AN AGP EPITOPE DISTINGUISHES A CENTRAL METAXYLEM INITIAL FROM OTHER VASCULAR INITIALS IN THE ARABIDOPSIS ROOT

The developmental expression of an arabinogalactan protein (AGP) recog nised by a monoclonal antibody, JIM 13, is described in Arabidopsis ro ots. It is expressed in the single initial of the central metaxylem ve ssel that lies immediately above the four central cells of the quiesce nt centre. AGP expression spreads in a non-clonal fashion to neighbour ing cell files as they mature. The AGP first appears in other pre-meta xylem elements and subsequently in the layer of parenchyma cells on ei ther side of the xylem axis. Later, it spreads to the entire ring of e ight endodermal cell files and to those pericycle cell files next to t he xylem pole. It is postulated that this epitope reflects an underlyi ng set of cell signalling events that may be involved in defining loca l positional values of cells important in establishing cellular patter ns in the root.