CSIR - ABC MODEL OF FLOWER DEVELOPMENT IMPORTANT TOPIC

Flowers are the fascinating organ of plants   but had you ever think that how flowers development occurs. And which environmental signals control flowering, and how are those signals perceived? In this blog we will tell this.

• How are environmental signals transduced to bring about the developmental changes associated with flowering?

Ultimately this process leads to the production of the floral organs—sepals, petals, stamens, and carpels.

FLORAL MERISTEMS AND FLORAL ORGAN DEVELOPMENT

Floral meristems usually can be distinguished from vegetative meristems, even in the early stages of reproductive development, by their larger size. 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. In the vegetative meristem, the cells of the central zone complete their division cycles slowly. 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. Recently, genetic and molecular studies have identified a network of genes that control floral morphogenesis in Arabidopsis, snapdragon (Antirrhinum), and other species. In this blog we will focus on floral development in Arabidopsis, which has been studied extensively in Figure . First we will outline the basic morphological changes that occur during the transition from the vegetative to the reproductive phase. Next we will consider the arrangement of the floral organs in four whorls on the meristem, and the types of genes that govern the normal pattern of floral development. According to the widely accepted ABC model, the specific locations of floral organs in the flower are regulated by the overlapping expression of three types of floral organ identity genes.

The Characteristics of Shoot Meristems in Arabidopsis Change with Development

During the vegetative phase of growth, the Arabidopsis vegetative apical meristem produces phytomeres with very short internodes, resulting in a basal rosette of leaves. As plants initiate reproductive development, the vegetative meristem is transformed into an indeterminate primary inflorescence meristem that produces floral meristems on its flanks . The lateral buds of the cauline leaves (inflorescence leaves) develop into secondary inflorescence meristems, and their activity repeats the pattern of development of the primary inflorescence meristem.

The Four Different Types of Floral Organs Are Initiated as Separate Whorls

Floral meristems initiate four different types of floral organs: sepals, petals, stamens, and carpels. These sets of organs are initiated in concentric rings, called whorls, around the flanks of the meristem. The initiation of the innermost organs, the carpels, consumes all of the meristematic cells in the apical dome, and only the floral organ primordia are present as the floral bud develops. In the wild-type Arabidopsis flower, the whorls are arranged as follows:

• The first (outermost) whorl consists of four sepals, which are green at maturity.

·         The second whorl is composed of four petals, which are white at maturity.

• The third whorl contains six stamens, two of which are shorter than the other four.

• The 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

Mutations have identified three classes of genes that regulate floral development: floral organ identity genes, cadastral genes, and meristem identity genes.

1. 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.

2. Cadastral genes act as spatial regulators of the floral organ identity genes by setting boundaries for their expression.

3. Meristem identity genes are necessary for the initial induction of the organ identity genes. These genes are the positive regulators of floral organ identity.

Meristem Identity Genes Regulate Meristem Function

Meristem identity genes must be active for the primordial formed at the flanks of the apical meristem to become floral meristems. (Recall that an apical meristem that is forming floral meristems on its flanks is known as an inflorescence meristem.) For example, mutants of Antirrhinum (snapdragon) that have a defect in the meristem identity gene FLORICAULA develop an inflorescence that does not produce flowers. Instead of causing floral meristems to form in the axils of the bracts, the mutant floricaula gene results in the development of additional inflorescence meristems at the bract axils. The wild-type floricaula (FLO) gene controls the determination step in which floral meristem identity is established.

In Arabidopsis, AGAMOUS-LIKE 201 (AGL20), APETALA1 (AP1), and LEAFY (LFY) are all critical genes in the genetic pathway that must be activated to establish floral meristem identity. LFY is the Arabidopsis version of the snapdragon FLO gene. AGL20 plays a central role in floral evocation by integrating signals from several different pathways involving both environmental and internal cues. AGL20 thus appears to serve as a master switch initiating floral development. Once activated, AGL20 triggers the expression of LFY, and LFY turns on the expression of AP1.

In Arabidopsis, LFY and AP1 are involved in a positive feedback loop; that is, AP1 expression also stimulates the expression of LFY.

