Abstract Embryogenic units of friable maize callus are formed as globular or oblong packets of tightly associated meristematic cells. These units are surrounded by conspic- uous cell walls visible in light microscopy after staining with basic fuchsin. Transmission electron microscopy re- vealed that embryogenic cells are rich in endoplasmic re- ticulum, polysomes and small protein bodies, and that the outermost layer of their cell walls is composed of fibrillar material. Electron microscopy has also shown that this ma- terial covers the surface of embryogenic cells as a distinct layer which we denote as extracellular matrix surface network (ECMSN). Employing histochemical staining with β-glucosyl Yariv phenylglycoside, we localized arab- inogalactan-proteins (AGPs) to the outer cell walls of em- bryogenic units including ECMSN. The most prominent staining was found in cell-cell junction domains. Large non-embryogenic callus cells were not stained with this AGP-specific dye. Immunofluorescence and silver-en- hanced immunogold labelling using monoclonal antibody JIM4 has shown that the ECMSN of embryogenic cells is equipped with JIM4 epitope, while non-embryogenic cal- lus cells are devoid of this epitope. We propose that some specific AGPs of the ECMSN might be relevant for cell- cell adhesion and recognition of embryogenic cells during early embryogenic stages, and that the JIM4 antibody can serve as an early marker of embryogenic competence in maize callus culture. Key words Arabinogalactan-proteins · Somatic embryogenesis · Zea mays Abbreviations AGP Arabinogalactan-protein · BSA Bo- vine serum albumin · ECMSN Extracellular matrix surface network · ER Endoplasmatic reticulum · PEM Proembryo- genic mass · PBS Phosphate-buffered saline · SB Stabiliz- ing buffer Introduction Arabinogalactan-proteins (AGPs) are known to play an im- portant though not fully elucidated role in plant morpho- genesis (for recent reviews see Knox 1996; Nothnagel 1997). Monoclonal antibodies against AGPs could be promising tools to study developmental and tissue-specific regulation of these plant proteoglycans (Knox 1997) in- cluding morphogenetic events in vitro. Somatic embryogenesis represents one of the most ex- citing models for studies of plant morphogenesis. It is well known that various AGPs might play an important role in this process (e.g. Knox 1996; Nothnagel 1997). The most extensively studied species is carrot, employing immuno- fluorescence microscopy with specific monoclonal anti- bodies against AGPs (Knox 1997). For example, the JIM4 epitope is enriched at surfaces of peripheral cells of carrot proembryogenic masses (PEMs) and in the dermal layer of young somatic embryos. Subsequently, during develop- ment of cotyledonary embryos, this epitope can be found in provascular strands, as in mother hypocotyl explants (Stacey et al. 1990). On the other hand, MAC 207 antibody recognizes its epitope within all cells of PEMs. Other AGP epitope recognized by antibody JIM8 was originally de- scribed as a marker of the very early transitional state within cells surrounding PEMs of embryogenic carrot sus- pensions (Pennell et al. 1992). Subsequently, it was shown that most somatic embryos develop from cells lacking JIM8 epitope (Toonen et al. 1996) and that the AGP frac- tion containing this epitope has an inhibitory effect on the Plant Cell Reports (1999) 18: 369–374 © Springer-Verlag 1999 Received: 13 March 1998 / Revision received: 6 June 1998 / Accepted: 1 July 1998 J. S ˇ amaj · F. Balus ˇka · M. Bobák · D. Volkmann Extracellular matrix surface network of embryogenic units of friable maize callus contains arabinogalactan-proteins recognized by monoclonal antibody JIM4 Communicated by H. Lörz J. S ˇ amaj () 1 · F. Balus ˇka · D. Volkmann Botanisches Institut, Universität Bonn, Venusberg 22, D-53115 Bonn, Germany Fax: +49-228-732677; e-mail: unb110@uni-bonn.de F. Balus ˇka Institute of Botany, Slovak Academy of Sciences, Dúbravská cesta 14, SK-84223 Bratislava, Slovakia M. Bobák Department of Plant Physiology, Comenius University Bratislava, Mlynská dolina B-2, SK-84215 Bratislava, Slovakia Present address: 1 Institute of Plant Genetics, Slovak Academy of Sciences, Akademická 2, SK-94901 Nitra, Slovakia