Intercellular communication during floral initiation and development - Intercellular Communication in Plants - page 185

and G. Coupland, unpublished results, 2004). GI regulates flowering-time at least in
part by the regulation of CO mRNA abundance; gi mutants contain less GI mRNA
(Suarez-Lopez et al. , 2001) while GI overexpressors show higher CO mRNA abun-
dance. The abundance of GI and CO mRNAs is circadian clock regulated. Under
days containing 16 h light, in which these genes promote early flowering, GI
mRNA abundance peaks around 10-12 h after dawn, whereas CO mRNA abun-
dance rises around 12 h after dawn and stays high throughout the night until the
following dawn (Fowler et al. , 1999; Park et al. , 1999; Suarez-Lopez et al. , 2001).
CO mRNA abundance is therefore high when plants are exposed to light at the end
of a long day. CO expression is also regulated at the post-transcriptional level, so
that the cryptochrome and phytochrome A photoreceptors act at the end of the day
to stabilize the CO protein (Valverde et al. , 2004), whereas in darkness the protein
is rapidly degraded, probably as a consequence of being ubiquitinated. Under short
days, the CO mRNA is expressed only in the dark, and so the protein would be
predicted never to accumulate. In agreement with these data, in wild-type plants
FT is activated by CO under long days, but not under short days (Suarez-Lopez
et al. , 2001; Yanovsky & Kay, 2002). Therefore, the combination of circadian clock
mediated regulation of CO mRNA abundance and stabilization of CO protein by
exposure to light can explain why CO promotes FT expression and thus flowering
only under long days.
The grafting experiments performed in Perilla and many other species indicated
that day length is perceived in the leaves. The observation that CO is a major part
of the molecular mechanism by which Arabidopsis discriminates between long and
short days suggests that CO may act in the leaf to regulate the transition to flowering
at the apex. The CO mRNA is present at very low abundance, but is expressed
widely. In situ hybridizations and RT-PCR detected the CO mRNA in the meristem,
young leaf primordia and whole seedling RNA. A more refined expression pattern
was identified using fusions of the CO promoter to the GUS marker gene (Takada
& Goto, 2003; An et al. , 2004). In CO:GUS plants, GUS expression was most
strongly detected in the phloem of cotyledons, leaves and stems, but also in the
protoxylem, young leaves and meristem (see Plate 7.1, following page 146). Several
recent observations suggest that CO acts in the vascular tissue and not the meristem
to promote flowering. The pattern of expression of the CO target gene, FT , has not
been described in wild-type plants, because of its low level of expression. However,
FT expression is increased in the early flowering terminal flower 2 ( tfl2 ) mutant, and
is present in the vascular tissue, suggesting that CO may activate its target gene in
these tissues (Takada & Goto, 2003). Consistent with this conclusion, FT expression
was reduced in tfl2 co-2 plants compared to tfl2 mutants. In an independent approach,
expression of CO from heterologous promoters specific to the phloem, such as that
of the SUCROSE TRANSPORTER 2 ( SUC2 ) gene, complemented the co-2 mutation,
butexpression of CO from meristem-specific promoters had no effect on flowering
(An et al. , 2004). Therefore, CO appears to act in the vascular tissue to regulate the
synthesis or transport of a long-distance signal that initiates floral development at
the apex.