Agriculture Reference
In-Depth Information
7
Intercellular communication during floral
initiation and development
George Coupland
7.1
Introduction
The adult organs of a plant are derived from groups of stem cells, or meristems, that
are formed during embryo development. These organs develop from primordia that
are formed repetitively on the flanks of the shoot apical meristem (SAM). The iden-
tity of the organs formed from these primordia can be influenced by environmental
conditions, illustrating the extreme flexibility of plant development. Furthermore,
analysis of genetic mosaics, in which the development of a tissue consisting of two
phenotypically distinct genotypes is followed (Huala & Sussex, 1993), or the ef-
fect of ablation of single cells within the developing root (Vandenberg
et al.
, 1995)
indicated that positional information produced by signaling from neighboring cells
can determine the fate of plant cells late in the development of an organ. Similarly,
the identity of the organ formed from a primordium or the behavior of axillary
meristems present at the junction between the shoot and leaf can be influenced by
long-distance signals formed in other tissues and transported to their site of action
(Bernier
et al.
, 1993; Booker
et al.
, 2003).
Short- and long-distance signaling appear to have important roles in the transi-
tion from vegetative development to flowering. This transition requires a change in
the behavior of meristems (Huala & Sussex, 1993). For example, in
Arabidopsis
the morphology of the SAM is altered irreversibly by increased rates of cell divi-
sion generating a larger, taller meristem called the inflorescence meristem (Vaughn,
1955), and inflorescences form from axillary meristems present in the axils of the
last few leaves to be formed during vegetative growth. New primordia formed on
the flanks of the inflorescence meristem give rise to flowers. In many species, in-
cluding
Arabidopsis
, the timing of the transition from vegetative to reproductive
development is determined by environmental conditions such as temperature, day
length and light quality. The environmental conditions that trigger flowering can be
perceived in different organs, so that changes in day length are detected in the leaves
(Zeevaart, 1976), while vernalization (extended exposures to low temperatures that
mimic winter conditions) is detected in the meristem (Michaels & Amasino, 2000).
Since flower development occurs at the meristem, then at least in the case of the
day-length response, long-distance signaling must be required to induce flower de-
velopment. More recently, analysis of developing flowers that are mosaics of mutant
and wild-type tissues demonstrated that the effect of mutations in regulatory genes
can be corrected by short-distance signaling from wild-type cells (Wu
et al.
, 2002).
