Development Supplement I, 1991, 157-167
Printed in Great Britain © The Company of Biologists Limited 1991
157
A genetic and molecular model for flower development in Arabidopsis
thaliana
ELLIOT M. MEYEROWITZ*, JOHN L. BOWMAN, LAURA L. BROCKMAN, GARY N. DREWS,
THOMAS JACK, LESLIE E. SIEBURTH and DETLEF WEIGEL
Division of Biology 156-29, California Institute of Technology, Pasadena, California 91125, USA
•Corresponding author
Summary
Cells in developing organisms do not only differentiate,
they differentiate in defined patterns. A striking example
is the differentiation of flowers, which in most plant
families consist of four types of organs: sepals, petals,
stamens and carpels, each composed of characteristic
cell types. In the families of flowering plants in which
these organs occur, they are patterned with the sepals in
the outermost whorl or whorls of the flower, with the
petals next closest to the center, the stamens even closer
to the center, and the carpels central. In each species of
flowering plant the disposition and number (or range of
numbers) of these organs is also specified, and the floral
'formula' is repeated in each of the flowers on each
individual plant of the species. We do not know how cells
in developing plants determine their position, and in
response to this determination differentiate to the cell
types appropriate for that position. While there have
been a number of speculative proposals for the
mechanism of organ specification in flowers (Goethe,
1790;
Goebel, 1900; Heslop-Harrison, 1964; Green,
1988),
recent genetic evidence is inconsistent with all of
them, at least in the forms in which they were originally
presented (Bowman
etal.
1989; Meyerowitz
etal.
1989).
We describe here a preliminary model, based on
experiments with Arabidopsis thaliana. The model is by
and large consistent with existing evidence, and has
predicted the results of a number of genetic and
molecular experiments that have been recently per-
formed.
Key words: Arabidopsis
thaliana,
pattern formation, floral
mutants.
Processes in flower development
From many descriptions of flower development (Payer,
1857;
Sattler, 1973), we can divide the developmental
processes that lead to flowers into four successive
stages. The first is floral induction. This is the process by
which the shoot apical meristem, which is the set of
dividing cells that gives rise to most of the plant parts
that are above the roots, decides that it is time to stop
making leaves, and to start making flowers (Bernier,
1988).
Induction is effected in different ways in
different plants; in Arabidopsis a combination of
internal and environmental signals, including age of the
plant, its nutritional state, day length, and in some
strains temperature, determines that the apical meri-
stem will change its fate from vegetative to floral. Once
this has happened the second stage of flower develop-
ment begins. This is the production of individual
flowers on the flanks of the shoot apical meristem.
These floral primordia arise, by patterned cell divisions,
in a defined temporal and spatial pattern (Fig. 1A). The
third stage is the formation of organ primordia on each
flower primordium. The wild-type Arabidopsis flower
contains
15
separate organs: four sepals, four petals, six
pollen-bearing stamens, and a pistil (Fig. 1). In
Arabidopsis, the third stage of floral development is
thus the formation of 15 undifferentiated organ
primordia (Fig. 1B,C), at stereotyped positions within
the flower primordium. These organ primordia are at
this time not only undifferentiated, but are also not
irreversibly determined, so that their fates are as yet
either partly or completely unspecified (Bowman et al.
1989).
The final stage of flower development is the
specification of the fates of the organ primordia, and
their consequent differentiation into organs appropri-
ate for their positions (Fig. ID). Each organ is
composed of
a
small number of defined cell types, some
of which (e.g. pollen sperm cells) are organ-specific,
and some of which (e.g. guard cells) are found in
several kinds of organs. Detailed descriptions of flower
development in Arabidopsis
thaliana
are available (Hill
and Lord, 1989; Smyth et al 1990).
That each of these stages of flower development is to
some degree a separate process, effected by a different
158 E. M. Meverowitz and others
Fig. 1. Scanning electron microscope views of the development of wild-type Arabidopsis flowers. (A) A florally-induced
shoot apical meristem (inflorescence meristem) surrounded by a series of flower primordia. The developmentally most
advanced flower primordium is at the right, and is showing the earliest stages of sepal development (stage 3). (B) An
individual flower primordium at stage 6, when organ primordia have formed, but are not yet fully specified. Two sepals
have been removed to reveal the underlying inner whorl primordia. (C) Another developing flower, at stage 7, with all of
the sepals removed to show the first signs of organ-specific differentiation of the six third whorl organs (which are
becoming stamens) and of the fourth-whorl ovary. (D) Developing flower at stage 11, when the organ-specific
differentiation of the epidermal cells of each organ type is evident. Sepals have been removed to reveal the inner organs.
