NMR of hydrated Arabidopsis seeds - Role of the seed mucilage

Myxospermous seeds accumulate complex polysaccharides in the epidermal cells of the seed coat during seed development and these are released on imbibition to form a gelatinous halo around the seed. This specialisation of the seed coat is found in a number of plant families such as Brasssiceae, Solanaceae, Linaceae and Plantaginaceae. The role of pectinaceous mucilage released from seeds on imbibition is unknown. Analyses of Arabidopsis seed mucilage structure and composition have shown that it is a complex mixture of polysaccharides present in two structurally distinct layers where rhamnogalacturonan I (RG-I) is the major component. The physico-chemical differences between the layers indicate that each has a different function. Proposed roles for seed mucilage include inhibition of germination by blocking oxygen diffusion to the seed, dispersal of seeds by its adhesion to animals, or keeping seeds moist and able to germinate under reduced water potentials, but none has been conclusively demonstrated in Arabidopsis. The reasons for the different layers and domains in Arabidopsis mucilage are also unknown, but it has been postulated that they each have a different function. The production of the complex polysaccharides that make up seed mucilage is, nonetheless, a significant metabolic investment during seed development and must impart a selective advantage. Mutants affected in mucilage structure have been employed for the identification of potential roles for different mucilage layers or domains and improving our understanding of the interactions that occur between different polysaccharide domain types. The rate of water uptake by Arabidopsis seeds has been analysed using the non-invasive technique of nuclear magnetic resonance (NMR). Measurements of the water T1 and T2 relaxation times have permitted the various water and oil compartments in seeds to be distinguished before and during imbibition. Analysis of the evolution over time of each T2 component, has led us to propose a role for the inner mucilage layer in modulating water uptake.