In situ, Chemical and Macromolecular Study of the Composition of Arabidopsis thaliana Seed Coat Mucilage

Abstract

A comprehensive analysis was carried out of the composition of seed coat mucilage from Arabidopsis thaliana using the Columbia-0 accession. Pectinaceous mucilage is released from myxospermous seeds upon imbibition, and in Arabidopsis consists of a water-soluble, outer layer and an adherent, inner layer. Analysis of monosaccharide composition in conjunction with digestion with pectolytic enzymes conclusively demonstrated that the principal pectic domain of both layers was rhamnogalacturonan I, and that in the outer layer this was unbranched. The macromolecular characteristics of the water-soluble mucilage indicated that the rhamnogalacturonan molecules in the outer layer were in a slightly expanded random-coil conformation. The inner, adherent layer remained attached to the seed, even after extraction with acid and alkali, suggesting that its integrity was maintained by covalent bonds. Confocal microscopy and monosaccharide composition analyses showed that the inner layer can be separated into two domains. The internal domain contained cellulose microfibrils, which could form a matrix with RGI and bind it to the seed. In effect, in the mum5-1 mutant where most of the inner and outer mucilage layers were water soluble, cellulose remained attached to the seed coat. Immunolabeling with anti-pectin antibodies indicated the presence of galactan and arabinan in the inner layer, with the latter only present in the non-cellulose-containing external domain. In addition, JIM5 and JIM7 antibodies labeled different domains of the inner layer, suggesting the presence of stretches of homogalacturonan with different levels of methyl esterification.

GALACTURONOSYLTRANSFERASE-LIKE5 Is Involved in the Production of Arabidopsis Seed Coat Mucilage

“…The ability of Arabidopsis mucilage to bind the antibodies JIM5 and JIM7 also suggests the presence of lowly and highly methyl-esterified HG (Western et al, 2000;Willats et al, 2001a;Macquet et al, 2007a). Small amounts of cellulose, arabinan, and galactan were also found in Arabidopsis seed mucilage (Willats et al, 2001a;Macquet et al, 2007a;Young et al, 2008).…”

LASSO modeling of the Arabidopsis thaliana seed/seedling transcriptome: a model case for detection of novel mucilage and pectin metabolism genes

“…In Arabidopsis thaliana, the seed coat is characterized by epidermal cells showing some specialized structures. Within the epidermal cells one can find the columella, which is a volcano-shaped secondary cell wall structure, 35 and which is surrounded by pectinaceous mucilage, arranged in a donut-shaped ring 36 under the primary cell wall separating the epidermal cell from outside. This mucilaginous material is released upon contact with water and then completely envelopes the seed.…”

“…This mucilaginous material is released upon contact with water and then completely envelopes the seed. 36,37 Thus, mucilage has been suggested to be important for seed hydration and germination, attachment to soil components and for preventing gas exchange. 38,39 Once released, mucilage is characterized by a denser, relatively insoluble, inner layer and a more soluble outer layer, composed of sparsely branched rhamnogalacturonan I (RGI), a polysaccharide formed by succession of L-rhamnose-D-galacturonic acid dimers with side chains of arabinose, galactose and arabinogalactan residues.…”

“…38,39 Once released, mucilage is characterized by a denser, relatively insoluble, inner layer and a more soluble outer layer, composed of sparsely branched rhamnogalacturonan I (RGI), a polysaccharide formed by succession of L-rhamnose-D-galacturonic acid dimers with side chains of arabinose, galactose and arabinogalactan residues. 36,37,40 Due to its composition, it can be considered as a model to study pectin biosynthesis. Unlike in other plant tissues, mucilage can be easily extracted from Arabidopsis seeds and Arabidopsis plants can tolerate the absence of mucilage under laboratory conditions.…”

“…45 A sub-pathway whose expression is at least partly positively regulated by MUM1 (also known as LUH or Leunig Homolog 46,47 ) has a key role in mucilage release and/or modification. This sub-pathway comprises MUM2 (also known as BGAL6 or beta-galactosidase 6 36,48 ) as well as BXL1 and SBT1.7 and is required for pectin modification. 46,[49][50][51] MUM4 is clearly responsible for mucilage biosynthesis as cloning of the underlying gene revealed this to be RHM2 (rhamnose biosynthesis 2) which codes for an UDP-L-rhamnose synthase.…”