The bars in panels A,B and C represent lOum, that in panel D 100/«n.
group of regulatory genes in Arabidopsis, is indicated
by the existence of many mutations that affect only one
or only a subset of the stages. That these processes are
interrelated is equally demonstrated by the fact that
many mutations affect more than one of them. The
mutations that interfere with the fourth stage are
homeotic: they cause normal organs to develop in
abnormal places (Pruitt et al. 1987; Meyerowitz et al.
1989).
Some of these mutants are normal up to the time
when the undifferentiated organ primordia are formed,
and these primordia generally appear in their wild-type
numbers and places. Other mutants affect organ
number and organ position as well as organ identity.
There is much more known about the fourth stage in
flower development than about the others: we have
done a series of genetic experiments that have led us to
a working hypothesis for the way in which three classes
of homeotic genes specify organ identity in each floral
whorl (Bowman et al. 1989; Bowman et al. 1991). This
model does not. however, explain the phenotypes of the
homeotic mutants that relate to organ number and
position. Nonetheless, the model has correctly pre-
dicted patterns of organ identity in doubly and triply
mutant strains, and predicts both the pattern of
expression of the RNA of one of the homeotic genes,
and the changes in that pattern that result from
mutations in one of the other homeotic genes.
A genetic model for organ identity in flower
development
Wild-type Arabidopsis flowers have four concentric sets
of organs, which consist of (from the outside of the
flower) four sepals, four petals, six stamens, and an
ovary (Figs 2A and 3). Mutations that alter this pattern
were first recognized in Arabidopsis by Braun (1873),
who recognized a homeotic phenotype in a wild
population of plants in Berlin. To begin our genetic and
molecular characterization of
Arabidopsis
flower devel-
opment, we collected from others, and produced
ourselves, a large series of mutations (induced by ethyl
methanesulfonate or X-rays) with abnormal floral
phenotypes. As described above, these mutations fall
into several classes, each apparently affecting one or
more processes in flower development. We have
concentrated on the homeotic mutations that affect the
fourth stage in flower development, including the
process by which unspecified organ primordia learn
their fate. Of the mutations initially screened, many
gave clear, reproducible homeotic phenotypes. Comp-
lementation analysis (by us and others) has shown that
most of these fall into four complementation groups,
and thus represent four genetic loci with major effects
on organ type specification (Pruitt
et
al.
1987; Meyerow-
itz,
1987; Komaki
et
al.
1988; Bowman
et
al.
1989; Kunst
etal. 1989; Meyerowitz
et
al.
1989; Yanofsky etal. 1990;
Bowman et al. 1991).
Four mutations in three
classes
These loci are agamous
{ag),
at position 37.6 cM on the
fourth chromosome in the morphological marker map
of Arabidopsis (Koornneef, 1989); apetalal (ap2) at
4-63.5;
pistillata (pi) at 5-23.6; and
apetala3
(ap3) at
3-73.9. All of the mutations at these loci are recessive
(except for one semidominant ap2 allele which we
recently obtained). Fig. 3 shows the map positions of
these four loci, and schematically represents the
phenotypes of one or more mutant alleles of each of the
genes.
To describe the phenotypes we will use the term
whorl to mean a region of the flower or flower
primordium (and not the collection of organs found in
that region). The first whorl is the outer ring of tissue,
from which the sepals arise in wild-type flowers. The
second whorl is the next inner concentric ring, from
which petals normally arise. The third whorl is the ring
from which stamens develop in wild type, and the
fourth whorl is the bull's-eye. It is the normal location
of the ovary.
Each of these mutations affects the organs of two
Fig. 2. Mature Arabidopsis flowers. (A) Wild type, showing the four petals, six stamens, and ovary.
