Side Chains Influence Pectin Properties
Šola et al. examine how galactose alters rhamnogalacturonan-I properties. Plant Cell https://doi.org/10.1105/tpc.18.000954
By Krešimir Šola and George W. Haughn
Department of Botany, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada
Background: Pectin is a component of the plant cell wall that has many important roles, such as support, the regulation of cell growth and cell-to-cell adhesion. There are three main types of pectin: homogalacturonan (HG), rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II). Specific biochemical roles are known for HG and RG-II, but not for RG-I. RG-I typically has galactose and arabinose sugars as side-chains. However, a form abundant in seed coat mucilage of Arabidopsis thaliana has few side-chains. Arabidopsis mucilage rapidly expands and extrudes when seeds are put in water. The ability of mucilage RG-I to expand is dependent on the absence of the galactose side-chains but the reasons for this are unknown.
Question: We wanted to understand how adding galactose to RG-I changes its properties.
Findings: Using Arabidopsis seed mucilage with galactose-rich RG-I, we identified a gene, RUBY, which prevents RG-I with galactose side-chains from expanding and is required for normal cell-to-cell adhesion between seed coat epidermal cells. We found that RUBY is important for galactose oxidase enzyme activity in seeds, which results in oxidation of galactose on mucilage RG-I. Our interpretation is that the oxidation of RG-I galactose promotes chemical bonds between RG-I and other cell wall components, strengthening cell-to-cell adhesion.
Next steps: Our work suggests a new way of changing pectin properties. It would be interesting to explore changes in bonding between components of the cell wall following pectin galactose oxidation using additional methods. We predict that expression of RUBY in tissues other than the seed coat should increase cell-to-cell adhesion. An analysis of the roles of RUBY-like genes that are active in other plant tissues may help answer such questions.
Krešimir Šola, Erin J. Gilchrist, David Ropartz, Lisa Wang, Ivo Feussner, Shawn D. Mansfield, Marie-Christine Ralet, George W. Haughn (2019). RUBY, a putative galactose oxidase, influences pectin properties and promotes cell-to-cell adhesion in the seed coat epidermis of Arabidopsis thaliana. Plant Cell; DOI: https://doi.org/10.1105/tpc.18.00954
Key words: cell wall, galactose oxidase, mucilage, pectin