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Molecular genetic analysis of seed coat mucilage mutants of arabidopsis thaliana Huang, Jun


During differentiation, the Arabidopsis seed coat epidermal cells produce copious amounts of mucilage that is extruded from the seed coat upon imbibition. Mucilage is composed primarily of pectin, a polysaccharide that is a main component of the cell wall. For this reason, the Arabidopsis seed coat is a good system for studying the biosynthesis, secretion and modification of pectin. Mutants with mucilage defects can be used to identify genes involved in the production of pectin. Mucilage-Modified mutants, including mum1, mum2 and mum4, were identified using screens of EMS mutagenized plants. Both mum1 and mum2 lack the ability to release the mucilage when mature seeds are imbibed. MUM2 encodes a β-galactosidase that modifies the mucilage structure in the apoplast. I have cloned the MUM1 gene and shown it to encode a putative transcription factor LEUNIG_HOMOLOG (LUH). Cellular localization and transcriptional assay results suggest that LUH/MUM1 is a nuclear-localized, transcriptional activator. LUH/MUM1 is expressed in all the tissues examined including the seed coat. qRT PCR data suggest that LUH/MUM1 is expressed throughout seed coat development, reaching peak expression late in differentiation. MUM2 expression in the luh/mum1 mutant was reduced dramatically, relative to that of wild type. Over-expression of MUM2 could partially rescue the mum1 phenotype. These data suggest that LUH/MUM1 is a positive regulator of MUM2. qRT PCR data revealed a similar expression level of LUH/MUM1 in wild type compared to plants homozygous for mutations in several genes encoding regulators of seed coat mucilage, namely APETALA2, TRANSPARENT TESTA GLABRA1 (TTG1), TTG2 and GLABRA2. Thus the LUH/MUM1-MUM2 regulatory pathway appears to be independent of other transcription factors known to regulate aspects of seed coat mucilage biology. Mutations in the MUM4 gene result in seeds that release little mucilage. A mum4 mutant was mutagenized and resulting M2 progeny screened for modifier mutants. Ten enhancers (mum4 enhancer (men)) and ten suppressors (mum4 suppressors (msu)) mutants were isolated and partially characterized genetically and phenotypically. Further studies are needed to characterize these mutants.

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