Genetic Analysis of Early Flowering Mutants in Arabidopsis Defines a Class of Pleiotropic Developmental Regulator Required for Expression of the Flowering-Time Switch Flowering Locus C

Abstract

The Arabidopsis flowering-repressor gene FLOWERING LOCUS C (FLC) is a developmental switch used to trigger floral induction after extended growth in the cold, a process termed vernalization. In vernalized plants, FLC becomes transcriptionally silenced through a process that involves an epigenetic mechanism. We identified recessive mutations designated vernalization independence (vip) that confer cold-independent flowering and suppression of FLC. These mutations also lead to developmental pleiotropy, including specific defects in floral morphology, indicating that the associated genes also have functions unrelated to flowering time. We identified the VIP3 gene by positional cloning and found that it encodes a protein consisting almost exclusively of repeated Trp-Asp (WD) motifs, suggesting that VIP3 could act as a platform to assemble a protein complex. Constitutive transgenic expression of VIP3 in vernalized plants is insufficient to activate FLC, and thus VIP3 probably participates in the regulation of FLC as one component of a more extensive mechanism. Consistent with this, genetic analyses revealed that the VIP loci define a functional gene class including at least six additional members. We suggest that VIP3 and other members of this gene class could represent a previously unrecognized flowering mechanism.

Polymerase II–Associated Factor 1 Complex-Regulated FLOWERING LOCUS C-Clade Genes Repress Flowering in Response to Chilling

“…Expression of FLC -clade genes is epigenetically regulated by a number of histone modifiers that are recruited to these loci by Polymerase II-Associated Factor 1 Complex (PAF1C; Zhang and Van Nocker, 2002 ; Zhang et al, 2003 ; Oh et al, 2004 ). In Arabidopsis, PAF1C components include CELL DIVISION CYCLE73 (CDC73)/PLANT HOMOLOGOUS TO PARAFIBROMIN (PHP), VERNALIZATION INDEPENDENCE2 (VIP2)/EARLY FLOWERING7 (ELF7), VIP3, VIP4, VIP5, and VIP6/ELF8 ( Zhang and Van Nocker, 2002 ; Zhang et al, 2003 ; He, 2009 ; Yu and Michaels, 2010 ).…”

“…Expression of FLC -clade genes is epigenetically regulated by a number of histone modifiers that are recruited to these loci by Polymerase II-Associated Factor 1 Complex (PAF1C; Zhang and Van Nocker, 2002 ; Zhang et al, 2003 ; Oh et al, 2004 ). In Arabidopsis, PAF1C components include CELL DIVISION CYCLE73 (CDC73)/PLANT HOMOLOGOUS TO PARAFIBROMIN (PHP), VERNALIZATION INDEPENDENCE2 (VIP2)/EARLY FLOWERING7 (ELF7), VIP3, VIP4, VIP5, and VIP6/ELF8 ( Zhang and Van Nocker, 2002 ; Zhang et al, 2003 ; He, 2009 ; Yu and Michaels, 2010 ). Components of PAF1C interact with RNA Polymerase II ( Betz et al, 2002 ) and recruit H3K4 methyltransferase to their target genes, primarily in the 5′ transcribed regions, thereby leading to the active expression of their target genes ( Ng et al, 2003b ).…”

“…Genetic studies revealed that lesions in these PAF1C components resulted in nearly identical early flowering phenotypes under standard growth conditions ( He et al, 2004 ; Oh et al, 2004 ). PAF1C is required for the enrichment of active epigenetic marks, primarily H3K4me3, at the chromatin of FLC and its homologs to maintain their expression; PAF1C deficiency results in reduced expression of these floral repressors, which eventually accelerates flowering ( Zhang et al, 2003 ; He et al, 2004 ; Yu and Michaels, 2010 ). For instance, loss of function of ELF7 and VIP6 results in the reduced expression of FLC , FLM / MAF1 , and MAF2 ( He et al, 2004 ).…”

“…Mutations in VIP3 strongly reduced FLC expression and strongly accelerated flowering. In particular, vip3 mutants flowered significantly earlier than flc mutants, suggesting that additional genes are involved in the early flowering of vip3 mutants ( Zhang et al, 2003 ). Loss of function of VIP4 and VIP5 also resulted in strong flowering acceleration that was comparable to that seen in vip3 single mutants.…”

Sexual and Apogamous Species of Woodferns Show Different Protein and Phytohormone Profiles

“…In our work, a homologue of MATERNAL EFFECT EMBRYO ARREST 59 (MEE59) is up regulated in the apogamous gametophyte, but its biological function is unknown. Furthermore, we found TFL2, a chromo domain-containing protein LHP1, structural component of heterochromatin, and the transducin/WD40 repeat-like superfamily protein, which is a component of the PAF1 complex (PAF1C), involved in epigenetic gene repression of several floral homeotic genes, such as FLT, that regulates flowering time, and is required for maintenance of vernalization-induced repression of FLC (Kotake et al, 2003 ; Zhang et al, 2003 ; Xu and Shen, 2008 ). An interesting finding is the ortologue of the transcriptional regulation protein Methyl-CpG-binding domain-containing protein 10, required for nucleolar dominance that leads to the silencing of rRNA genes inherited from one progenitor in interspecific hybrids (Preuss et al, 2008 ).…”