title: Helix-loop-helix/basic helix-loop-helix transcription factor network represses cell elongation in Arabidopsis through an apparent incoherent feed-forward loop.
abstract: Cell elongation is promoted by different environmental and hormonal signals, involving light, temperature, brassinosteroid (BR), and gibberellin, that inhibit the atypical basic helix-loop-helix (bHLH) transcription factor INCREASED LEAF INCLINATION1 BINDING bHLH1 (IBH1). Ectopic accumulation of IBH1 causes a severe dwarf phenotype, but the cell elongation suppression mechanism is still not well understood. Here, we identified a close homolog of IBH1, IBH1-LIKE1 (IBL1), that also antagonized BR responses and cell elongation. Genome-wide expression analyses showed that IBH1 and IBL1 act interdependently downstream of the BRASSINAZOLE-RESISTANT1 (BZR1)-PHYTOCHROME-INTERACTING FACTOR 4 (PIF4)-DELLA module. Although characterized as non-DNA binding, IBH1 repressed direct IBL1 transcription, and they both acted in tandem to suppress the expression of a common downstream helix-loop-helix (HLH)/bHLH network, thus forming an incoherent feed-forward loop. IBH1 and IBL1 together repressed the expression of PIF4, known to stimulate skotomorphogenesis synergistically with BZR1. Strikingly, PIF4 bound all direct and down-regulated HLH/bHLH targets of IBH1 and IBL1. Additional genome-wide comparisons suggested a model in which IBH1 antagonized PIF4 but not the PIF4-BZR1 dimer.
authors: Department of Plant Systems Biology, VIB, B-9052 Ghent, Belgium.
authors: Miroslava K
authors: MK
authors: Zhiponova
authors: Kengo
authors: K
authors: Morohashi
authors: Isabelle
authors: I
authors: Vanhoutte
authors: Katja
authors: K
authors: Machemer-Noonan
authors: Miglena
authors: M
authors: Revalska
authors: Marc
authors: M
authors: Van Montagu
authors: Erich
authors: E
authors: Grotewold
authors: Eugenia
authors: E
authors: Russinova
mesh_terms: D017360:Arabidopsis / Q000166:cytology / Q000254:growth & development*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D001483:Base Sequence; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism*; D049108:Cell Enlargement; D047369:Chromatin Immunoprecipitation; D017931:DNA Primers / Q000235:genetics; D005453:Fluorescence; D020869:Gene Expression Profiling; D053263:Gene Regulatory Networks / Q000235:genetics / Q000502:physiology*; D008954:Models, Biological; D008969:Molecular Sequence Data; D009024:Morphogenesis / Q000502:physiology*; D036226:Seedlings / Q000254:growth & development; D017423:Sequence Analysis, RNA; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D017931:DNA Primers; C582892:IBH1 protein, Arabidopsis; C587818:IBL1 protein, Arabidopsis; C459466:PIF4 protein, Arabidopsis
keywords: development; growth; plant