Plant Development Biology

First section has discussed Cadastral genes and their role in floral meristem patterning. It has given a brief discussion of the floral patterning mentioning the areas where cadastral genes are involved. The second section has dealt with the establishment of floral meristem identity in Arabidopsis thaliana and Antirrhinum plant species. The section has outlined the genes responsible for establishment of floral meristem identity and compared the differences and similarities between the two species.

The third section is about the ABC model of floral patterning, how the transcription factors are defined and the establishment of the domain of expression of the transcription factors.
The last section has discussed the establishment of the flower shape in Antirrhinum majus and the importance of that discovery to the study of other plant species shape establishment.

1) Cadastral Genes and their Role in Floral Meristem Patterning
The ABC model of flower development rests on Goethes foliar theory of the flower which assumes that all floral organs are modified leaves. The floral organs are produced by specific gene expressions in specific whorls and organ primordial territory. According to the ABC model, sepals are produced in whorl 1 where A genes are expressed, petals are produced in whorl 2 where A and C class genes are expressed, stamens are produced in whorl 3 where B and C genes are expressed. The expression of A class genes in whorl 3 is down regulated (Glover, 2007).

Carpels are produced in whorl 4 where only C class genes are expressed. Production of the floral organs in the correct places depends on defined expression patterns of the ABC class genes. Some of the expression patterns are determined by mutual inhibition for example in the case of whorl 1 and 3 where the expression of C down regulates the expression of A genes (Glover, 2007).

In some instances however, other factors are involved. There are genes that demarcate organ primordial and whorl territory but do not have specific functions of their own. It is such genes that are referred to as cadastral genes (Glover, 2007). Cadastral genes are therefore boundary demarcating genes that ensure floral organs are produced in the right places but with no defined function to demarcate the boundaries. The genes demarcate the boundaries by either using the inhibition process of cell proliferation in the boundaries or through spatial regulation of other genes expression patterns (Glover, 2007).

The role of cadastral genes in patterning the floral meristem As indicated above, the floral meristem develops into sepals, petals, stamens and carpels. The patterning is determined by the expression genes which are expressed in specific whorls. A gene is expressed in whorl 1, B in whorl 2 and 3, and C in whorl 3 and 4. Floral meristem patterning only depends on specific gene expression and not the whorls.

The whorls determine the position of a specific organ. For a sepal to develop in whorl 1, only A genes have to be expressed. For a stamen to develop in whorl 3 only B and C genes have to be expressed. Cadastral genes help in the regulation of the expression of the genes that determine the development of a floral organ. For example, SUPERMAN (SUP), a cadastral gene of Arabidopsis, negatively regulates the expression of B function genes in whorl 4 (Glover, 2007).

The gene restricts expression of B gene in whorl 2 and 3 where its expression supports the development of petals. If B genes are expressed in whorl 4, it means that the leaves will develop into petals instead of stamens. According to Glover, SUP is only produced upon detection of B gene transcripts in the Stamen primordial boundary (2007).

There are several other cadastral genes. In controlling the expression of C genes and restricting them to whorl 3, the A gene (AP2) and a number of cadastral genes are involved. AP2 represses the expression of AG (the C gene) in whorl 1 and 2. Cadastral genes that encode SEUSS (SEU), LEU-NIG (LUG) and STERILE APETALA also interact with AP2 to repress AG expression (Glover, 2007).
2). Floral meristem Identity Establishment in Arabidopsis thaliana and Antirrhinum majus
Establishment of floral meristem identity is by development of specific genes that guide the development of the flower organs and patterns. These genes are referred to as floral meristem identity genes. In Antirrhinum, mutations on the gene FLORICAULA (FLO) produced shoots instead of flowers and in Arabidopsis, mutations on the gene LEAFY (LFY) also led to the formation of shoots instead of floral structures.

The development of floral structures was later observed in such mutants indicating that the two genes are not the sole determinants of floral structure development in the species respectively. Researchers Carpenter et. al (1995) as noted by Weigel  Clark found out that there are other overlapping actions of some other genes that cause the floral structure development (1996).

