Regulation of Floral Development (Part I)

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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)

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