The Arabidopsis MUM2 Gene Encodes a β-Galactosidase Required for the Production of Seed Coat Mucilage with Correct Hydration Properties

Abstract

Seed coat development in Arabidopsis thaliana involves a complex pathway where cells of the outer integument differentiate into a highly specialized cell type after fertilization. One aspect of this developmental process involves the secretion of a large amount of pectinaceous mucilage into the apoplast. When the mature seed coat is exposed to water, this mucilage expands to break the primary cell wall and encapsulate the seed. The mucilage-modified2 (mum2) mutant is characterized by a failure to extrude mucilage on hydration, although mucilage is produced as normal during development. The defect in mum2 appears to reside in the mucilage itself, as mucilage fails to expand even when the barrier of the primary cell wall is removed. We have cloned the MUM2 gene and expressed recombinant MUM2 protein, which has b-galactosidase activity. Biochemical analysis of the mum2 mucilage reveals alterations in pectins that are consistent with a defect in b-galactosidase activity, and we have demonstrated that MUM2 is localized to the cell wall. We propose that MUM2 is involved in modifying mucilage to allow it to expand upon hydration, establishing a link between the galactosyl side-chain structure of pectin and its physical properties.

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

“…Consistent with previous studies (Penfield et al, 2001;Dean et al, 2007), the predominant linkages in the mucilage of both the wild type and atgatl5-1 are 2-linked Rha (2-Rhap) and 4-linked GalA (4-GalAp); both of these linkages decreased slightly in atgatl5-1 mucilage compared with the wild type (Table II). Consistent with the monosaccharide analysis data in Table I, no detectable changes in the glycosyl linkage composition of other sugars were found in atgatl5-1 mucilage.…”

“…CELLULOSE SYNTHASE5 (CESA5) was found to be involved in cellulose synthesis (Sullivan et al, 2011), while MUCILAGE-MODIFIED4/RHAMNOSE SYNTHESIS2 (MUM4/RHM2) encodes a UDP-Rha synthase (Usadel et al, 2004;Western et al, 2004;Oka et al, 2007). The mum2, atsbt1.7, atbxl1, pmei6, fly1, and per36 mutants were found to be defective in pectin modification (Dean et al, 2007;Macquet et al, 2007b;Rautengarten et al, 2008;Arsovski et al, 2009;Saez-Aguayo et al, 2013;Voiniciuc et al, 2013) or in the degradation of the outer cell wall of the outer integument (Kunieda et al, 2013), all of which show a mucilage-release defect.…”

“…Arabidopsis (Arabidopsis thaliana) is one of the plant species that produces myxospermous seed, where the epidermal cells of the seed coat accumulate a large quantity of mucilage that is composed mainly of relatively unbranched RG-I, which is occasionally substituted with 5-arabinans and nonreducing terminal galactosyl residues at the Rha O-3 position (Penfield et al, 2001;Western et al, 2004;Dean et al, 2007). 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).…”

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

“…Mucilage separates into two fractions: a loose nonadherent fraction and an inner adherent fraction that can only be released by vigorous shaking, strong bases, or glycosidases (for review, see North et al, 2014). Galactans and arabinans are also present in mucilage, and their regulation by glycosidases is required for correct mucilage hydration (Dean et al, 2007;Macquet et al, 2007b;Arsovski et al, 2009). For example, b-XYLOSIDASE1 encodes a bifunctional b-D-xylosidase/a-L-arabinofuranosidase required for arabinan modification in mucilage, and b-xylosidase1 mutant seeds have a delayed mucilage release phenotype (Arsovski et al, 2009).…”

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

“…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.…”