REVIEW
Floral Initiation and Inflorescence Architecture: A Comparative View
REYES BENLLOCH
†
, ANA BERBEL, ANTONIO SERRANO-MISLATA and
FRANCISCO MADUEN
˜
O*
1
Instituto de Biologı´a Molecular y Celular de Plantas, CSIC-UPV, CPI, Ingeniero Fausto Elio, 46022 Valencia, Spain
Received: 10 March 2007 Returned for revision: 16 April 2007 Accepted: 15 June 2007 Published electronically: 6 August 2007
† Background A huge variety of plant forms can be found in nature. This is particularly noticeable for inflores-
cences, the region of the plant that contains the flowers. The architecture of the inflorescence depends on its branch-
ing pattern and on the relative position where flowers are formed. In model species such as Arabidopsis thaliana or
Antirrhinum majus the key genes that regulate the initiation of flowers have been studied in detail and much is known
about how they work. Studies being carried out in other species of higher plants indicate that the homologues of
these genes are also key regulators of the development of their reproductive structures. Further, changes in these
gene expression patterns and/or function play a crucial role in the generation of different plant architectures.
† Scope In this review we aim to present a summarized view on what is known about floral initiation genes in differ-
ent plants, particularly dicotyledonous species, and aim to emphasize their contribution to plant architecture.
Key words: Plant architecture, inflorescence development, compound inflorescence, floral meristem identity, LEAFY,
APETALA1, TERMINAL FLOWER1, legume, VEG1, DET.
INTRODUCTION: THE ARCHITECTURE
OF INFLORESCENCES
A striking feature of plants is the huge variety of forms
that can be found in nature. This enormous diversity is
due to variation in the shape and size of different plant
organs, basically leaves, shoots and flowers (later fruits),
and in the proportion of the different kinds of organs and
the position where they appear in the plant. The number
and arrangement of plant organs are the basis of plant
architecture.
Flowers tend to appear clustered in a region of the plant
called the inflorescence (Weberling, 1989a). Inflorescence
form varies enormously among different species and
seems to play a determinant role in reproductive success
as it has a strong effect on pollination and fruit set
(Wyatt, 1982). Whilst particular forms of inflorescences
frequently typify some plant families, the same type of
inflorescence architecture can also be found in unrelated
families, suggesting that adaptive selection has probably
played a role in the evolution of inflorescences (Tucker
and Grimes, 1999)
All the aerial organs of the plant derive from the shoot
apical meristem (SAM). This meristem generates leaves
and shoots during the vegetative phase, and in the reproduc-
tive phase – after the floral transition – it becomes an
inflorescence meristem and flowers are produced. The
architecture of the inflorescence depends on its branching
pattern and the position of the flowers: on when and
where flowers are formed.
Inflorescence types have been classified following
several criteria (Weberling, 1989a). A main parameter for
the classification is whether the shoot apices end in terminal
flowers or not. When they do not terminate, the inflores-
cences are classified as indeterminate. A typical example
of an indeterminate inflorescence is the raceme, present in
species such as Arabidopsis thaliana or Antirrhinum
majus. In this type of inflorescence, the apical meristem
is able to grow indefinitely, generating a continuous main
axis that laterally produces floral meristems (Fig. 1A–C).
On the other hand, inflorescences that form terminal
flowers are called determinate. A classical type of determi-
nate inflorescence is the cyme. Cymose inflorescences lack
a main axis: the main shoot terminates in a flower, while
growth continues through lateral axes produced below the
terminal flower (Fig. 1D–F). These lateral axes again
form terminal flowers and this process is reiterated several
times. Data on the developmental control of cymose inflor-
escences is available for several species such as Silene lati-
folia or tobacco (Nicotiana tabacum; Fig. 1D, E).
A variation of the cymose pattern is found, for example,
in tomato (Solanum lycopersicum; Fig. 1F); the inflores-
cence of this species is also a cyme but, in this case, after
the main axis generates the terminal inflorescence, a new
axis of growth develops from an axillary meristem that pro-
duces a certain number of leaves before again terminating
in an inflorescence. This process repeats indefinitely, gener-
ating a plant with an apparently continuous growing axis in
which the production of leaves and ‘lateral inflorescences’
alternates. This kind of plant architecture is called a sympo-
dium. Finally, as pointed out in an elegant modelling analy-
sis of inflorescence development (Prusinkiewicz et al.,
2007), a third main kind of inflorescence architecture,
also determinate, is the panicle (Fig. 1G). In contrast to
the cyme, in this type of inflorescence a clear main shoot
axis exists but this is terminated by a flower, as also
occurs in the series of lateral branches produced by the
main shoot.
