Mr Yagnit Vadhel and Ms Aneesha Sreekanth
St. Xavier's College (Autonomous),
Mumbai, Maharashtra, India
“Oh! Look at this lovely and spectacular tulip!”
“This daisy has bloomed throughout a single night!”
“This rose is just the perfect shade of red!”
How often do we make these observations in our life? Almost every day, right? However, apart from simply admiring the beauty of the natural world, do we ever stop to think about how exactly this process of flower development unfolds itself, and how different types of flowers and inflorescences develop? Surely, your minds would be pondering these phenomena too.
So, fasten your seat belts as this article takes you on a journey through the fascinating course of flower development, exploring the various complex interactions of genes associated with this process.
The entire process of flower development commences with the sequential interplay of different genes and their products.
Formation of floral meristem from inflorescence meristem
During the formation of the floral meristem from the inflorescence meristem, the activity of a range of genes is required.
Firstly, the process commences with the activation of the LFY gene, which is subsequently followed by the action of genes AP1, AP2 CAL, and CLV1. However, in stark contrast to this phenomenon, the Gtfl1 gene is responsible for down-regulating the process of floral meristem development by converting it back to inflorescence meristem.
To understand the process of flower development, various models have been proposed by many botanists over the years. One of the most promising models to describe the mechanism of this phenomenon is the ‘ABC’ model of floral organ initiation.
The ABC model of Flower organ initiation in plants
This model postulates that three major classes of genes, namely A, B, and C, interact in different combinations among themselves to dictate the development of the four whorls of the flower. The sepals (whorl 1) are formed by the expression of genes of class A, which includes the AP2 gene. Petals (whorl 2) are formed by the combined effect of AP2 as well as class B genes (AP3 and PI). In the third whorl, that is, the stamens, the combined interplay of class B genes along with the class C gene, AG, is very essential. Finally, in the fourth and innermost whorl which is composed of carpels, the sole action of the class C gene is required.
The model further signifies that the genes AP2 and AG are antagonistic to each other in action. As a result, the transcript abundance of class A genes is limited to whorls 1 and 2, while the activity of the gene AG is limited to whorls 3 and 4.
Hence, the development of a diverse range of mutants, including the famous double flower that we come across in horticulture literature, is only possible due to a mutation resulting in the permanent downregulation and/or silencing of the AG gene of class C. This mutant floral form lacks stamens and carpels and appears to be solely composed of sepals and petals.
Figure 1: Schematic representation of the ABC model of floral organ initiation
The ’quartet’ model of floral development
Figure 2: Schematic diagram of the different floral parts of Arabidopsis thaliana. (Kram et al., 2009)
Over the years, a new class ‘E’ of function genes was added to the model, which is responsible for the formation of all the floral organ types, making it the ‘ABCE’ model. While many experimental studies support most of the postulates of the aforementioned model, some deviations do occur and have been discovered in model species as well. For example, the AP2 gene in Arabidopsis thaliana, a model plant for floral developmental studies on a molecular level, does not directly play a role in dictating the differentiation of the plant tissue for sepal formation. It is involved in the process of ensuring that the floral primordial tissues orient themselves in the right way concerning each other, a phenomenon also known as organ patterning. However, it does inhibit the action of class C genes and hence adheres to the ABCE model in this respect.
In addition to deviations like these, the observation of a comparatively larger family of MADS-domain transcription factors in flowering plants relative to other eukaryotes, indicated the need for a slight revision of the ABCE model, leading to the proposal of the ‘quartet’ model of floral development (MADS-domain transcription factors are special proteins responsible for the specification of domains of whorls during the development of a flower).
According to this revised model, the concept of the products of the classes of genes acting as dimers was replaced by the idea that the proteins (identity factors) encoded by these genes bind in four different combinations as tetramers which act as regulatory complexes essential for the differentiation of the four floral whorls. The formation of these complexes has been proved experimentally, along with the possibility of the presence of unidentified co-factors that help in the process and whose exact functions have not yet been investigated.
Future directions and conclusion
Currently, our increasing knowledge about the process of flower development is only restricted to some species of flowering plants. However, on account of the revolutionary advancements in the domain of plant biology, especially in the subdisciplines of molecular biology and plant genetics, the adoption of various bioinformatic processes has been successful in discovering new genes and their products - including various transcription factors and regulatory proteins.
Additionally, studies performed to observe the mechanism of floral development in specific plant species will also assist horticulturists and botanists across the world to develop novel varieties of flowering plants by mutating the existing varieties.
Hence, research in this field has a long way to go. To further understand this immensely fascinating and varied phenomenon, studies should also focus on the role of various abiotic factors in flower development and flower initiation across a wide range of families. The information we shall gain by the detailed examination of the molecular mechanism of floral development in more plant families shall prove to be of great economic as well as ecological significance in the years to come.
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