In Antirrhinum, the gene responsible for the overlapping action was found to be the SQUAMOSA (SQUA) while in Arabidopsis, the gene responsible was found to be APETALA1 (AP1). Without AP1 in the case of Arabidopsis, there is no normal flower development even though LEAFY can induce its activity in an ectopic position.  Additionally, without SQUAMOSA, there is no normal development of flowers in Antirrhinum even though FLO can induce its activity in an ectopic position. Research analysis revealed that either gene has the capability of inducing the others activity in ectopic positions but for normal floral development, both genes activities were required in each species (Weigel  Clark, 1996).

Floral meristem identity development leads to the induction of a set of floral homeotic genes which are responsible for flower patterning. In both species of plants, that is in Arabidopsis and Antirrhinum, the set of homoetic genes are ABCDE. The original set of genes however is ABC as discovered before (Weigel  Clark, 1996).

In Arabidopsis, the A class of genes are APETALA1 and APETALA 2. The genes play specific roles in determination of floral meristem identity and organ identity. AP2 however does not participate in floral meristem identity establishment (Weigel  Clark, 1996).  Class B (APEPTALA3 (AP3) and PISTILLATA (PI) are required for the development of stamen and petals in whorls 2 and 4. The C class of genes (AGAMOUS (AG)) is required for carpel and stamen identity but is also found in floral meristem determinacy specification. The D class is the same as the SEEDSTICK gene and the E class has four paralogs, which are SEPALLATA 1-4 (Ainsworth, 2006)

Similarities between the two plant species in the ABC model is in the C class and B class of genes. Antirrhinum PLENA (PLE), the C class genes and the DEFICIENS (DEF), The B class genes, have the same functions as Arabidopsis C and B class genes. Antirrhinum A class genes however do not serve the same function as A class genes in Arabidopsis but the two genes LIP1 and LIP2 which are homologous of AP2, play some role in organ identity development (Ainsworth, 2006).
Another similarity is in the SQUAMOSA which is AP1 like, and AP1 which functions in floral meristem identity but has no function in organ identity. AP1 of Arabidopsis however has functions in floral meristem identity as well as floral organ identity (Glover, 2007). The development of floral meristem identity and the ABC class of genes generally suggests that floral meristem identity and organ identity in flowers is determined by certain genes and that different species of flowers have genes responsible for floral meristem development (Ainsworth, 2006).

3) The Main Class of Transcription Factors Involved in Flower Patterning
The main class of transcription factors that are involved in patterning of the flower is ABC as originally established. More research has been conducted and more factors added to the class that determines the position of the developing organs and the development of the flower organs. D and E function genes have also been identified to play a part in flower patterning.  The floral meristem Identity genes act sequentially to accomplish the formation of the floral state from the lateral meristems of the inflorescence meristem.  The process involves meristems indeterminacy inhibition to ensure that proliferation is stopped. WUS, which maintains cell division within the meristem is down regulated hence stopping proliferation (Glover, 2007).

Floral state formation also involves activation of floral organ identity genes expression.  These encode transcription factors and also activate the different structural genes necessary for organ formation and genes involved in controlling important processes of flower development for example pigment synthesis (Glover, 2007). The floral organ identity genes are the ones that belong to a specific class responsible for the patterning of the flower.

As noted earlier, the class is ABC but has other added factors found out later after more research on floral patterning.  The A function genes are expressed in whorl 1 which is the outer most whorl. These genes are responsible for the formation of sepals. The A function genes are expressed in addition to  other  leaf developmental program genes and their presence lead to modifications on the normal leaf development program on the primordia leading to the formation of sepals (Glover, 2007).
The B function genes are expressed in whorl 2. In this whorl, both B and A function genes are expressed in addition to other leaf developmental program genes.  In this whorl, the primordia undergoes dramatic changes that result into the development of petals (Glover, 2007).

C function genes are expressed in whorl three alongside the expression of B function genes. In this whorl, A function genes are no longer expressed. They are down regulated by the expression of C function genes. The presence of B and C function genes leads to the development of stamens through modification of the primordia. C function genes are also expressed in whorl 4 where it serves to develop the carpels. The C function genes lead to more dramatic modification of the primordia for the formation of the carpels.

