To access and download PowerPoint presentation on 'Regulation of Floral Development' click on the link below:
Parts of a flower
•The flower consists of several leaf-like
structures
attached to a specialized region of the stem called the receptacle.
Calyx (unit: sepal):
•It represents the outermost whorl and
protects the inner whorls in buds.
•They can photosynthesize.
Corolla (unit: petal):
•It
primarily
attracts insects to serve as pollinators
and
are often showy and brightly-colored
appearance.
Androecium (unit: stamen):
•It is the
male sexual structure.
•The stamen
consists of a narrow stalk called the filament and a chambered structure called
the anther.
•The anther
contains tissue that gives rise to pollen grains.
Gynoecium (unit: carpel):
•It
is
the
female sexual structure.
•The carpel consists
of the stigma (the tip where pollen lands during
pollination), the style (an
elongated structure), and the basal
ovary.
•The ovary encloses one
or more ovules.
•Each ovule,
in turn, contains an embryo sac,
the structure that gives rise to the female gamete, the egg.
Flower is a modified determinate shoot
Homology of the floral bud with a vegetative bud
•Floral
and vegetative buds both emerge either in terminal or in axillary position
•The
floral buds may sometimes get modified into vegetative buds or bulbils (e.g. Agave, Allium).
Thus proving that the two are analogous structures.
Axis nature of receptacle
•The
internodes
in floral axis remain highly reduced yet in a number of plants such as Capparis,
Passiflora
etc.
the receptacle shows prominent nodes and internodes.
•In
certain plants (eg.,
rose, pear, etc.), sometimes the receptacle
of the flower continues its growth even after producing all the four types of
floral appendages and then produces normal foliage leaves.
•In
Michelia
champaca
the
thalamus
can
elongate like an ordinary stem beyond carpels and
bears aggregate fruit.
Foliage nature of floral appendages
•Foliage
leaves (phyllotaxy)
and floral appendages (aestivation)
have an identical arrangement on the stem.
•Sometimes,
sepal
can
be modified to an
enlarged leaf-like structure as seen in Mussaenda.
Transition of floral leaves
•In
nature, in many cases, such as Nymphaea
(water lily) all degrees of transition from sepals to petals and from petals to
stamens can be seen.
•In
Canna, the
stamens and the style become petaloid.
•In
Zinnia,
some of the stamens and carpels become petaloid
or sepaloid.
Floral development
•Transition to flowering involves major changes in the pattern of morphogenesis and cell differentiation at the shoot apical meristem leading to the formation of the floral organs.
•These events are collectively referred to as floral evocation.
•The developmental signals that bring about floral evocation include:
Ø Endogenous factors: circadian rhythms, phase change, and hormones
Ø External factors: day length (photoperiod) and temperature
(vernalization)
•In the case of photoperiodism, transmissible signals from the leaves collectively referred to as the floral stimulus, are translocated to the shoot apical meristem.
•The interactions
of these endogenous and external factors enable plants to synchronize their
reproductive development with the environment.
Floral meristems and floral organ development
•The transition
from vegetative to reproductive development is marked by an increase in the frequency of cell divisions within the central zone
of the shoot apical meristem.
•As reproductive development commences,
the increase in the size of the meristem is largely a result of the increased division rate of
these central cells.
Four
Different Types of Floral Organs
Are Initiated as Separate Whorls
•Floral meristems initiate four different
types of floral organs in concentric
rings
(called whorls): sepals, petals, stamens, and carpels.
•In Arabidopsis
flower,
the whorls are arranged as follows:
- ØThe first (outermost) whorl consists of four sepals, which are green at maturity.
- ØThe second whorl is composed of four petals, which are white at maturity.
- ØThe third whorl contains six stamens, two of which are shorter than the other four.
- ØThe fourth whorl is a single complex organ, the gynoecium or pistil, which is composed of an ovary with two fused carpels, each containing numerous ovules, and a short style capped with a stigma.
Three Types of Genes Regulate Floral Development
Three
classes
of genes regulate floral
development:
•Meristem identity genes are
necessary for the initial induction of the organ identity genes. These genes
are the positive regulators of floral organ identity.
