Collaborative Research: Ligule development in the proximal-distal axis of the maize leaf

在作物驯化过程中，植物结构的改变至关重要。通过一代又一代的育种来提高产量，农作物已经选择了各种各样的分支模式，这些分支模式可以改变分支的数量、长度或角度。在玉米中，叶角是一个重要的分支性状，可能有助于提高玉米产量。叶片的角度是由叶舌区决定的，叶舌区形成于叶片和鞘的交界处。叶片向后倾斜，以吸收来自太阳的能量，而鞘紧紧抓住茎。叶片和叶鞘之间的角度是一种可遗传的性状，可以通过选择来改变：角度的减小使植株更直立，以适应更紧凑的田地，而角度的增加使叶片表面积优化，以获得更多的能量。因此，了解叶片角度形成的基本分子机制将为调控玉米结构提供新的工具。该项目旨在阐明决定舌区何时以及如何形成的复杂因素网络。该项目将首先确定和表征在发育的早期阶段定义舌部边界的基因的表达。分子、遗传和基因组的方法将被用来研究舌形区域是如何形成的。研究结果将与包括育种者、科学界和学生在内的不同受众分享。为此，将制作YouTube视频，向普通观众介绍玉米发展的科学，并为学生教室制作额外的教育材料。所有资源将通过实验室网站和玉米社区网站（http://www.maizegdb.org）向公众提供。本项目研究舌状区是如何在未分化的叶原基中建立一个明确的器官边界的。先前的工作使用解剖学上独特的舌叶作为转录组分析的位点：前叶叶、前叶和前鞘细胞被明确地鉴定为激光捕获，然后进行RNA测序。由于捕获的特异性，确定了一组候选基因，这些基因在舌叶区域独特而差异地表达。其他器官边界的共同表达模式表明舌叶重复发生在茎尖分生组织和穗枝边界的侧枝起始程序。因此，不同的器官边界被假设共享基因，并使用共同的机制来定义和限制发育途径。为了验证这一假设，将建立舌瓣发育早期阶段的基因网络，并进一步研究舌瓣起始的机制。第一个具体目标是通过捕获可获得的最年轻的叶原基细胞来确定叶舌形成的最早决定因素。候选基因将根据其在其他语言特异性突变体中的表达进行优先排序。第二个目的是研究支持knottted1 （KN1）样同源盒转录因子和伴侣蛋白在建立舌瓣边界中的作用的三个有希望的候选基因途径。突变体将分析这些基因的舌形和分支表型。第三个目的是验证由KN1结合的bZIP转录因子LIGULELESS2 （LG2）参与定位叶片/鞘边界的假设。蛋白质伴侣将被确定为下游目标，以确定LG2在建立叶片鞘边界中的作用。

Alteration in plant architecture has been critically important during crop domestication. Through generations of breeding to increase yield, crop plants have been selected for a variety of branching patterns that change the number, length or angle of branches. In maize, leaf angle has been an important branching trait that has likely contributed to increasing corn yields. The angle of the leaf is determined by the ligule region, which forms at the boundary between the blade and sheath. The blade tilts back to absorb energy from the sun while the sheath holds tight to the stem. This angle between the blade and sheath is a heritable trait that can be modified through selection: decreased angle makes plants more upright for more compact fields, whereas increased angle optimizes the surface area of the blade for more energy capture. Thus, understanding the basic molecular mechanisms involved in establishing the leaf angle will provide new tools for manipulating maize architecture. This project aims to clarify the complex network of factors that determine when and how the ligule region forms. This project will first identify and characterize the expression of genes that define the ligule boundary during the earliest stages of development. Molecular, genetic and genomic methods will then be used to investigate how the ligule region forms. The results will be shared with diverse audiences including breeders, the scientific community and with students. To that end, YouTube videos will be produced that describe the science of corn development to a general audience and additional educational materials will be generated for student classrooms. All resources will be made publically available through lab websites and the maize community website (http://www.maizegdb.org). This project investigates how the ligular region is established as a defined organ boundary in the undifferentiated leaf primordium. Previous work used the anatomically distinct ligule as a site for transcriptome analysis: preligule, preblade and presheath cells were clearly identified for laser capture followed by RNA sequencing. Due to the specificity of capture, a collection of candidate genes was identified that is uniquely and differentially expressed in the ligule region. Shared expression patterns in other organ boundaries suggest that the ligule reiterates the lateral organ initiation program that occurs at the shoot apical meristem and at tassel branch boundaries. Thus, distinctly different organ boundaries are hypothesized to share genes and use common mechanisms to define and restrict developmental pathways. To test this hypothesis, gene networks will be established for early stages of ligule development and mechanisms of ligule initiation further investigated. The first specific aim is to identify the earliest determinants of ligule formation by capturing cells from the youngest accessible leaf primordia. Candidate genes will be prioritized based on their expression in other ligule-specific mutants. The second aim investigates three promising candidate gene pathways that support the role of KNOTTED1 (KN1)-like homeobox transcription factors and partner proteins in establishing the ligule boundary. Mutants will be analyzed in these genes for ligule and branching phenotypes. The third aim tests the hypothesis that the bZIP transcription factor LIGULELESS2 (LG2), bound by KN1, is involved in positioning the blade/sheath boundary. Protein partners will be identified as will down-stream targets to determine the role of LG2 in establishing the blade sheath boundary.