Not surprisingly, there are mutants defective in floral evocation, and their study helps us
understand some of the molecular requirements for floral evocation. TheConstans(CO)
gene fromArabidopsisencodes azinc-finger transcription factorwhose mRNA levels
rise and fall with a circadian rhythm. CO turns on a number of genes, including
FLOWERING LOCUS T(FT), a gene known to also be involved in floral evocation. The
FT protein binds to and activates other transcriptional regulators such as FD and LEAFY
in the nucleus of the meristem cells. FD and LEAFY are considered to be master switches
that “turn on” expression of genes needed for flowering. Thus, CO protein accumulation,
controlled by the circadian rhythm, make trigger a cascade of events that results in flower-
ing. More recent studies also indicate that increased CO protein expressed only in the leaves
of transgenic plants can stimulate early flowering inArabidopsis. As mentioned, LEAFY is
a transcription factor involved in the switch from the inflorescence to floral meristem.Leafy
mutants have a delay in floral meristem development and flowers are replaced by leaflike or
flowerlike shoots, suggesting that the function of LEAFY is to promote floral meristem
identity. Indeed, ectopic expression of LEAFY in transgenic aspen trees can speed up
the flowering process in these trees, presumably by promoting floral meristem identity.
Another important floral meristem mutant containing the opposite phenotype is theterm-
inal flower(tfl) mutant. Thesetflmutants flower early and have a determinate inflorescence
which means that the inflorescence meristem is transformed into a terminal flower. Thus the
function of the TFL protein is to promote inflorescence identity (Krizek and Fletcher 2005;
Bernier and Perilleux 2005; Corbesier and Coupland 2006).
4.5.2 Floral Organ Identity and the ABC Model
After floral evocation and development of a floral meristem committed to the process of
flowering, the individual organs present in the flower develop. A flower consists of four
concentric whorls containing flower organs that in most dicots like Arabidopsis are
arranged this way: sepals(Se),petals(P), stamens(St), andcarpels (C) (Fig. 4.8).
Sometimes, one of the whorls is not well developed or is repeated (like the petals in a
tea rose), or sometimes one whorl is dominant so that the rest of the organs are not notice-
able. On closer inspection, however, one can usually distinguish the four types of organs.
Figure 4.8.Flower development.Arabidopsis(a) wild-type, (b)ap2,(c)pi,(d)ag, and (e)sep
flowers. Below each photo is a rendering of the ABC model as it functions in that flower. [Reprinted
from Krizek and Fletcher (2005), with permission from Nature Publishing.] See color insert.
4.5. FLOWER DEVELOPMENT 99