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

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

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.