Isolation and Characterization of Mutants Defective in Seed Coat Mucilage Secretory Cell Development in Arabidopsis

Abstract

In Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type producing extracellular pectinaceous mucilage and a volcano-shaped secondary cell wall. Differentiation involves a regulated series of cytological events including growth, cytoplasmic rearrangement, mucilage synthesis, and secondary cell wall production. We have tested the potential of Arabidopsis seed coat epidermal cells as a model system for the genetic analysis of these processes. A screen for mutants defective in seed mucilage identified five novel genes (MUCILAGE-MODIFIED [MUM]1-5). The seed coat development of these mutants, and that of three previously identified ones (TRANSPARENT TESTA GLABRA1, GLABRA2, and APETALA2) were characterized. Our results show that the genes identified define several events in seed coat differentiation. Although APETALA2 is needed for differentiation of both outer layers of the seed coat, TRANSPARENT TESTA GLABRA1, GLABRA2, and MUM4 are required for complete mucilage synthesis and cytoplasmic rearrangement. MUM3 and MUM5 may be involved in the regulation of mucilage composition, whereas MUM1 and MUM2 appear to play novel roles in post-synthesis cell wall modifications necessary for mucilage extrusion.Fertilization of the angiosperm ovule not only results in the development of the embryo and endosperm, but also initiates differentiation of the ovule integuments to form the seed coat. The seed coat consists of multiple specialized cell layers that play important roles in embryo protection and the regulation of germination. One specialization is known as myxospermy, a property of epidermal cells whereby they produce large quantities of pectic polysaccharide (mucilage; Frey-Wyssling, 1976; Grubert, 1981; Boesewinkel and Bouman, 1995). Myxospermy is commonly found in species of the Brassicaceae, Solanaceae, Linaceae, and Plantaginaceae, where mucilage forms a gel-like capsule surrounding the seed upon imbibition. Proposed roles for mucilage include facilitating seed hydration and/or dispersal. Mucilages are also found in the root cap and transmitting tract (Frey-Wyssling, 1976; Esau, 1977), where they foster root tip and pollen tube growth, respectively.Mucilages are largely composed of pectins, a heterogeneous group of complex, acidic polysaccharides that also comprise the majority of the plant cell wall matrix. Dicotyledonous pectins largely consist of poly-GalUA (PGA) and rhamnogalacturonan I (RG I; Brett and Waldron, 1990; Carpita and Gibeaut, 1993; Cosgrove, 1997). PGA is composed of an unbranched chain of ␣1,4-linked GalUA residues, whereas RG I is a highly branched polysaccharide with a backbone of alternating ␣1,4-linked GalUA and ␣1,2-linked rhamnose (Rha), with sugar side chains attached to the Rha residues (Brett and Waldron, 1990). The degree of gelling of pectins is largely dependent on ionic bonding between PGA molecules and free divalent calcium. Thus, cell wall fluidity is affected by the degree of methyl esterification of...

Cortical microtubules mark the mucilage secretion domain of the plasma membrane in Arabidopsis seed coat cells

“…Ruthenium red was used as in Western et al (2001) to qualitatively compare the volume of mucilage extruded from seeds, as it stains negative charges (i.e., carboxyl groups of pectin in mucilage). Wild-type and mor1-1 seeds grown at both permissive and restrictive temperatures were examined.…”

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

“…This is in line with what has been observed for several mutants such as mum3, mum5 and sbt1. 7,45,51 where treatment with metal chelators promoted the mucilage release as in our mutants (Fig. 5J, K and L).…”

“…50 Finally, mum3 and mum5 mutants show mucilage with altered composition. 45 It is noteworthy to observe that among this collection of cloned loci, only a few have been associated with the early biosynthesis steps of seed coat mucilage, but much more has been discovered about the upstream signaling cascades (Fig. 1).…”

“…44 Furthermore, through screening of mucilage-defective mutants, five ''MUcilage-Modified'' (MUM) loci have been identified, which seem to act specifically in certain steps of mucilage production and release. 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.…”

Diverse roles of Groucho/Tup1 co-repressors in plant growth and development

“…24,25 This extrusion defect was not caused by a failure to produce mucilage, as examination of developing luh seeds revealed normal mucilage synthesis and secretion by the epidermal cells of the seed coat. 25,28 However, examination of mucilage release following chemical weakening of the cell wall of the luh seed coat clearly demonstrated a reduced capacity to swell. As swelling of the mucilage generates the force necessary to rupture the epidermal cell wall of the seed coat, it is likely that the altered hydration properties of luh mucilage account for its failure to be released from the seed.…”