Abstract
Arabidopsis seed coat epidermal cells produce a large quantity of mucilage that is extruded upon exposure to water. Chemical analyses and cell biological techniques suggest that this mucilage represents a specialized type of secondary cell wall composed primarily of pectin with lesser amounts of cellulose and xyloglucan. Once extruded, the mucilage capsule has a distinctive structure with an outer non-adherent layer that is easily removed by shaking in water, and an inner adherent layer that can only be removed with strong acid or base. Most of the cellulose in the mucilage is present in the inner layer and is responsible at least in part for its adherence to the seed. There are also differences in the pectin composition between the two layers that could contribute to the difference in adherence. The Arabidopsis seed coat epidermis and its mucilage are not essential for seed viability or germination. This dispensability, combined with the fact that the epidermal cells synthesize an accessible pectin-rich cell wall at a specific time in development, makes them well suited as a genetic model for studying cell wall biogenesis, function, and regulation. Mutants defective in seed mucilage identified by both forward and reverse genetic analyses are proving useful in establishing connections between carbohydrate structure and cell wall properties in vivo. In the future, genetic engineering of seed coat mucilage carbohydrates should prove useful for testing hypotheses concerning cell wall structure and function.
Highlights
When wetted, seeds of Arabidopsis thaliana (Arabidopsis), like those of many angiosperm species, become surrounded by a gelatinous capsule called mucilage (Figure 1A).The mucilage originates from the specialized epidermal cells of the seed coat
Research using chemical analyses and cell biological techniques has revealed that the mucilage has a complex composition and structure and as we argue below, could be considered a specialized pectin-rich secondary cell wall
Because the seed coat is so amenable to molecular–genetic manipulations and the mucilage so accessible, seed coat mucilage provides a unique system for studying many aspects of cell wall structure and function
Summary
Seeds of Arabidopsis thaliana (Arabidopsis), like those of many angiosperm species, become surrounded by a gelatinous capsule called mucilage (Figure 1A). The mucilage originates from the specialized epidermal cells of the seed coat. During seed coat differentiation following cessation of cell growth, mucilage is synthesized and secreted to a region of the apoplast adjacent to the radial and outer tangential cell walls, forming a donut-shaped pocket between the membrane and the cell wall. A thick cellulose-rich cell wall (columella) is synthesized beneath the mucilage pocket, along the outer tangential and radial sides of the cell that completely displaces the cytoplasm by the end of seed development. The epidermal cells consist of a volcano-shaped cellulosic cell wall surrounded by a donut-shaped mucilage pocket, all of which are contained within the primary cell wall (Figure 1C). We review the current literature pertaining to mucilage composition and structure and discuss its potential for enhancing cell wall research
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