ABSTRACT
In indeterminate inflorescences, floral meristems develop on the flanks of the shoot apical meristem, at positions determined by auxin maxima. The floral identity of these meristems is conferred by a handful of genes called floral meristem identity genes, among which the LEAFY (LFY) transcription factor plays a prominent role. However, the molecular mechanism controlling the early emergence of floral meristems remains unknown. A body of evidence indicates that LFY may contribute to this developmental shift, but a direct effect of LFY on meristem emergence has not been demonstrated. We have generated a LFY allele with reduced floral function and revealed its ability to stimulate axillary meristem growth. This role is barely detectable in the lfy single mutant but becomes obvious in several double mutant backgrounds and plants ectopically expressing LFY. We show that this role requires the ability of LFY to bind DNA, and is mediated by direct induction of REGULATOR OF AXILLARY MERISTEMS1 (RAX1) by LFY. We propose that this function unifies the diverse roles described for LFY in multiple angiosperm species, ranging from monocot inflorescence identity to legume leaf development, and that it probably pre-dates the origin of angiosperms.
Subject(s)
Arabidopsis Proteins/genetics , Arabidopsis/genetics , Gene Expression Regulation, Developmental , Meristem/genetics , Transcription Factors/genetics , Alleles , Arabidopsis/growth & development , Arabidopsis/metabolism , Arabidopsis Proteins/chemistry , Arabidopsis Proteins/metabolism , Crystallography , DNA-Binding Proteins , Flowers/genetics , Flowers/growth & development , Flowers/metabolism , Gene Expression Regulation, Plant , Meristem/growth & development , Meristem/metabolism , Models, Biological , Mutation , Nucleotide Motifs , Plant Leaves/genetics , Plant Leaves/growth & development , Plant Leaves/metabolism , Plants, Genetically Modified , Protein Multimerization , Protein Structure, Tertiary , Seedlings/genetics , Seedlings/growth & development , Seedlings/metabolism , Transcription Factors/chemistry , Transcription Factors/metabolism , Two-Hybrid System TechniquesABSTRACT
Axillary meristems form in the leaf axils during post-embryonic development. In order to initiate the genetic dissection of axillary meristem development, we have characterized the late-flowering branchless ecotype of Arabidopsis thaliana (L.) Heynh., Zu-0. The first-formed rosette leaves of Zu-0 plants all initiate axillary meristems, but later-formed leaves of the rosette remain branchless. Alteration in the meristem development is axillary meristem-specific because the shoot apical and floral meristems develop normally. Scanning electron microscopy, histology and RNA in situ analysis with SHOOTMERISTEMLESS ( STM), a marker for meristematic tissues, show that a mound of cells form and STM mRNA accumulates in barren leaf axils, indicating that axillary meristems initiate but arrest in their development prior to organizing a meristem proper. Expression and retention of the STM RNA in barren leaf axils further suggests that STM expression is not sufficient for the establishment of the axillary meristem proper.