MUCILAGE-MODIFIED4 Encodes a Putative Pectin Biosynthetic Enzyme Developmentally Regulated by APETALA2, TRANSPARENT TESTA GLABRA1, and GLABRA2 in the Arabidopsis Seed Coat1

The Arabidopsis seed coat epidermis undergoes a complex process of differentiation that includes the biosynthesis and secretion of large quantities of pectinaceous mucilage, cytoplasmic rearrangement, and secondary cell wall biosynthesis. Mutations in MUM4 (MUCILAGE-MODIFIED4) lead to a decrease in seed coat mucilage and incomplete cytoplasmic rearrangement. We show that MUM4 encodes a putative NDP-l-rhamnose synthase, an enzyme required for the synthesis of the pectin rhamnogalacturonan I, the major component of Arabidopsis mucilage. This result suggests that the synthesis of monosaccharide substrates is a limiting factor in the biosynthesis of pectinaceous seed coat mucilage. In addition, the reduced cytoplasmic rearrangement observed in the absence of a key enzyme in pectin biosynthesis in mum4 mutants establishes a causal link between mucilage production and cellular morphogenesis. The cellular phenotype seen in mum4 mutants is similar to that of several transcription factors (AP2 [APETALA2], TTG1 [TRANSPARENT TESTA GLABRA1], TTG2 MYB61, and GL2 [GLABRA2]). Expression studies suggest that MUM4 is developmentally regulated in the seed coat by AP2, TTG1, and GL2, whereas TTG2 and MYB61 appear to be regulating mucilage production through alternate pathway(s). Our results provide a framework for the regulation of mucilage production and secretory cell differentiation.

[1]  S. Clough,et al.  Floral dip: a simplified method for Agrobacterium-mediated transformation of Arabidopsis thaliana. , 1998, The Plant journal : for cell and molecular biology.

[2]  R. Last,et al.  Arabidopsis Map-Based Cloning in the Post-Genome Era , 2002, Plant Physiology.

[3]  J. Mendenhall,et al.  Arabidopsis seed coat development: morphological differentiation of the outer integument. , 2000, The Plant journal : for cell and molecular biology.

[4]  W. Ruhland.  Encyclopedia of plant physiology. , 1958 .

[5]  K. Adler,et al.  Ectopic expression of the Arabidopsis AtMYB23 gene induces differentiation of trichome cells. , 2001, Developmental biology.

[6]  C. Whitfield,et al.  Characterization of dTDP-4-dehydrorhamnose 3,5-Epimerase and dTDP-4-dehydrorhamnose Reductase, Required for dTDP-l-rhamnose Biosynthesis in Salmonella enterica Serovar Typhimurium LT2* , 1999, The Journal of Biological Chemistry.

[7]  Christopher P. Bonin,et al.  A bifunctional epimerase-reductase acts downstream of the MUR1 gene product and completes the de novo synthesis of GDP-L-fucose in Arabidopsis. , 2000, The Plant journal : for cell and molecular biology.

[8]  G. Haughn,et al.  AP2 Gene Determines the Identity of Perianth Organs in Flowers of Arabidopsis thaliana. , 1989, The Plant cell.

[9]  M. Bevan,et al.  MYB61 Is Required for Mucilage Deposition and Extrusion in the Arabidopsis Seed Coat Article, publication date, and citation information can be found at www.plantcell.org/cgi/doi/10.1105/tpc.010265. , 2001, The Plant Cell Online.

[10]  Michael Y. Galperin,et al.  The COG database: a tool for genome-scale analysis of protein functions and evolution , 2000, Nucleic Acids Res..

[11]  J. Dumville,et al.  Uronic acid-containing oligosaccharins : Their biosynthesis, degradation and signalling roles in non-diseased plant tissues , 2000 .

[12]  C. Whitfield,et al.  The crystal structure of dTDP-D-Glucose 4,6-dehydratase (RmlB) from Salmonella enterica serovar Typhimurium, the second enzyme in the dTDP-l-rhamnose pathway. , 2001, Journal of molecular biology.

