An AGP epitope distinguishes a central metaxylem initial from other vascular initials in the Arabidopsis root

The developmental expression of an arabinogalactan protein (AGP) recognised by a monoclonal antibody, JIM 13, is described in Arabidopsis roots. It is expressed in the single initial of the central metaxylem vessel that lies immediately above the four central cells of the quiescent centre. AGP expression spreads in a non-clonal fashion to neighbouring cell files as they mature. The AGP first appears in other pre-metaxylem elements and subsequently in the layer of parenchyma cells on either side of the xylem axis. Later, it spreads to the entire ring of eight endodermal cell files and to those pericycle cell files next to the xylem pole. It is postulated that this epitope reflects an underlying set of cell signalling events that may be involved in defining local positional values of cells important in establishing cellular patterns in the root.