Analysis of GL3 and TTG1 Function in Trichome Development

了解细胞命运的决定过程是了解有机体个体发育的基础。这个项目的长期目标是了解细胞命运的决定是如何在发育中的植物表皮上做出和协调的。在拟南芥叶片发育过程中，植物原胚层细胞必须经过有限数量的转换点才能形成成熟的组织系统。其中一个开关决定了一个细胞是否会成为毛发细胞。在拟南芥中，遗传学研究表明TTG1、GL1和GL3在调节细胞命运的决定中发挥重要作用。最近的初步工作已经确定GL3编码一个碱性-螺旋-环-螺旋(BHLH)myc类型的蛋白，与玉米的花青素调节因子R同源。就像R的过度表达一样，GL3的过度表达会导致野生型植物中多余的毛状体的产生，并将抑制TTG1的突变，TTG1是一种包含WD40重复序列的蛋白质。然而，与R不同的是，GL3是毛状体缺陷的弱抑制因子，这表明GL3通常需要TTG1来实现完整的功能。同样像R一样，GL3将在转基因植物中与GL1相互作用，GL1是一种MYB类型的蛋白，两者都过度表达。酵母双杂交研究表明，GL3通过bHLH域以外的氨基区域与GL1相互作用，GL3通过单独的氨基区域与TTG1相互作用。然而，GL1不与TTG1相互作用。GL3还可以通过bHLH区域与其他bHLH蛋白发生同源二聚和异源二聚。为了验证2-杂交分析中发现的蛋白质-蛋白质相互作用如何介导TTG1对GL3的调节的假说，进一步的工作将集中在确定TTG1和GL3在野生型和突变植物中的表达模式和亚细胞位置。虽然ttg1-1和GL1-1突变几乎完全没有毛状体，但GL3突变形成的毛状体比发育较差的野生型要少。除了GL3，在拟南芥中至少还有一个高度同源的bHLH基因，它能够在过表达时抑制ttg1，并与GL3进行相同的相互作用。基因实验将被用来试图确定GL3是否是部分冗余的基因座，或者GL3突变不是零。理解细胞命运决定是如何调控的，是理解一般发育的基础，目前的研究将增加这一领域迅速扩大的知识库。

Understanding the process of cell-fate determination is fundamental to understanding the ontogeny of an organism. The long term goal of this project is to understand how cell-fate decisions are made and coordinated on the developing plant epidermis. During the process of Arabidopsis leaf development, plant protodermal cells must pass through a limited number of switch points on the way to a mature tissue system. One of those switches determines whether a cell will become a trichome (hair) cell. In Arabidopsis thaliana, genetic studies indicate important roles for TTG1, GL1 and GL3 in regulating this cell-fate decision. Recent preliminary work has determined that GL3 encodes a basic-Helix-Loop-Helix (bHLH) myc-type protein with homology to the anthocyanin regulator, R, from maize. Much like R overexpression, GL3 overexpression leads to supernumerary trichome production in wild type plants and it will suppress mutations in TTG1, a WD40 repeat containing protein. However, unlike R, GL3 is a weak suppressor of the trichome defect indicating that GL3 normally requires TTG1 for full functionality. Also like R, GL3 will interact with GL1, a myb-type protein, in transgenic plants overexpressing both. Yeast 2-hybrid studies indicate that GL3 interacts with GL1 through its amino region outside of the bHLH domain and that GL3 interacts with TTG1 through a separate amino region domain. However, GL1 does not interact with TTG1. GL3 can also homodimerize and heterodimerize with other bHLH proteins through the bHLH region. In order to test hypotheses about how protein-protein interactions uncovered in the 2-hybrid analysis may mediate the regulation of GL3 by TTG1, further work will focus on defining the expression patterns and subcellular locations of TTG1 and GL3 in wild type and mutant plants. While the ttg1-1 and gl1-1 mutations are almost completely devoid of trichomes, the gl3 mutations make fewer trichomes than wild type that are less well developed. In addition to GL3, there is at least one other highly homologous bHLH gene in Arabidopsis that is able to suppress ttg1 when overexpressed and make the same interactions as GL3. Genetic experiments will be employed to try to determine whether GL3 is a partially redundant locus or that the gl3 mutations are not null. The understanding of how cell-fate decisions are regulated is fundamental to the understanding of development in general and the present studies will add to the rapidly expanding knowledge-base in this area.