Arabidopsis irregular xylem8andirregular xylem9: Implications for the Complexity of Glucuronoxylan Biosynthesis

Abstract

Mutations of Arabidopsis thaliana IRREGULAR XYLEM8 (IRX8) and IRX9 were previously shown to cause a collapsed xylem phenotype and decreases in xylose and cellulose in cell walls. In this study, we characterized IRX8 and IRX9 and performed chemical and structural analyses of glucuronoxylan (GX) from irx8 and irx9 plants. IRX8 and IRX9 are expressed specifically in cells undergoing secondary wall thickening, and their encoded proteins are targeted to the Golgi, where GX is synthesized. 1 H-NMR spectroscopy showed that the reducing end of Arabidopsis GX contains the glycosyl sequence 4-b-D-, which was previously identified in birch (Betula verrucosa) and spruce (Picea abies) GX. This indicates that the reducing end structure of GXs is evolutionarily conserved in woody and herbaceous plants. This sequence is more abundant in irx9 GX than in the wild type, whereas irx8 and fragile fiber8 (fra8) plants are nearly devoid of it. The number of GX chains increased and the GX chain length decreased in irx9 plants. Conversely, the number of GX chains decreased and the chain length heterodispersity increased in irx8 and fra8 plants. Our results suggest that IRX9 is required for normal GX elongation and indicate roles for IRX8 and FRA8 in the synthesis of the glycosyl sequence at the GX reducing end.

GALACTURONOSYLTRANSFERASE-LIKE5 Is Involved in the Production of Arabidopsis Seed Coat Mucilage

“…The idea of a terminal oligosaccharide playing a role in the regulation of polysaccharide size has been proposed for glucuronoxylan synthesis in Arabidopsis (York and O'Neill, 2008). It was found that a normal chain length of glucuronoxylan requires the presence of a GalA-containing tetrasaccharide sequence that is located at the reducing end of the polysaccharide, and mutation of the GAUT12/IRREGLULAR XYLEM8 (IRX8) gene results in lower levels of xylan synthesis and altered polymer length (Peña et al, 2007;Persson et al, 2007). If such a primer/terminator exists on RG-I, it would only account for a small percentage of the total mass of the mucilage, and its absence or alteration would be difficult to detect by any of the methods applied in this study.…”

The Biochemistry and Structural Biology of Plant Cell Wall Deconstruction

“…The biological rationale for this dual specificity is unclear. It has been proposed that in model plants, such as Arabidopsis, in rapidly dividing cells there are significant quantities of unmethylated GlcA (Pena et al, 2007). This has led to the hypothesis that, by targeting unmethylated GlcA, the CBM is directing enzymes to more open structures that are particularly susceptible to enzyme degradation.…”

The Genetics and Transcriptional Profiles of the Cellulose Synthase-Like HvCslF Gene Family in Barley

“…However, it has been shown recently that GAUT1, a type II galacturonyltransferase of the GT8 family, is capable of mediating the synthesis of the homogalacturonan backbone of pectic polysaccharides (Sterling et al, 2006). Similarly, type II GTs have been implicated in the synthesis of the (1,4)-b-D-xylan backbone of heteroxylans of the wall (Lee et al, 2007;Mitchell et al, 2007;Pena et al, 2007).…”