* For correspondence. E-mail madueno@ibmcp.upv.es
† Present address: Laboratoire DRDC / PCV, UMR CEA – CNRS 5168
– INRA1200 – UJF CEA, 17 rue des Martyrs, baˆt. C2 – 38054
GRENOBLE Cedex 9, France
# The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
For Permissions, please email: journals.permissions@oxfordjournals.org
Annals of Botany 100: 659–676, 2007
doi:10.1093/aob/mcm146, available online at www.aob.oxfordjournals.org

Inflorescences are also classified according to the com-
plexity of their branching. Those inflorescences where
flowers are directly formed from the main axis are called
simple inflorescences, while compound inflorescences are
those where flowers are formed from secondary or higher-
order branches. An example of a compound inflorescence
is the compound or double raceme present in many
Leguminosae species, such as pea (Pisum sativum),
Medicago truncatula or Lotus japonicus (Fig. 1C). The
inflorescences of arabidopsis and antirrhinum are simple
racemes (Fig. 1A, B).
Although the evolution of inflorescences is poorly under-
stood, it is generally accepted that the most primitive inflor-
escences would have had terminal flowers. This, in part,
derives from the idea that the flower is a specialized
shoot, and the transition of a vegetative apex to a flower
would be direct in a primitive inflorescence. As discussed
by Tucker and Grimes (1999), the first authors speculating
about inflorescence evolution favoured the idea that a
solitary terminal flower would be the ancestral inflores-
cence form (Parkin, 1914); this supported the idea of
woody trees, such as those of Magnoliaceae, being among
the most primitive families. However, the primitiveness of
the Magnolia type of flower has been challenged by
several authors, such as Stebbins (1974), based on questions
such as the high complexity of its vasculature, and a more
recent view is that the ancestral angiosperms would have
had simple cymose inflorescences.
As explained above, the architecture of the inflorescence
depends on which meristems give rise to shoots and which
to flowers (Coen and Nugent, 1994). The genetic control
of the specification of floral meristems has largely been
studied in model species, mainly in antirrhinum and
arabidopsis, and the main factors have been identified and
a lot of information about how they work is available.
In recent years, the homologues of these and other genes
with related functions have been identified and studied in
many other plant species. These studies suggest that the
functioning of the genetic network controlling the initiation
of flowers is largely conserved among flowering plants,
with key differences often relating to the different inflores-
cence architecture of each species. In this review we aim to
present a summarized view on what is known about floral
initiation genes in different species, and we try to empha-
size their role in directing plant architecture.
CONTROL OF FLORAL INITIATION: HOW IT
WORKS IN ARABIDOPSIS
As for many genetic processes in plants, the genetic control
of floral initiation is best known in the model plant arabi-
dopsis. However, the aim of this article is not to describe
in detail how the specification of floral meristems is con-
trolled in arabidopsis, a question that has been treated
in several excellent reviews (Jack, 2004; Vijayraghavan
et al., 2005; Bla´zquez et al., 2006), but to try to describe
and compare what is known about the genes controlling
this process in other species. Therefore, we will briefly
introduce the key elements of the genetic network in arabi-
dopsis as a basis for the comparison.
In arabidopsis, during the vegetative phase the SAM pro-
duces on its flanks vegetative primordia that will form
leaves with shoot meristems in their axils. Upon transition
to the reproductive phase, the SAM becomes an inflores-
cence meristem (IM) and the new lateral primordia pro-
duced after that point develop as floral meristems (FM).
Therefore, with the floral transition the fate of these
lateral primordia has to be reprogrammed so that they
acquire the identity of floral meristems.
In arabidopsis, the acquisition of floral meristem identity
(FMI) by these primordia is controlled by the interaction of
positive and negative regulators. Although several other
genes have also been shown to play important roles in the
regulation of floral meristem identity in arabidopsis, we
will concentrate on LEAFY (LFY), APETALA1 (AP1) and
TERMINAL FLOWER1 (TFL1). These genes seem to
form the backbone of the network and, consequently, they
are the ones whose role in the process has been best ana-
lysed in arabidopsis and whose homologues have been
studied most in many other species.
LEAFY
The LFY gene is required for the specification of FMI in
arabidopsis. This is clearly deduced from the phenotype of
lfy mutant plants, where the flowers are replaced by struc-
tures with shoot characteristics (Fig. 2A; Schultz and
Haughn, 1991; Huala and Sussex, 1992; Weigel et al.,
1992). The shoot character of the lfy ‘flowers’ is more
marked in the first positions in the inflorescence, while
structures formed in more apical positions progressively
acquire an increasing degree of floral identity due to inde-
pendent activation of other floral meristem identity genes
FIG. 1. Diagrams of different types of inflorescences. (A–C)
Indeterminate inflorescences: (A) the simple raceme of Arabidopsis thali-
ana and (B) Antirrhinum majus, and (C) the compound raceme of pea.
(D–G) Determinate inflorescences: (D) the dichasium of Silene latifolia;
(E) the tobacco cyme; (F) the sympodium of tomato; and (G) a panicle.
Open circles represent flowers and arrows represent indeterminate shoots.
Grey triangles in (C) represent stubs.
Benlloch et al. — Genetic Regulation of Inflorescence Architecture660