ABSTRACT
The role of the margin in leaf development has been debated over a number of years. To investigate the molecular basis of events in the margin, we performed an enhancer trap screen to identify genes specifically expressed in this tissue. Analysis of one of these lines revealed abnormal differentiation in the margin, accompanied by an abnormal leaf size and shape. Further analysis revealed that this phenotype was due to insertion of the trap into DWF4, which encodes a key enzyme in brassinolide biosynthesis. Transcripts for this gene accumulated in a specific and dynamic pattern in the epidermis of young leaf primordia. Targeted expression of DWF4 to a subset of these cells (the leaf margin) in a dwf4 mutant background led to both restoration of differentiation of a specific group of leaf cells (margin cells) and restoration of wild-type leaf shape (but not leaf size). Ablation of these cells led to abrogation of leaf development and the formation of small round leaves. These data support the hypothesis that events in the margin play an essential role in leaf morphogenesis, and implicate brassinolide in the margin as a key mediator in the control of leaf shape, separable from a general function of this growth factor in the control of organ size.
Subject(s)
Arabidopsis Proteins/genetics , Arabidopsis/genetics , Cytochrome P-450 Enzyme System/genetics , Gene Expression Regulation, Plant , Plant Leaves/genetics , Arabidopsis/growth & development , Arabidopsis/ultrastructure , Arabidopsis Proteins/physiology , Brassinosteroids , Cholestanols/metabolism , Cytochrome P-450 Enzyme System/physiology , In Situ Hybridization , Microscopy, Confocal , Microscopy, Electron, Scanning , Microscopy, Electron, Transmission , Phenotype , Plant Leaves/growth & development , Plant Leaves/ultrastructure , Plants, Genetically Modified , Reverse Transcriptase Polymerase Chain Reaction , Seedlings/genetics , Seedlings/growth & development , Seedlings/metabolism , Steroids, Heterocyclic/metabolismABSTRACT
Stabilization of beta-catenin by inhibiting the activity of glycogen synthase kinase-3beta has been shown to initiate axis formation or axial patterning processes in many bilaterians. In hydra, the head organizer is located in the hypostome, the apical portion of the head. Treatment of hydra with alsterpaullone, a specific inhibitor of glycogen synthase kinase-3beta, results in the body column acquiring characteristics of the head organizer, as measured by transplantation experiments, and by the expression of genes associated with the head organizer. Hence, the role of the canonical Wnt pathway for the initiation of axis formation was established early in metazoan evolution.
Subject(s)
Head/growth & development , Hydra/growth & development , Hydra/metabolism , Intercellular Signaling Peptides and Proteins/metabolism , Signal Transduction , Animals , Benzazepines/pharmacology , Cell Cycle/drug effects , Cell Membrane/drug effects , Cell Membrane/metabolism , Cell Nucleus/drug effects , Cell Nucleus/metabolism , Cytoskeletal Proteins/metabolism , Glycogen Synthase Kinase 3/antagonists & inhibitors , Glycogen Synthase Kinase 3/genetics , Glycogen Synthase Kinase 3/metabolism , Glycogen Synthase Kinase 3 beta , Head/physiology , Hydra/cytology , Hydra/drug effects , Indoles/pharmacology , Intercellular Signaling Peptides and Proteins/genetics , Intracellular Signaling Peptides and Proteins/pharmacology , Signal Transduction/drug effects , Trans-Activators/metabolism , Wnt Proteins , beta CateninABSTRACT
Developmental gradients play a central role in axial patterning in hydra. As part of the effort towards elucidating the molecular basis of these gradients as well as investigating the evolution of the mechanisms underlying axial patterning, genes encoding signaling molecules are under investigation. We report the isolation and characterization of HyBMP5-8b, a BMP5-8 orthologue, from hydra. Processes governing axial patterning are continuously active in adult hydra. Expression patterns of HyBMP5-8b in normal animals and during bud formation, hydra's asexual form of reproduction, were examined. These patterns, coupled with changes in patterns of expression in manipulated tissues during head regeneration, foot regeneration as well as under conditions that alter the positional value gradient indicate that the gene is active in two different processes. The gene plays a role in tentacle formation and in patterning the lower end of the body axis.