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Summary. The auxin-binding protein ABP-1 was localisedimmunocytochemically in coleoptiles and immature embryos of Zea mays. Two primary polyclonal antibodies raised against ABP-1 and secondary antibodies were either labelled with FITC or 10 nm gold particles for light microscopy, and with 10 nm gold particles for transmission electron microscopy. Light microscopy revealed that ABP-1 was localised in the epidermal cells of etiolated maize coleoptiles, in subepidermal parenchymatic mesophyll cells of the coleoptile and in the companion cells of the vascular bundles. Most labelling was found in the cytoplasm, less in nuclei and vacuoles and cell walls appeared negative. The region of the plasma membrane exhibited prominent labelling. Embryos showed low labelling throughout their tissues just after excision, but after culture for 7 days intensive labelling was found in the epidermis of the scutellum. Quantitative electron microscopy confirmed that ABP-1 was present in the cytoplasm of epidermal, mesophyll, and companion cells of coleoptiles. Gold particles were neither found in cell walls nor in the cuticle. Areas with ER and dictyosomes within epidermal and mesophyll ceils of coleoptiles had a denser labelling with gold particles than elsewhere. Labelling at the plasma membrane, being the site where the auxin binds to the ABP, was observed at low levels in all cells examined, which is due to the method applied. Epidermal cells of embryos cultured for 5 days exhibited high levels of gold particles in ER and nuclei, and lower levels in the cytoplasm. The distribution is only partly in accordance with the model in which ABP is thought to cycle through the plant cell from the ER via the Golgi system towards the plasma membrane.

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