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Xylogenic cultures of zinnia (Zinnia elegans) provide a unique opportunity to study signaling pathways of tracheary element (TE) differentiation. In vitro TEs differentiate into either protoxylem (PX)-like TEs characterized by annular/helical secondary wall thickening or metaxylem (MX)-like TEs with reticulate/scalariform/pitted thickening. The factors that determine these different cell fates are largely unknown. We show here that supplementing zinnia cultures with exogenous galactoglucomannan oligosaccharides (GGMOs) derived from spruce (Picea abies) xylem had two major effects: an increase in cell population density and a decrease in the ratio of PX to MX TEs. In an attempt to link these two effects, the consequence of the plane of cell division on PX-MX differentiation was assessed. Although GGMOs did not affect the plane of cell division per se, they significantly increased the proportion of longitudinally divided cells differentiating into MX. To test the biological significance of these findings, we have determined the presence of mannan-containing oligosaccharides in zinnia cultures in vitro. Immunoblot assays indicated that b-1,4-mannosyl epitopes accumulate specifically in TE-inductive media. These epitopes were homogeneously distributed within the thickened secondary walls of TEs when the primary cell wall was weakly labeled. Using polysaccharide analysis carbohydrate gel electrophoresis, glucomannans were specifically detected in cell walls of differentiating zinnia cultures. Finally, zinnia macroarrays probed with cDNAs from cells cultured in the presence or absence of GGMOs indicated that significantly more genes were down-regulated rather than up-regulated by GGMOs. This study constitutes a major step in the elucidation of signaling mechanisms of PX-and MX-specific genetic programs in zinnia.Xylogenesis is one of the most remarkable examples of cell specialization in higher plants. The xylem is the principal water-conducting tissue, transporting water from the root system to the aerial portions of the plant. To ensure this critical function, long files of cells divide and elongate, secondary cell wall material is deposited, the end walls between cells are hydrolyzed, and cell content is destroyed. In angiosperms, the resulting hollow structure, or xylem vessel, is composed of single units called tracheary elements (TEs). TEs are characterized by the different types of secondary wall thickening that are laid down: annular, helical, reticulate, and pitted. Protoxylem (PX) TEs with annular or helical secondary cell wall thickening differentiate while an organ is still expanding, whereas metaxylem (MX) TEs with reticulate or pitted secondary cell wall thickening differentiate after organ expansion has ceased (Esau, 1977). The relative proportions of PX and MX TEs that make up a given vascular bundle vary among plant organs and within a given organ throughout plant growth (Fahn, 1990;Pesquet et al., 2003). Until now, the underlying molecular mechanisms, signaling events, and positional information dict...