The Questions of Developmental Biology

dependent, vernalization-dependent, and autonomous pathways that regulate the floral transition
have been genetically dissected.
The ancestral angiosperm is believed to have formed a terminal flower directly from the terminal
shoot apex (Stebbins 1974). In modern angiosperms, a variety of flowering patterns exist in
which the terminal shoot apex is indeterminate, but axillary buds produce flowers. This
observation introduces an intermediate step into the reproductive process: the transition of a
vegetative meristem to an inflorescence meristem, which initiates axillary meristems that can
produce floral organs, but does not directly produce floral parts itself. The inflorescence is the
reproductive backbone (stem) that displays the flowers (see Figure 20.20). The inflorescence
meristem probably arises through the action of a gene that suppresses terminal flower formation.
The CENTRORADIALUS (CEN) gene in snapdragons suppresses terminal flower formation
(Bradley et al. 1996). It suppresses expression of FLORICAULA (FLO), which specifies floral
meristem identity. Curiously, the expression of FLO is necessary for CEN to be turned on. The
Arabidopsis homologue of CEN (TERMINAL FLOWER 1 or TFL1) is expressed during the
vegetative phase of development as well, and has the additional function of delaying the
commitment to inflorescence development (Bradley et al. 1997). Overexpression of TFL1 in
transgenic Arabidopsis extends the time before a terminal flower forms (Ratcliffe et al. 1998).
TFL1 must delay the reproductive transition. Garden peas branch one more time than
snapdragons do before forming a flower. That is, the axillary meristem does not directly produce
a flower, but acts as an inflorescence meristem that initiates floral meristems. Two genes, DET
and VEG1, are responsible for this more complex inflorescence, and only when both are
nonfunctional is a terminal flower formed (Figure 20.31; Singer et al. 1996).
The next step in the reproductive process is the specification of floral meristems those
meristems that will actually produce flowers (Weigel 1995). In Arabidopsis,LEAFY (LFY),
APETALA 1 (AP1), and CAULIFLOWER (CAL) are floral meristem identity genes (Figure
20.32). LFY is the homologue of FLO in snapdragons, and its upregulation during development is
key to the transition to reproductive development (Blázquez et al. 1997). Expression of these
genes is necessary for the transition from an inflorescence meristem to a floral meristem. Mutants
(lfy) tend to form leafy shoots in the axils where flowers form in wild-type plants; they are unable
to make the transition to floral development. If LFY is overexpressed, flowering occurs early. For
example, when aspen was transformed with an LFY gene that was expressed throughout the plant,
the time to flowering was dramatically shortened from years to months (Weigel and Nilsson
1995). AP1 and CAL are closely related and redundant genes. The cal mutant looks like the wildtype
plant, but ap1 cal double mutants produce inflorescences that look like cauliflower heads
(Figure 20.33)
Floral meristem identity genes initiate a cascade of gene expression that turns on regionspecifying
(cadastral) genes, which further specify pattern by initiating transcription of floral
organ identity genes (Weigel 1995). SUPERMAN (SUP) is an example of a cadastral gene in
Arabidopsis that plays a role in specifying boundaries for organ identity gene expression. Three
classes (A, B, and C) of organ identity genes are necessary to specify the four whorls of floral
organs (Figures 20.34 and 20.35; Coen and Meyerowitz 1991). They are homeotic genes (but not
Hox genes) and include AP2,AGAMOUS (AG), AP3, and PISTILLATA (PI) in Arabidopsis. Class
A genes (AP2) alone specify sepal development. Class A genes and class B genes (AP3 and PI)
together specify petals. Class B and class C (AG) genes are necessary for stamen formation; class
C genes alone specify carpel formation. When all of these homeotic genes are not expressed in a
developing flower, floral parts become leaflike. The ABC genes code for transcription factors that
initiate a cascade of events leading to the actual production of floral parts. In addition to the ABC