Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 182

conditions accelerates flowering (Blazquez et al. , 2003; Cerdan & Chory, 2003).
The genes identified by mutagenesis and by allelic variation between accessions
were placed in pathways based on genetic criteria and their effect on the response of
flowering-time to different environmental cues (Koornneef et al. , 1998). The major
features of this model were later confirmed by the cloning of the genes and analysis
of their expression patterns in wild-type and mutant plants (Mouradov et al. , 2002;
Simpson & Dean, 2002).
Within this model, four major pathways control flowering-time and converge to
regulate the expression of genes that integrate the information received from the
different pathways (Fig. 7.1). One pathway controls the response to day-length,
and specifically promotes flowering in response to long days (Hayama & Coupland,
2003; Yanovsky & Kay, 2003; Searle & Coupland, 2004). Mutations in this pathway
can either delay flowering under long days or accelerate flowering under short days.
The last gene that is specifically involved in this pathway is CONSTANS ( CO ),
which encodes a zinc finger protein that promotes transcription of downstream

Photoperiod
pathway

Autonomous
pathway

Gibberellin
pathway Vernalization

Cryptochromes
Phytochromes

FCA VIN3

PHYA
CRY2

GA

FLC

clock
LHY, CCA1
TOC1

GI

CO

FT
SOC1

PHYB PFT1

Vegetative
phase

Reproductive
phase

Figure 7.1 A network of four major pathways controls flowering-time in Arabidopsis . The
photoperiod pathway promotes flowering specifically under long days. The transcription of the GI and
CO genes is regulated by the circadian clock, whereas the photoreceptors PHYA, CRY2 and PHYB
regulate CO protein abundance. Flowering is also influenced by light quality; in particular, low ratios of
red to far-red light resembling the shaded conditions formed by vegetation promote flowering. This
response probably acts partly by stabilization of CO protein and partly independently of CO through
PFT1. The autonomous pathway negatively regulates the abundance of the mRNA of the floral
repressor FLC, and FCA is included as a representative of this pathway. Vernalization also promotes
flowering by repressing FLC mRNA levels and acts independently of the autonomous pathway.
VERNALIZATION INSENSITIVE 3 (VIN3) is shown as a representative of this pathway (Sung &
Amasino, 2004). Finally, gibberellin promotes flowering of Arabidopsis , particularly under short days.
All four pathways appear to converge on the transcriptional regulation of the FT and SOC1 genes,
which are often referred to as floral integrators and promote flowering. These pathways are described in
more detail in the text and in recent reviews (Michaels & Amasino, 2000; Mouradov et al. , 2002;
Simpson & Dean, 2002; Searle & Coupland, 2004).