(B)
agamous-3
homozygote, showing the extra petals and sepals, as well as the absence of stamens and
ovary. (C)
apetala3-l
homozygote, with the organs of the outer two whorls removed to show the third
whorl organs, which develop as carpels rather than stamens, as in wild type. (D)
apelala2-2/apeiala2-l
heterozygous flower, showing first whorl carpels, absence of second whorl organs, and normal organ
identity (though not number) in the third whorl. The fourth whorl is normal. (E)
apetala2-l
homozygous
flowers, showing first whorl leaves and second whorl stamens. The third and fourth whorl organs are
normal.
Fig. 5. Flowers of doubly and triply mutant strains. (A) Flowers homozygous for both ap2-l and ap3-l. All organs are carpelloid.
(B) Rowers homozygous for both ap3-l and ag-I. All floral organs are sepals. (C) Flower homozygous for ap2-I as well as ag-l.
showing that the whorl 1 and 4 organs are leaves, and the whorl 2 and 3 organs are intermediate between petals and stamens.
(D) Triply mutant inflorescence, homozygous for the three mutations
ap2-I.
ag-I. and pi-1. In each flower, all of the floral organs
have developed as leaves.
Fig. 6. Flower of a plant homozygous for
supennan-I. Note the four sepals, four
petals.
10 stamens, and reduced ovary.
Arabidopsis flower development
159
adjacent whorls. The agamous mutations (of which
three alleles have been studied, i.e.
ag-1,
ag-2
and
ag-3)
affect only whorls three and four. The six organs of
whorl three develop as petals rather than stamens, and
the whorl four primordium forms four sepals, inside of
which are additional whorls of petals and sepals. Thus,
if the normal pattern of
sepals,
petals, stamens, ovary is
called an ABCD pattern, agamous mutants have an
ABBA(BBABBA...) pattern (Fig. 2B). We have ex-
tensively studied three pi alleles, pi-1, pi-2 and pi-3
(Bowman el
al.
1989; Bowman et
al.
1991) and two ap3
alleles (one is described by Bowman et
al.
1989), and
both pi and
ap3
affect whorls two and three only, having
essentially the same effects (except for some details of
the
pi-1
third whorl, where the organ primordia do not
form normally). Whorl two contains four sepals rather
than the wild-type four petals, and instead of the wild-
type pattern of six third whorl stamens, the mutant
whorl three consists of six carpels (the subunit of the
ovary; the normal Arabidopsis ovary consists of two
carpels). These mutations thus have the phenotype
AADD (Fig. 2C). Apetala2 mutants, four alleles of
which have been studied in our laboratory (Pruitt et al.
1987;
Bowman
et
al.
1989;
Bowman etal. 1991), and five
more by others (Komaki et
al.
1988; Kunst et
al.
1989),
affect organ identity in whorls one and two. In most
alleles (the single exception will be explained below)
the organs of the first whorl are carpels, and the second
whorl consists of four (or sometimes fewer) stamens in
the places normally occupied by the four petals. Most
alleles also have effects on organ number in the first,
second and third whorls, and on the position
of
initiation of organ primordia in whorls 1, 2 and 3. Since
we have only succeeded in producing a model for organ
identity, and not for organ number, we will for present
purposes consider the relevant pattern of most alleles to
be DCCD (Fig. 2D). Discussion of the effects of AP2
on organ number and position will be reserved for later.
A working
hypothesis,
in the form of
a
model
We have proposed a simple model to explain the wild-
type functions of the homeotic genes (Bowman et al.
1989;
Bowman et
al.
1991). It proposes that these four
genes fall into three classes, each class affecting
a
different pair of
whorls.
APETA LA2 is the only known
gene in the first class, which affects whorls
1
and 2; A
P3
and PI constitute the presently-known second class,
with effects in whorls
2
and
3;
and AG is the only known
member of the final class, which affects whorls 3 and 4.
Each class
of
genes
is
expressed
in
the developing
flower in those whorls affected in the mutants. Whorl
one is thus characterized by expression of AP2\ whorl
two by the combination AP2, PI and AP3; whorl three
by the combination AG, PI and AP3; and whorl four by
the expression of AG. Each of the products of these
genes has as its function the induction of expression of
a
series of downstream genes, the expression of which
LINKAGE GROUP
sepal
apetala3-l apetala2-2 apetala2-l
agamoua-1
pistillata-2
Fig. 3. Schematic representation of the phenotypes of some mutant alleles of the four best-studied homeotic genes. The
structure of a wild-type Arabidopsis flower, and the genetic map positions of the homeotic genes, are also depicted.