Analysis of GL3 and TTG1 Function in Trichome Development

理解细胞命运决定的过程是理解生物体个体发生的基础。该项目的长期目标是了解细胞命运是如何在发育中的植物表皮上决定和协调的。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开关点才能进入成熟的组织系统。其中一个开关决定细胞是否会变成毛状细胞。在拟南芥中，遗传学研究表明TTG1、GL1和GL3在调节这种细胞命运决定中起重要作用。最近的初步工作已经确定GL3编码一种碱性螺旋-环-螺旋（bHLH） myc型蛋白，与玉米花青素调节因子R同源。与R过表达非常相似，GL3过表达导致野生型植物产生多余的毛状体，并抑制TTG1（一种含有WD40重复序列的蛋白）的突变。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才能发挥全部功能。与R一样，GL3也会与GL1相互作用，GL1是一种myb型蛋白，在两者都过表达的转基因植物中。酵母2杂交研究表明，GL3通过bHLH结构域外的氨基区与GL1相互作用，GL3通过单独的氨基区与TTG1相互作用。然而，GL1不与TTG1相互作用。GL3也可以通过bHLH区域与其他bHLH蛋白进行同二聚和异二聚。为了验证在2-杂交分析中发现的蛋白-蛋白相互作用如何介导TTG1对GL3的调控，进一步的工作将集中在确定TTG1和GL3在野生型和突变型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突变产生的毛状体比野生型少，后者的毛状体发育不太好。除了GL3，在拟南芥中至少还有一个高度同源的bHLH基因在过表达时能够抑制ttg1，并产生与GL3相同的相互作用。遗传学实验将用于确定GL3是否部分冗余位点或GL3突变不是零。了解细胞命运的决定是如何被调节的，对于理解一般的发育是至关重要的，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.