Extracellular matrix surface network of embryogenic units of friable maizecallus contains arabinogalactan-proteins recognized by monoclonal antibodyJIM4

Embryogenic units of friable maize callus are formed as globular or oblong packets of tightly associated meristematic cells. These units are surrounde d by conspicuous cell walls visible in light microscopy after staining with basic fuchsin. Transmission electron microscopy revealed that embryogenic cells are rich in endoplasmic reticulum, 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 material covers the surface of embryogenic cells as a distinct layer which we denote as extrac ellular matrix surface network (ECMSN). Employing histochemical staining wi th beta-glucosyl Yariv phenylglycoside, we localized arabinogalactan-protei ns (AGPs) to the outer cell walls of embryogenic 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. Immu nofluorescence and silver-enhanced immunogold labelling using monoclonal an tibody JIM4 has shown that the ECMSN of embryogenic cells is equipped with JIM4 epitope, while non-embryogenic callus cells are devoid of this epitope . We propose that some specific AGPs of the ECMSN might be relevant for cel l-cell adhesion and recognition of embryogenic cells during early embryogen ic stages, and that the JIM4 antibody can serve as an early marker of embry ogenic competence in maize callus culture.