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...

PECTIN METHYLESTERASE48 Is Involved in Arabidopsis Pollen Grain Germination

“…The mucilage modified2 (mum2) mutant (impaired in the expression of a b-galactosidase) failed to properly hydrate the seed mucilage (Macquet et al, 2007). Interestingly, in the mucilage of mum2, the content of methylesterified HG increased compared with the wild type, suggesting that (1) b-galactosidase may be required for PME activity (Western et al, 2001;Walker et al, 2011) and (2) PME activity is required for the hydration of HGs. This observation may explain why, in the pme482/2 mutant lines, the imbibition and germination of pollen grains were strongly delayed, as the DM of the HG in the intine of pme482/2 is almost 2.5-fold higher than in the wild type.…”

MYB5 and MYB14 Play Pivotal Roles in Seed Coat Polymer Biosynthesis in Medicago truncatula

“…To test whether MtMYB5 is also involved in mucilage biosynthesis in M. truncatula we stained the myb5 mutant seeds with Ruthenium Red, a dye used to detect mucilage (Western et al, 2001). Similar to the Arabidopsis myb5 mutant, M. truncatula myb5 mutant seeds exhibited much lighter Ruthenium Red staining than wild-type seeds, indicating that the mutant seeds accumulate much less mucilage in the seed coat ( Fig.…”

SALT-OVERLY SENSITIVE5 Mediates Arabidopsis Seed Coat Mucilage Adherence and Organization through Pectins

“…Seeds were germinated on plates with Arabidopsis medium (Haughn and Somerville, 1986) and 7% (w/v) agar, and seedlings were transferred to soil (Sunshine Mix 4; SunGro, Kelowna) after 7 d. Plants were grown with continuous fluorescent illumination of 80 to 140 mEm 22 s 21 at 20°C to 22°C. Developing seeds were staged as previously described (Western et al, 2001). All double mutants (cesa5-1 sos5-2, cesa5-1 mum2-1, sos5-2 mum2-1, cesa5-1 fly1-1, and sos5-2 fly1-2) were isolated from an F2 population (n = 72) from a cross between the two respective single-mutants parents.…”