[13]  T. Payne,et al.  GL 3 Encodes a bHLH Protein That Regulates Trichome Development in Arabidopsis Through Interaction With GL 1 and TTG 1 C . , 2000 .

[14]  Fan Zhang,et al.  A network of redundant bHLH proteins functions in all TTG1-dependent pathways of Arabidopsis , 2003, Development.

[15]  J. Giraudat,et al.  A Brassica napus transcript encoding a protein related to the Künitz protease inhibitor family accumulates upon water stress in leaves, not in seeds. , 1992, The Plant journal : for cell and molecular biology.

[16]  G. Vanzin,et al.  Molecular genetics of nucleotide sugar interconversion pathways in plants. , 2001 .

[17]  D. Smyth,et al.  TRANSPARENT TESTA GLABRA2, a Trichome and Seed Coat Development Gene of Arabidopsis, Encodes a WRKY Transcription Factor Article, publication date, and citation information can be found at www.plantcell.org/cgi/doi/10.1105/tpc.001404. , 2002, The Plant Cell Online.

[18]  R. Hellens,et al.  pGreen: a versatile and flexible binary Ti vector for Agrobacterium-mediated plant transformation , 2000, Plant Molecular Biology.

[19]  W. Willats,et al.  In-situ analysis of pectic polysaccharides in seed mucilage and at the root surface of Arabidopsis thaliana , 2001, Planta.

[20]  B. Ridley,et al.  Pectins: structure, biosynthesis, and oligogalacturonide-related signaling. , 2001, Phytochemistry.

[21]  B. Moffatt,et al.  Isolation and characterization of mutants defective in seed coat mucilage secretory cell development in Arabidopsis. , 2001, Plant physiology.

[22]  L. Staehelin,et al.  Domain-specific and cell type-specific localization of two types of cell wall matrix polysaccharides in the clover root tip , 1992, The Journal of cell biology.

[23]  L. Sturla,et al.  The metabolism of 6-deoxyhexoses in bacterial and animal cells. , 1998, Biochimie.

[24]  F. Bouman,et al.  The seed: structure and function , 1995 .

[25]  D. Feingold.  Aldo (and Keto) Hexoses and Uronic Acids , 1982 .

[26]  The Arabidopsis Genome Initiative.  Analysis of the genome sequence of the flowering plant Arabidopsis thaliana , 2000, Nature.

[27]  J. Schiefelbein.  Cell-fate specification in the epidermis: a common patterning mechanism in the root and shoot. , 2003, Current opinion in plant biology.

[28]  B. Usadel,et al.  RHM2 Is Involved in Mucilage Pectin Synthesis and Is Required for the Development of the Seed Coat in Arabidopsis , 2004, Plant Physiology.

[29]  G. Haughn,et al.  Differentiation of mucilage secretory cells of the Arabidopsis seed coat. , 2000, Plant physiology.

[30]  K. Feldmann,et al.  Agrobacterium-mediated transformation of germinating seeds of Arabidopsis thaliana: A non-tissue culture approach , 1987, Molecular and General Genetics MGG.

[31]  M. Koornneef.  The complex syndrome of ttg mutanis , 1981 .

[32]  G. Vanzin,et al.  Molecular genetics of nucleotide sugar interconversion pathways in plants , 2004, Plant Molecular Biology.

[33]  M. Van Montagu,et al.  Control of Arabidopsis flower and seed development by the homeotic gene APETALA2. , 1994, The Plant cell.

[34]  Fan Zhang,et al.  GL3 encodes a bHLH protein that regulates trichome development in arabidopsis through interaction with GL1 and TTG1. , 2000, Genetics.

[35]  Dirk Inzé,et al.  Histological Study of Seed Coat Development in Arabidopsis thaliana , 2000, Journal of Plant Research.