Arabinogalactan-proteins (AGPs) are extracellular proteoglycans, which are presumed to participate in the regulation of cell shape, thus contributing to the excellent mechanical properties of plants. AGPs consist of a hydroxyprolinerich core-protein and large arabinogalactan (AG) sugar chains, called type II AGs. These AGs have a β-1,3-galactan backbone and β-1,6-galactan side chains, to which other sugars are attached. The structure of type II AG differs depending on source plant, tissue, and age. Type II AGs obtained from woody plants in large quantity as represented by gum arabic and larch AG, here designated gum arabic-subclass, have a β-1,3;1,6-galactan structure in which the β-1,3-galactan backbone is highly substituted with short β-1,6-galactan side chains. On the other hand, it is unclear whether type II AGs found as the glycan part of AGPs from herbaceous plants, here designated AGP-subclass, also have conserved β-1,3:1,6-galactan structural features. In the present study we explore similarities of type II AG structures in the AGP-subclass. Type II AGs in fractions obtained from spinach, broccoli, bok choy, komatsuna, and cucumber were hydrolyzed into galactose and β-1,6-galactooligosaccharides by specific enzymes. Based on the proportion of these sugars, the substitution ratio of the β-1,3-galactan backbone was calculated as 46-58% in the five vegetables, which is consistently lower than what is seen in gum arabic and larch AG. Although most side chains were short, long chains such as β-1,6-galactohexaose chains were also observed in these vegetables. The results suggest a conserved β-1,3;1,6-galactan structure in the AGP-subclass that distinguishes it from the gum arabic-subclass.
Arabinogalactan-proteins (AGPs) are mysterious extracellular glycoproteins in plants. Although AGPs are highly conserved, their molecular functions remain obscure. The physiological importance of AGPs has been extensively demonstrated with β-Yariv reagent, which specifically binds to AGPs and upon introduction into cells, causes various deleterious effects including growth inhibition and programmed cell death. However, structural features of AGPs that determine their functions have not been identified with β-Yariv reagent. It is known that AGPs are decorated with large type II arabinogalactans (AGs), which are necessary for their functions. Type II AGs consist of a β-1,3-galactan main chain and β-1,6-galactan side chains with auxiliary sugar residues such as L-arabinose and 4-O-methyl-glucuronic acid. While most side chains are short, long side chains such as β-1,6-galactohexaose (β-1,6-Gal6) also exist in type II AGs. To gain insight into the structures important for AGP functions, in vivo structural modification of β-1,6-galactan side chains was performed in Arabidopsis. We generated transgenic Arabidopsis plants expressing a fungal endo-β-1,6-galactanase, Tv6GAL, that degrades long side chains specifically under the control of dexamethasone (Dex). Two of 6 transgenic lines obtained showed more than 40 times activity of endo-β-1,6-galactanase when treated with Dex. Structural analysis indicated that long side chains such as β-1,6-Gal5 and β-1,6-Gal6 were significantly reduced compared to wild-type plants. Tv6GAL induction caused retarded growth of seedlings, which had a reduced amount of cellulose in cell walls. These results suggest that long β-1,6-galactan side chains are necessary for normal cellulose synthesis and/or deposition as their defect affects cell growth in plants.
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