The specification of floral organ identity begins during floral evocation, for example,
when the LEAFY protein acts to turn on gene expression. We have learned the most
about floral organ identity fromArabidopsishomeotic mutants. Floral homeotic mutants
were isolated that contain a transformation of one organ into another. To understand
these mutants and the resulting ABC model of floral organ identity genes, one must be fam-
iliar with the normal arrangement of organs in theArabidopsisflower (Fig. 4.8). This flower
contains an outer whorl of four green sepals, four white petals, four to five yellow stamens,
and two fused carpels.Agamous(ag) mutants are homeotic mutants that are very striking
and contain an outer whorl of sepals, followed by petals, and then sepals again.
Comparison ofagflowers to wild-type flowers indicates thatagmutants have lost infor-
mation required to make stamens and carpels in whorls 3 and 4, and have replaced this
with petals and sepals, respectively. Mutants in agalso contain a reiteration of this
pattern resulting in an indeterminate meristem and extra rows of petals and sepals. This
finding indicates that AG function is required for whorls 3 and 4 (stamen and carpel) iden-
tity. In contrast to AG, theapetela2(ap2) mutants have sepals transformed into carpels in
the first whorl, and petals transformed into stamens in the second whorl, followed by
stamens and carpels in the next two whorls as usual. This indicates that AP2 is required
for identity of whorls 1 and 2 (sepals and petals). Finally, two different mutants with the
same phenotype, thepistillata(pi)orapetela3(ap3) mutants, contain a transformation
of petals to sepals in the second whorl, and of stamens to carpels in the third whorl.
This indicates that the PI and AP3 proteins function in identity of whorls 3 and 4.
Together, results from these homeotic mutants suggest that three separate types of genes
(denoted A, B, and C), function in floral organ identity (Fig. 4.8). The A function is con-
trolled by the AP2 gene product and must be required for both sepals and petals in whorls 1
and 2. AP3 and PI are gene products with a B function and are required in whorls two and
three to help specify petals and stamens. Finally, the C function is controlled by AG, which
helps specify whorl 3 (stamens) and whorl 4 (carpels). Important to this model is the antag-
onism of A/C function, such that if one is lost, the other expands its function into the two
whorls where it would not normally function. Another caveat is that B function must
necessarily be present in combination with either A or C to specify the petals and
stamens. By drawing out each mutant’s observed pattern, one can see that the mutant
data “fit” exactly to this model.
This elegant model can also be used to predict the phenotypes of double and triple
mutants, which, for the most part, verify the model. For example, if both A and B functions
are lost, this model predicts that C function will expand to all four whorls, and that carpels
should be present in each whorl. The resulting double mutant is found to contain a leaflike
structure in whorl 1, carpelloid leaves in whorl 2, and carpels in whorls 3 and 4, a close
approximation of what the model predicts. A triple mutant that has lost A, B, and C func-
tions is predicted to contain no floral organ identity. The observed mutant is found to
contain carpelloid leaves in each whorl, which suggests that the ground state of the
flower is not totally vegetative (i.e., leaflike).
A new dimension to the ABC model has recently been discovered that involves a group
of four genes, calledSepellata(Sep) genes, which are required to specify each whorl in
addition to the ABC genes. Loss of this E function through a quadruple mutant lacking
all four genes results in whorls of carpelloid leaves, similar to the mutant lacking
ABC function.
Thus, our understanding of flower development starts with CO and LEAFY transcrip-
tional function to begin the developmental program and results in the production of
100 PLANT DEVELOPMENT AND PHYSIOLOGY