•Floral organ identity genes directly
control floral identity. The proteins encoded by these genes are transcription
factors that likely control the expression of other genes whose products are
involved in the formation and/or function of floral organs.
•Cadastral genes act
as spatial regulators of the floral organ identity genes by setting boundaries
for their expression.
Meristem Identity Genes Regulate Meristem Function
•Meristem identity
genes must be active for the proper floral meristem development.
•In Arabidopsis,
three
genes must be activated to establish floral
meristem identity:
ØAGAMOUS-LIKE 20, also called SUPPRESSOR OF
OVEREXPRESSION OF CONSTANS 1 (AGL20/SOC1)
ØAPETALA1 (AP1)
ØLEAFY (LFY)
•AGL20 serves as
a master switch initiating floral development
by integrating signals from both
environmental
and internal cues.
•Once activated,
AGL20 triggers the expression of LFY, and LFY turns on the expression of AP1.
•AP1 expression
also stimulates the expression of LFY
(positive
feedback loop).
Homeotic Mutations Led to the Identification of Floral Organ Identity Genes
•The genes that determine floral organ
identity was discovered as floral homeotic mutants.
•Mutations in these genes resulted in development of the floral organs in the wrong place.
•Because mutations in these genes change
floral organ identity without affecting the initiation of flowers, they are homeotic genes.
•Five different
genes are known to specify floral organ identity in Arabidopsis:
ØAPETALA1 (AP1)
ØAPETALA2 (AP2)
ØAPETALA3 (AP3)
ØPISTILLATA (PI)
ØAGAMOUS (AG)
•The homeotic
genes encode transcription factors—proteins
that control the expression of other genes.
•Most plant
homeotic genes belong to MADS box genes.
•The MADS domain enables these
transcription factors to bind to specific
DNA motifs.
•Not all genes containing the MADS box
domain are homeotic genes.
•For example,
AGL20 is a MADS box gene, but it functions as a meristem identity gene.
•The organ
identity genes initially were identified through homeotic
mutations.
The ABC model for floral development
•In 1991 the ABC model was proposed to explain how homeotic genes control organ identity.
•The ABC
model for the acquisition of floral organ identity is based on the interactions
of the three different
types of activities of floral homeotic genes: A, B, and C.
ØActivity of type A alone specifies sepals.
ØActivities of both A and B are required for the formation of petals.
ØActivities of B and C form stamens.
ØActivity of C alone specifies carpels.
Type A activity:
•Encoded by AP2
•Controls organ
identity in the first and second whorls.
•Loss of activity
results
in the formation of carpels instead of sepals in the first whorl, and of
stamens instead of petals in the second whorl.
Type B activity:
•Encoded by
AP3 and PI
•Controls organ
determination in the second and third whorls
•Loss of activity
results
in the formation of sepals instead of petals in the second whorl, and of
carpels instead of stamens in the third whorl.
Type C activity:
•Encoded by AG
•Controls events
in the third and fourth whorls.
•Loss of activity
results
in the formation of petals instead of stamens in the third whorl, and
replacement of the fourth whorl by a new flower such that the fourth whorl of
the ag
mutant flower is occupied by sepals.
•Quadruple-mutant plants (ap1, ap2, ap3/pi, and ag)
produce floral meristems that develop as pseudoflowers.
All the floral organs are replaced with green leaflike structures,
although these organs are produced with a whorled phyllotaxy.
•This demonstrates that the floral organs are highly modified leaves.
The ABCE model (Quartet model)
•A
class genes specify sepals in the first whorl.
•A
and B class genes specify petals within the second whorl.
•B
and C class genes specify stamens within the third whorl
•C
class gene function specifies carpel identity within the fourth whorl.
•The E class genes (SEPALLATA 1-4) are
active within all four whorls.
•Combinatorial
interactions of floral organ identity factors within each whorl form tetrameric complexes.
•These
floral organ identity factors can act as pioneer factors, influencing chromatin accessibility
throughout flower development.
•Additionally, class D genes
regulate ovule development.
Thomson et al. 2017 (Plant Physiology)
(Images kindly shared by Oxford University Press and are being used for teaching purpose only)
