Yariv phenylglycosides [1,3,5-tri(p-glycosyloxyphenylazo)-2,4,6-trihydroxybenzene] are a group of chemical compounds that selectively bind to arabinogalactan proteins (AGPs), a type of plant proteoglycan. Yariv phenylglycosides are widely used as cytochemical reagents to perturb the molecular functions of AGPs as well as for the detection, quantification, purification, and staining of AGPs. However, the target structure in AGPs to which Yariv phenylglycosides bind has not been determined. Here, we identify the structural element of AGPs required for the interaction with Yariv phenylglycosides by stepwise trimming of the arabinogalactan moieties using combinations of specific glycoside hydrolases. Whereas the precipitation with Yariv phenylglycosides (Yariv reactivity) of radish (Raphanus sativus) root AGP was not reduced after enzyme treatment to remove a-L-arabinofuranosyl and b-glucuronosyl residues and b-1,6-galactan side chains, it was completely lost after degradation of the b-1,3-galactan main chains. In addition, Yariv reactivity of gum arabic, a commercial product of acacia (Acacia senegal) AGPs, increased rather than decreased during the repeated degradation of b-1,6-galactan side chains by Smith degradation. Among various oligosaccharides corresponding to partial structures of AGPs, b-1,3-galactooligosaccharides longer than b-1,3-galactoheptaose exhibited significant precipitation with Yariv in a radial diffusion assay on agar. A pull-down assay using oligosaccharides cross linked to hydrazine beads detected an interaction of b-1,3-galactooligosaccharides longer than b-1,3-galactopentaose with Yariv phenylglycoside. To the contrary, no interaction with Yariv was detected for b-1,6-galactooligosaccharides of any length. Therefore, we conclude that Yariv phenylglycosides should be considered specific binding reagents for b-1,3-galactan chains longer than five residues, and seven residues are sufficient for cross linking, leading to precipitation of the Yariv phenylglycosides.Arabinogalactan proteins (AGPs) are a type of plant proteoglycans consisting of a Hyp-rich core protein and large arabinogalactan (AG) moieties (Fincher et al., 1983;Nothnagel, 1997). Although there are many molecular species of AGP differentiated by their core proteins, the AG moieties commonly comprise b-1,3-galactan main chains and b-1,6-galactan side chains, to which L-Ara and other auxiliary sugars, such as GlcA, 4-O-methyl-GlcA, L-Fuc, L-Rha, and Xyl, are attached (Fincher et al., 1983;Nothnagel, 1997;Seifert and Roberts, 2007). A commercial product of AGPs prepared from the acacia (Acacia senegal) tree is known as gum arabic and utilized as a food stabilizer. In the Japanese herbal remedy Juzen-Taiho-To, AGs from Astragalus membranaceus are the active ingredient (Majewska-Sawka and Nothnagel, 2000;Kiyohara et al., 2002). In intact plants, AGPs are implicated in various physiological events and serve as extracellular constituents and signaling molecules. For instance, an AGP from stylar transmitting tissue attracts pollen tub...