75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

165 The Arabidopsis TALE Homeobox Gene ATH1 Controls Flowering Time by Regulating FLC
Levels
Bas Rutjens, Marco Brand, Sjef Smeekens, Marcel Proveniers
Molecular Plant Physiology, Utrecht University, Padualaan 8 3584 CH Utrecht, The Netherlands
Floral induction is controlled by a large number of genes acting in different pathways that either repress or promote
the reproductive transition at the shoot apical meristem. Here we show that altered expression of the light-regulated
ARABIDOPSIS THALIANA HOMEOBOX 1 gene (ATH1), a member of the BEL family, results in a flowering time
phenotype. We found that ATH1 specifically affects mRNA levels of FLOWERING LOCUS C (FLC), the most prominent
floral repressor in Arabidopsis. Basal levels of FLC are attenuated by ath1mutations resulting in early flowering.
Interestingly, constitutive ectopic expression (OE) of ATH1 does not increase FLC levels unconditionally. We noticed
that weak FLC alleles are most effectively upregulated by ATH1-OE as is the case in backgrounds that carry the strong
FLC activator FRIGIDA (FRI). Likewise, introduction of an ath1 mutation in plants containing dominant FRI and FLC
alleles partially suppresses the ability of FRI to increase FLC levels. Currently we are pinpointing the position of ATH1
in the web of flowering time genes.
166 Two Novel Cycling Dof (DNA binding with one finger) Transcription Factors are involved in
Flowering Time Regulation
Nicolas Sauerbrunn 1 , Takato Imaizumi 2 , Steve A. Kay 2 , Richard Amasino 3
1
Max Planck Institute for Plant Breeding, Carl-von-Linne Weg 10, D-50829 Cologne, Germany, 2 Institute,
10550 North Torrey Pines Road, La Jolla, California 92037, USA , 3 Department of Biochemistry University of
Wisconsin Madison, WI 53706-1544
We identified two novel DOF-domain proteins (CDF5 and CDF6) which are closely related to CDF1 (Cycling Dof-
Factor 1). A constitutive over-expression of CDF5 and CDF6 using the CaMV 35S-promoter leads to a late flowering
phenotype whereas T-DNA insertion alleles of CDF5 showed an early flowering phenotype in long day conditions.
The mRNA-levels of CDF5 and CDF6 remain rhythmic in constant light conditions indicating that both genes are
controlled by the circadian clock. Transgenic lines expressing CDF5 and CDF6-promoter:GUS constructs showed a
GUS-expression in the root and leaf vascular tissues and in the stomata which was also the site of expression for CDF1.
The CDF1-protein was shown to bind the CONSTANS (CO)-promoter and to act as a repressor of CO. The zinc-finger
protein CO is a key regulator for the photoperiodic flowering pathway. In a Yeast two Hybrid assay CDF5 and CDF6
interacted with the FKF1-kelch domain (FLAVIN-BINDING, KELCH REPEAT F-BOX 1) indicating that CDF5 and
CDF6 might be regulated by FKF1 at the protein level as has been shown for CDF1. FKF1 is thought to be a component
of a Skp1-Cullin-F-box (SCF) E3 ubiquitin ligase complex and was shown to be involved in the degradation of CDF1.
Our current data suggests that CDF5 and CDF6 are novel members of a transcription factor family which is redundantly
involved in the flowering time control in Arabidopsis thaliana.