Homeotic Mutations Led to the Identification of Floral Organ Identity Genes

The genes that determine floral organ identity were discovered as floral homeotic mutants (see my video on you Tube Channel ). Mutations in the fruit fly, Drosophila, led to the identification of a set of homeotic genes encoding transcription factors that determine the locations at which specific structures develop. Such genes act as major developmental switches that activate the entire genetic program for a particular structure. The expression of homeotic genes thus gives organs their identity. As we have seen already in this chapter, dicot flowers consist of successive whorls of organs that form as a result of the activity of floral meristems: sepals, petals, stamens, and carpels. These organs are produced when and where they are because of the orderly, patterned expression and interactions of a small group of homeotic genes that specify floral organ identity.

The floral organ identity genes were identified through homeotic mutations that altered floral organ identity so that some of the floral organs appeared in the wrong place. For example, Arabidopsis plants with mutations in the APETALA2 (AP2) gene produce flowers with carpels where sepals should be, and stamens where petals normally appear. The homeotic genes that have been cloned so far encode transcription factors—proteins that control the expression of other genes. Most plant homeotic genes belong to a class of related sequences known as MADS box genes, whereas animal homeotic genes contain sequences called homeoboxes .

Many of the genes that determine floral organ identity are MADS box genes, including the DEFICIENS gene of snapdragon and the AGAMOUS, PISTILLATA1, and APETALA3 genes of Arabidopsis. The MADS box genes share a characteristic, conserved nucleotide sequence known as a MADS box, which encodes a protein structure known as the MADS domain. The MADS domain enables these transcription factors to bind to DNA that has a specific nucleotide sequence. 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.

Three Types of Homeotic Genes Control Floral Organ Identity

Five different genes are known to specify floral organ identity in Arabidopsis:

Type A gene    APETALA1 (AP1),  APETALA2 (AP2),

 Type B  gene   APETALA3 (AP3),  PISTILLATA (PI),

Type C  gene  AGAMOUS (AG)

The organ identity genes initially were identified through mutations that dramatically alter the structure and thus the identity of the floral organs produced in two adjacent whorls. For example, plants with the ap2 mutation lack sepals and petals (see Figure). Plants bearing ap3 or pi mutations produce sepals instead of petals in the second whorl, and carpels instead of stamens in the third whorl. And plants homozygous for the ag mutation lack both stamens and carpels . Because mutations in these genes change floral organ identity without affecting the initiation of flowers, they are homeotic genes. These homeotic genes fall into three classes—types A, B, and C—defining three different kinds of activities .

1. Type A activity, encoded by AP1 and AP2, controls organ identity in the first and second whorls. Loss of type A activity results in the formation of carpels instead of sepals in the first whorl, and of stamens instead of petals in the second whorl.

2. Type B activity, encoded by AP3 and PI, controls organ determination in the second and third whorls. Loss of type B activity results in the formation of sepals instead of petals in the second whorl, and of carpels instead of stamens in the third whorl.

3. Type C activity, encoded by AG, controls events in the third and fourth whorls. Loss of type C 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.

The role of the organ identity genes in floral development is dramatically illustrated by experiments in which two or three activities are eliminated by loss-of-function mutations

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.

The ABC Model Explains the Determination of Floral Organ Identity

In 1991 the ABC model was proposed to explain how homeotic genes control organ identity. The ABC model postulates that organ identity in each whorl is determined by a unique combination of the three organ identity gene activities:

• Activity of type A alone specifies sepals.

• Activities of both A and B are required for the formation of petals.

• Activities of B and C form stamens.

• Activity of C alone specifies carpels.

The model further proposes that activities A and C mutually repress each other ; that is, both A- and C-type genes have cadastral function in addition to their function in determining organ identity. The patterns of organ formation in the wild type and most of the mutant phenotypes are predicted and explained by this model . The challenge now is to understand how the expression pattern of these organ identity genes is controlled by cadastral genes; how organ identity genes, which encode transcription factors, alter the pattern of other genes expressed in the developing organ; and finally how this altered pattern of gene expression  results in the development of a specific floral organ.

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