The presence of fucogalactoxyloglucan and its synthesis in rice indicates conserved functional importance in plants

Abstract

The predominant structure of the hemicellulose xyloglucan (XyG) found in the cell walls of dicots is a fucogalactoXyG with an XXXG core motif, whereas in the Poaceae (grasses and cereals), the structure of XyG is less xylosylated (XXGGn core motif) and lacks fucosyl residues. However, specialized tissues of rice (Oryza sativa) also contain fucogalactoXyG. Orthologous genes of the fucogalactoXyG biosynthetic machinery of Arabidopsis (Arabidopsis thaliana) are present in the rice genome. Expression of these rice genes, including fucosyl-, galactosyl-, and acetyltransferases, in the corresponding Arabidopsis mutants confirmed their activity and substrate specificity, indicating that plants in the Poaceae family have the ability to synthesize fucogalactoXyG in vivo. The data presented here provide support for a functional conservation of XyG structure in higher plants.

Figure 1.

Schematic structures of two types of XyGs and known biosynthetic proteins in Arabidopsis (Hsieh and Harris, 2009; Pauly et al., 2013). The corresponding one-letter code for XyG is shown below the pictograms (Fry et al., 1993; Tuomivaara et al., 2015).

Figure 2.

XyG OLIMP of Arabidopsis leaf and various tissues from rice. The m/z of significant ion signals and suggested corresponding structures are labeled. Detailed structural information can be found in Supplemental Table S1.

Figure 3.

Immunofluorescent labeling of 3-d-old rice root transverse sections with CCRC-M1. Root section in bright field (A), autofluorescence (B), CCRC-M1 (C), and merged signal of bright field and CCRC-M1 fluorescent signal (D). Root hairs in bright field (E) and CCRC-M1 fluorescent signal (F). Bars = 25 µm.

Figure 4.

XyG structures found in various Arabidopsis mutants and Arabidopsis transformants overexpressing particular rice genes. The XyG oligosaccharide mass profile is derived from 4-week-old leaves. The average relative ratios of fucosylated (A; XXFG, XXFG, XLFG, and XLFG), acetylated (B; XXLG, XXFG, and XLFG), and galactosylated (C and D; XXLG/XLXG, XXLG, XXFG, XXFG, XLLG, XLFG, and XLFG) oligosaccharides are shown. OX indicates an independent overexpression line. One-way ANOVA statistical analysis was performed. Different lowercase letters indicate significant differences. Error bars indicate sd (n ≥ 4).

Figure 5.

Phenotype of rice galactosyltransferase transformants. Representative 4-week-old plants of OsMUR3 (A) and 7-week-old plants of OsMUR3 (B) or OsXLT2 (C) transformed into the xlt2 mur3.1 mutant. Bars = 2 cm. Average height of 7-week-old plants of OsMUR3 (D) and OsXLT2 (E) transformed into the xlt2 mur3.1 mutant. OX indicates an independent overexpression line. One-way ANOVA statistical analysis was performed. Different lowercase letters indicate significant differences. Error bars indicate sd (n = 12).