Mucilage attachment to seeds provides evidence of a novel link between cellulose and pectin

In certain plant species, including Arabidopsis, seed coat epidermal cells accumulate a large quantity of polysaccharides that are released as mucilage upon imbibition. Once released, Arabidopsis seed coat mucilage appears as two distinct layers, the outer non-adherent composed of unsubstitutedrhamnogalacturonan I (RG-I), and the inner adherent one mainly made of RG-I and cellulose. CELLULOSE SYNTHASE5 and MUCILAGE-MODIFIED (MUM)5 have both been shown to be required for anchoring the mucilage pectin to the seed coat surface [1-5]. The identity and molecular role of MUM5 remained to be determined. A comparison of sequentially extracted seed mucilage components from cesa5 and mum5 mutants was carried out. Both mutants showed a redistribution of mucilage pectin from the inner adherent layer to the outer soluble one. cesa5 further showed a clear reduction in cellulose content, whereas mum5 had a wild-type cellulose content, but had reduced Xyl contents in the outer mucilage layer. Accordingly, MUM5 was identified as a putative xylosyltransferase. Biochemical and in vitro binding assay data demonstrated that xylan chains are attached to RG-I chains and mediate the adsorption of mucilage to cellulose microfibrils. The importance of having both a correctly synthesized cellulose scaffold and pectin structure will be discussed with regard to mucilage partitioning into non-adherent and adherent layers