Arabidopsis seed coats as a model system for genetic analysis of plant cell wall biosynthesis and modification

在拟南芥种皮表皮的分化过程中，细胞产生并分泌大量的粘液，主要由果胶组成。 随后，大量的次生细胞壁被合成。 当成熟种子暴露于水时，粘液膨胀以包裹种子，从而在发芽之前提供保护和水分。与许多专门的细胞类型不同，种皮表皮对于生存力不是必需的，允许分离在其分化的各个方面有缺陷的突变体。因此，拟南芥种皮代表了一个很好的模型系统的细胞壁生物发生的遗传解剖，特别是相对于果胶。我们一直在使用这个系统来识别和克隆编码粘液结构和功能所需的蛋白质的基因。 这些基因中的三个，MUM1，MUM2和FLY都是当暴露于水时从种皮中适当挤出粘液所必需的。 我们相信，研究这些基因的作用将有助于了解植物如何调节果胶产生生长和发育过程中所需的不同物理特性。 我们最近对MUM2基因的克隆表明，它编码一种水解酶，该酶在分泌后修饰细胞外的粘液，并且可能类似于果实成熟过程。MUM1编码一种转录因子，它可以通过调节MUM2等基因来控制粘液的挤出，MUM2是修饰粘液结构所必需的。 我们即将克隆FLY基因。我们建议1）继续分析所有三个基因及其蛋白产物的作用，2）更详细地研究种皮粘液的生化结构，3）分离和表征新一代种皮突变体。在完成这项工作后，我们希望更好地了解果胶结构以及如何对其进行修饰以改变其物理性质。 这项工作应该产生的信息和工具，需要在体内操纵果胶的结构和功能。

During differentiation of the Arabidopsis seed coat epidermis, a large amount of mucilage, comprised primarily of pectin, is produced and secreted from the cells. Subsequently an extensive secondary cell wall is synthesized. Upon exposure of mature seed to water, the mucilage expands to envelop the seed providing protection and moisture prior to germination. Unlike many specialized cell types the seed coat epidermis is not essential for viability, allowing for the isolation of mutants defective in various aspects of its differentiation. Thus, the Arabidopsis seed coat represents an excellent model system for genetic dissection of cell wall biogenesis, especially with respect to pectin. We have been using this system to identify and clone genes encoding proteins needed for mucilage structure and function. Three of these genes, MUM1, MUM2 and FLY are all required for proper mucilage extrusion from the seed coat when exposed to water. We believe that investigating the role of these genes will contribute to understanding how plants modulate pectin to produce different physical properties required during growth and development. Our recent cloning of the MUM2 gene indicates that it encodes a hydrolytic enzyme that modifies the mucilage outside the cell following secretion and may be analogous to processes involved in fruit ripening. MUM1 encodes a transcription factor that may control mucilage extrusion by regulating genes like MUM2 needed to modify mucilage structure. We are close to cloning the FLY gene. We propose here to 1) continue our analysis of the role of all three of the genes and their protein products, 2) study the biochemical structure of the seed coat mucilage in more detail and 3) isolate and characterize a new generation of seed coat mutants. Upon completion of this work we hope to better understand pectin structure and how it can be modified to alter its physical properties. This work should generate information and tools needed to manipulate pectin structure and function in vivo.