The B function genes in this whorl are down regulated. The process of flower patterning is guided by the specific genes responsible for specific structures expression. Expression of certain genes for example of class C leads to the formation of carpels and C and B functions together to form the stamen. It is therefore the genes that define the class of transcription factors (Glover, 2007).

Establishment of the domain of expression of the transcription factors in the floral meristem is done by regulation of the expression of the genes responsible for the formation of specific organs of the flower. As noted before, the pattern of expression of the floral identity genes determine the positions of the organs that should develop.  The patterns of expression are also determined by regulation which is done by cadastral genes. These genes act to specify the domain of expression of other genes. It means that it specifies the domains of expression of B genes for example to be only expressed in whorl 2 and 3 and not whorl 4 and 1 (Glover, 2007).

4) Establishment of Flower Shape in Antirrhinum Majus
Mutant analysis has revealed that the shape of Antirrhinum majus is controlled by genes. Weigel, indicated that genes acting differentially along the dorsoventral axis of the flower control its asymmetric shape. In Antirrhinum, floral meristem development leads to formation of flowers with sepals, stamens, petals and carpels all arranged in concentric whorls. The first whorl has five sepals, the second whorl five petals, the third whorl four stamens and the last whorl two carpels (Weigel, 1998).

These organs differ in shape according to their positions in relation to the dorsoventral axis. The whole flower has one line of symmetry which also determines the pattern of the differences among the organs. Two axes forming the different patterns in the flower are revealed in Antirrhinum majus which are the dorsoventral axis and the radial axis. There are two systems of genes that have been found through mutant analysis that determine the dorsoventral or the radial patterns (Almeida, Rocheta  Galego, 1997).

Mutant analysis involves removal of the suspected gene by inhibiting its transcription or by inactivation. When the genes determining the radial pattern in the flowers are inactivated, the differences between the whorls reduce but the patterns achieved from dorsoventral axis remain. When the genes determining the dorsoventral pattern are inactivated, the differences between the organs within a whorl reduce but the radial patterns remain. The genes determining the dorsovemtral pattern therefore are responsible for the development of different organ shapes in specific whorls while those responsible for the radial pattern give the flower its overall shape, which is the shape of each whorl (Almeida, Rocheta  Galego, 1997).

Example of the genes that control the shape of the Antirrhinum majus are cycloidea (cyc), dichotoma (dich) and divaricata (div) (Almeida, Rocheta  Galego, 1997).  According to a research carried out by Almeida, Rocheta  Galego to determine the genetic control of the shape of Antirrhinum majus, dichotoma and cycloidea negatively regulate divaricata during normal development of the flowers petals (1997).

This was shown by studying mutations of all the three genes. The results showed that the triple mutants had radial symmetric flowers with petals having lateral identity.  This clearly revealed that the mutant div gene is semi-dominant, which means that the domain that the div gene affects in wild type flower extends all round the flower when dich and cyc are mutants (Almeida, Rocheta  Galego, 1997).
Mutants of dorsoventral genes produce ventralised phenotypes. The wild type Antirrhinum majus has five petals with three identities. One petal is ventral, two of them are dorsal and the rest two laterals. In the mutant case, the flower produces all the five petals identical ventrally which indicates that they have been ventralized (Almeida, Rocheta  Galego, 1997).

Flower development is very diverse and floral shapes and forms differ among so many different types of flowers and even in closely related species. It is not easy to understand how the flowers obtain their shapes and forms. Recent research advances however show that this can be easily understood by studying the genetic basis of floral induction, floral shape and floral form (Weigel, 1998).

Identification of the genes responsible for flower shape formation as revealed by Almeida, Rocheta  Galego, is a step forward to identifying other genes responsible for the development of specific shapes in other flowers.

In understanding this diversity, it is important to identify common mechanisms underlying the processes for example the one identified above, then isolating the regulatory genes from models used in research and using the results to study other species. Plant models that have been used include Antirrhinum majus and Arabidopsis thaliana. Mutant analyses that have been used to determine the genetic control of flower shape can be used to find out the other genes responsible for shape determination in other flowers (Weigel, 1998).


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