The Identification of Novel Genes Involved in the Synthesis, Secretion and Modification of Cell Wall Components in the Seed Coat of Arabidopsis thaliana.

摘要

Plant cells are encased within a complex polysaccharide wall that not only strengthens the plant, but also has key roles in plant growth, cell differentiation, and defence. The plant cell wall is comprised of a network of cellulose microfibrils interconnected by hemicelluloses; this framework is embedded in a more soluble pectin matrix. This dynamic structure is under continual modification during plant growth and development, and its synthesis and modification requires the activity of a myriad of enzymes. Recent research has provided insight into how plants manufacture and regulate the cell wall during development, but much remains unknown. The mucilage secretory cells (MSCs) of Arabidopsis thaliana are used as a model to discover novel genes involved in the synthesis, secretion and modification of cell wall components, particularly pectin. These cells synthesize copious amounts of pectinaceous mucilage during development and, upon hydration of the desiccated seed, this mucilage rapidly swells, bursts from the MSCs and surrounds the seed in a gelatinous capsule. The patchy (pty)/beta-xylosidase1(bxl1) mutant has a peculiar phenotype where mucilage release is patchy and slow, and mutant seeds are delayed in germination. Cloning of the mutant locus revealed a lesion in an encoded bifunctional beta-xylosidase/alpha-arabinofuranosidase. Chemical and immunological analyses indicate an increase in 1,5-linked arabinans, suggesting the action of AtBXL1 is required for the trimming of these chains to allow correct mucilage release. In addition to the study of AtBXL1, an enhancer/suppressor screen of the mum4 reduced mucilage mutant was performed in order to identify novel genes involved in mucilage secretory cell differentiation. The screen identified six novel mutants named mum enhancer (men) 1-6. Characterization of men mum4 double mutants revealed two distinct groups, those that produced similar amounts of mucilage to mum4 but failed to release it (men2, 6), and those that produced a further reduced amount of mucilage compared to mum4 (men1, 3, 4, 5). Of these, the men4 mutant was chosen for additional study as it had a single mutant phenotype of reduced mucilage. Characterization of men4 seed development, chemical analysis of men4 mucilage, and the use of pectin-specific antibodies suggest that MEN4 has a role in mucilage synthesis or secretion. Molecular mapping places the MEN4 locus on the upper arm on chromosome 5, away from known mucilage production or cell wall-related genes. Finally, the role of mucilage during seed germination was investigated using a series of mucilage mutants affected in either mucilage production or release upon seed hydration. Preliminary results indicate that the production of mucilage may be more important than its release in the promotion of germination.