1546. Cyr RJ, Bustos MM, Guiltinan M J, Fosket DE. 1987. Developmental modulation of tubulin protein and mRNA levels during somatic embryogenesis in cultured carrot cells. Planta 171: 365-376. Dijak M, Simmonds DH. 1988. Microtubule organization during early embryogenesis from mesophyll protoplasts ofMedicago sativa L. Plant Science 58:183-191. Domon J-M, Meyer Y, Faye L, David A, David H. 1994. Extracellular (glyco)proteins in embryogenic and non-embryogenic cell lines of Carribean pine. Comparison betweem phenotypes of stage one somatic embryos. Plant Physiology and Biochemistry 32:137-147. Du It, Clarke AE, Bacie A. 1996. Arabinogalactanproteins: a class of extracellular matrix proteoglycans involved in plant growth and development. Trends in Cell Biology 6:411-414. Dubois T, Guedira M, Dubois J, Vasseur J. 1990. Direct somatic embryogenesis in roots of Cichorium: Is callose an early marker? Annals of Botany 65: 539-545. Dubois T, Guedira M, Dubois J, Vasseur J. 1991. Direct somatic embryogenesis in leaves of Cichorium. A histological and SEM study of early stages. Protoplasma 162: 120-129. Dubois T, Dubois J, Guedira M, Diop A, Vasseur J. 1992. SEM characterisation of an extracellular matrix 125
M. OVECKA ,~ M. BOBAK around somatic proembryos in roots of Cichorium. Annals of Botany 70:119-124. Dudits D, Brge L, Gyrgyey J. 1991. Molecular and cellular approaches to the analysis of plant embryo development from somatic cells in vitro. Journal of Cell Science 99: 475-484. Emons AMC. 1994. Somatic embryogenesis: cell biological aspects. Acta Botanica Neerlandica 43: 1-14. Fahn A. 1979. Secretory tissues in plants. London: Academic Press. Goodner B, Quatrano RS. 1993. Fucus embryogenesis: a model to study the establishment of polarity. Plant Cell 5: 1471-1481. Hilbert JL, Dubois T, Vasseur J. 1992. Detection of embryogenesis-related proteins during somatic embryo formation in Cichorium. Plant Physiology and Biochemistry 30: 733-741. Knox JP. 1990. Emerging patterns of organisation at the plant cell surface. Journal of Cell Science 96: 557-561. Knox JP. 1992a. Molecular probes for the plant cell surface. Protoplasma 167: 1-9. Knox JP. 1992b. Cell adhesion, cell separation and plant morphogenesis. Plant Journal 2: 137-141. Mariani P, Raseio N, Baldan B, Paiero P, Urso T. 1988. Epidermal mucilage cells in leaves of Salix species. Flora 181: 137-145. Mo LH, Egertsdotter U, yon Arnold S. 1996. Secretion of specific extracellular proteins by somatic embryos of Picea abies is dependent on embryo morphology. Annals of Botany 77:143-152. Murashige T, Skoog F. 1962. A revised medium for rapid growth and bioassays with tobacco tissue cultures. Physiologia Plantarum 15: 473-497. Ove~ka M, Bobfik M, Erdelsk~ K, gamaj J, Blehovfi A, Kri]tin J.1996. Morphology and conversion ability of somatic embryos in long-term embryogenic callus culture of Papaver somniferum. Biologia (Bratislava) 51: 417-423. Ove6ka M, Bobfik M, Samaj J. 1997. Development of shoot primordia in tissue culture of Papaver somniferum L. Biologia Plantarum 39: 499-506. Ove~:ka M, Bobfik M, Blehovfi A, Kri~tin J. 1997/98. Papaver somniferum regeneration by somatic embryogenesis and shoot organogenesis. Biologia Plantarum 40: 321-328. Poethig RS. 1989. Genetic mosaics and cell lineage analysis in plants. Trends Genetics 5: 273-277. Poulsen GB, Frugis G, Albrechtsen M, Mariotti D. 1996. Synthesis of extracellular proteins in embryogenic and non-embryogenic cell cultures of alfalfa. Plant Cell Tissue and Organ Culture 44: 257-260. Reuzeau C, Pont-Lezica RF. 1995. Comparing plant and animal extracellular matrix-cytoskeleton connections - are they alike? Protoplasma 186: 113-121. Roberts K.1989. The plant extracellular matrix. Current Opinion in Cell Biology 1: 1020-1027. Roberts K. 1994. The plant extracellular matrix: in a new expansive mood. Current Opinion in Cell Biology 6: 688-694. Sawidis T. 1991. A histochemical study of nectaries of Hibiscus rosa-sinensis. Journal of Experimental Botany 42: 1477-1487. Sondahl MR, Salisbury JL, Sharp WR. 1979. SEM characterization of embryogenic tissue and globular embryos during high frequency somatic embryogenesis in coffee callus cells. Zeitschrift fr Pflanzenphysiologie 94: t85-188. Stacey N J, Roberts K, Knox JP. 1990. Patterns of expression of the JIM4 arabinogalactan-protein epitope in cell cultures and during somatic embryogenesis in Daucus carota. Planta 180: 285-292. Sterk P, Booij H, Schellekens GA, Van Kammen A, De Vries SC. 1991. Cell-specific expression of the carrot EP2 lipid transfer protein gene. Plant Cell 3: 907-921. Samaj J, Bobfik M, Erdelsk~ K. 1990. Histologicalanatomical studies of the structure of the organogenic callus in Papaver somniferum L. Biologia Plantarum 32: 14-18. ~amaj J, Bobfik M, Blehovfi A, Kri~tin J, Auxtovfi- Samajovfi O. 1995. Developmental SEM observations on an extracellular matrix in embryogenic ca]li of Drosera rotundifolia and Zea mays. Protoplasma I86: 45-49. Thorpe TA, Murashige T. 1968. Starch accumulation in shoot-forming tobacco callus cultures. Science 160: 421-422. Verdus M-C, Dubois T, Dubois J, Vasseur J. 1993. Ultrastructural changes in leaves of Cichorium during somatic embryogenesis. Annals of Botany 72: 375-383. Received May 26, 1998; accepted December 21, 1998 126
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