ABSTRACT
Plants, similarly to animals, form polarized axes during embryogenesis on which cell differentiation and organ patterning programs are orchestrated. During Arabidopsis embryogenesis, establishment of the shoot and root stem cell populations occurs at opposite ends of an apical-basal axis. Recent work has identified the PLETHORA (PLT) genes as master regulators of basal/root fate, whereas the master regulators of apical/shoot fate have remained elusive. Here we show that the PLT1 and PLT2 genes are direct targets of the transcriptional co-repressor TOPLESS (TPL) and that PLT1/2 are necessary for the homeotic conversion of shoots to roots in tpl-1 mutants. Using tpl-1 as a genetic tool, we identify the CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors as master regulators of embryonic apical fate, and show they are sufficient to drive the conversion of the embryonic root pole into a second shoot pole. Furthermore, genetic and misexpression studies show an antagonistic relationship between the PLT and HD-ZIP III genes in specifying the root and shoot poles.
Subject(s)
Arabidopsis Proteins/metabolism , Arabidopsis/embryology , Arabidopsis/metabolism , Body Patterning/physiology , Transcription Factors/metabolism , Arabidopsis/genetics , Arabidopsis Proteins/antagonists & inhibitors , Arabidopsis Proteins/genetics , Body Patterning/genetics , Cell Differentiation/genetics , Cell Lineage/genetics , Gene Expression Regulation, Plant , Genes, Plant/genetics , Homeodomain Proteins , Leucine Zippers , Plant Roots/cytology , Plant Roots/embryology , Plant Shoots/cytology , Plant Shoots/embryology , Stem Cells/cytology , Transcription Factors/antagonists & inhibitors , Transcription Factors/geneticsABSTRACT
The embryos of seed plants develop with an apical shoot pole and a basal root pole. In Arabidopsis, the topless-1 (tpl-1) mutation transforms the shoot pole into a second root pole. Here, we show that TPL resembles known transcriptional corepressors and that tpl-1 acts as a dominant negative mutation for multiple TPL-related proteins. Mutations in the putative coactivator HISTONE ACETYLTRANSFERASE GNAT SUPERFAMILY1 suppress the tpl-1 phenotype. Mutations in HISTONE DEACETYLASE19, a putative corepressor, increase the penetrance of tpl-1 and display similar apical defects. These data point to a transcriptional repression mechanism that prevents root formation in the shoot pole during Arabidopsis embryogenesis.
Subject(s)
Arabidopsis Proteins/physiology , Arabidopsis/embryology , Repressor Proteins/physiology , Alleles , Arabidopsis/genetics , Arabidopsis Proteins/genetics , Cell Polarity , Chromosome Mapping , Chromosomes, Plant , Gene Expression Regulation, Plant , Histone Deacetylases/genetics , Histone Deacetylases/physiology , Mutation , Plant Roots/embryology , Plant Shoots/embryology , Repressor Proteins/genetics , SeedsABSTRACT
The transition from vegetative growth to reproductive development in Arabidopsis is regulated by multiple floral induction pathways, including the photoperiodic, the autonomous, the vernalization, and the hormonal pathways. These pathways converge to regulate the expression of a small set of genes critical for floral initiation and different signal transduction pathways can interact to govern the time to flower. One important regulator of floral initiation is the MADS-box transcription factor FLC, which acts as a negative regulator of flowering in response to both endogenous and environmental signals. In this report, we describe a study of the flowering-time gene, FLK [flowering locus K homology (KH) domain] that encodes a putative RNA-binding protein with three KH domains. The flk mutations cause delayed flowering without a significant effect on the photoperiodic or vernalization responses. FLK functions primarily as a repressor of FLC expression, although it also modestly affects expression of genes associated with the photoperiodic pathway. In addition to FLK, the expression of two other KH domain genes are modestly affected by the flk mutation, suggesting a possible involvement of more than one KH domain protein in the regulation of flowering time in Arabidopsis.