Rhamnogalacturonan I, a pectic polysaccharide that is a component of monocot cell-walls

Abstract

Pectic polysaccharides were solubilized from the primary walls of suspension-cultured cells of maize (Zea mays) and rice (Oryza sativa), and shown by glycosyl-linkage analysis to be similar to the pectic polysaccharide rhamnogalacturonan I (RG-I) that had previously been solubilized from walls of suspension-cultured sycamore (Acer pseudoplatanus) cells. Maize and rice are monocots; sycamore is a dicot. The backbone of sycamore RG-I had previously been shown to consist of the disaccharide repeating unit →4)-α- d-Gal pA-(1→2)-α- l-Rha p-(1→. Side chains containing arabinosyl, galactosyl, and fucosyl residues are attached through O-4 of about half of the backbone rhamnosyl residues. To compare the structures of monocot and dicot RG-I, galactosyluronic acid residues in the maize and rice polysaccharides were selectively cleaved by treatment with lithium in ethylenediamine, and the structural characteristics of the oligoglycosylalditol products compared to those of oligoglycosylalditols previously isolated from sycamore RG-I. The strong resemblance of the glycosyl-linkage compositions of lithium-treated RG-I from maize, rice, and sycamore indicated the overall structural likeness of the three polysaccharides. Spectra obtained by fast-atom-bombardment mass spectrometry (f.a.b.-m.s.) of lithium-treated maize and sycamore RG-I showed that most of the oligoglycosylalditols derived from the side chains of the two polysaccharides were the same. Nine oligoglycosylalditols partially purified from maize RG-I were similar to the corresponding components of sycamore RG-I. F.a.b.-m.s. of the maize reaction-products indicated the presence in maize RG-I of an additional eight oligoglycosylalditols previously isolated from sycamore RG-I. On the other hand, seven oligoglycosylalditols were partially purified from maize RG-I that had not been observed in previous studies of sycamore RG-I, and maize RG-I was shown not to contain fucosyl residues (as does sycamore RG-I). The results described here suggest that, despite the observed differences, many of the structural features of RG-I have been conserved in the primary cell-walls of monocots and dicots.