Abstract
A monoclonal antibody (JIM4) has been derived that recognizes a series of glycoproteins associated with the plasma membrane of a suspension-cultured carrot cell line and also an arabinogalactan proteoglycan secreted by the cultured cells. Immunocytochemistry indicated that the plasma membrane antigen(s) recognized by JIM4 are specific to certain cells of Daucus carota L. seedlings. In the root apex JIM4 labelled two segments of the stele. These were centred upon the poles of the protoxylem. An axis of unlabelled cells connected the two phloem regions. Two sections of the pericycle with characteristic oblique longitudinal divisions were particularly reactive with JIM4. This pattern of reactive cells, reflecting cell position rather than a specific future cell type, would appear to be a unique observation in plants. The association of JIM4 antigens with these vascular tissues is maintained through the transition from root to the shoot tissue of the cotyledons and the mature plant. Examination of JIM4 labelling upon ultrathin frozen sections of the carrot seedling root apical meristem indicated that the expression of the antigen is a very early event in root development. Cells express the surface epitope, within one or two cells of the dome of apical initials, before the pattern of future vascular tissue can be discerned and well before its actual differentiation.
