A meristem is a region of a plant containing actively dividing plant cells which form bud, leaf or root initials. A floral meristem is the cluster of cells that gives rise to flowers. Floral meristems can be identified from a vegetative meristem by their relatively larger size which is a product of rapid cellular division and growth. Floral meristems cannot form without a floral stimulus (such as photoperiod or vernalisation) which produces hormonal and molecular changes and finally the activation of meristem identity genes.
There are three key types of genes which control floral development:
- Floral organ identity genes
- Cadastral genes
- Meristem identity genes
Meristem identity genes must be active for floral initiation. If the meristem identity genes are not active, an inflorescence or vegetative meristem cannot produce floral meristems.
From a clump of meristematic cells, a complex flower is produced.
Flowers are composed of four different floral organ types:
- Stamens (anther and filament, otherwise known as the androecium [male])
- Carpels (stigma, style and ovary, otherwise known as the gynoecium [female])
In recent years there has been a considerable amount of research focussed on the ways that plants control "floral identity" or the means by which floral organs develop on each whorl.
By altering a few key genes in a plant the type of organs that develop on each whorl can be altered. In many cases, the outcomes can also be predicted. Two plants that have received particular attention; Arabidopsis thaliana and Antirrhinum majus.
MADS Box Genes
Floral organ identity genes control the formation of specific organs in flowers. Many of the floral organ identity genes belong to a group called the MADS box genes.
The MADS box is a conserved sequence motif found in genes belonging to the MADS-box gene family. (The term MADS is derived from the first letter of the genes MCM1, AGAMOUS, DEFICIENS, and SRF). The MADS box encodes the DNA-binding MADS domain which binds to DNA sequences of high similarity to the motif CC[A/T]6GG, often referred-to as the CArG-box. MADS-domain proteins are generally transcription factors. A typical MADS box gene is 168-180 base pairs in lenth with the encoded MADS domain having a length of 56-60 amino acids. The genomes of flowering plants have around 100 MADS-box genes.
Arabidopsis thaliana is one of the most-studied 'model plants'. In A. thaliana, the MADS box gene AGAMOUS has been shown to be critical for determinate floral growth. Expression of AGAMOUS mRNA in the central region of floral meristems has been found to rely on the partially-overlapping functions of the LEAFY (LFY) and APETALA1 (AP1) genes, which promote initial floral meristem identity.
Additionally, floral promotion pathways in A. thaliana such as the photoperiod and gibberellin (GA) pathway increase the expression levels of floral integrator genes, such as FLOWERING LOCUS T (FT), TWIN SISTER OF FT (TSF), SUPPRESSOR OF CONSTANS 1 (SOC1)/AGAMOUS-LIKE 20 (AGL20), AGAMOUS-LIKE 24 (AGL24), and LEAFY (LFY), the expression of which is somewhat regulated by various MADS box transcription factors. For example, MADS transcription factors such as SHORT VEGETATIVE PHASE and FLOWERING LOCUS C bind directly to target euchromatin to repress specific loci including FLOWERING LOCUS T (FT) and FD. The AP2-domain transcription factor TEMPRANILLO 1 has also been shown to directly repress FT by binding to its 5'-UTR.
In essence, various MADS box genes and transcription factors work together in a complex manner to control when a plant flowers, based on whether the species has been exposed to other required stimuli.
Most of the genes which code for floral organ identity (that is, genes that cause a specific organ to be made, such as a stamen or petal) are MADS box genes.
This is significant because it indicates that the mechanisms for floral identity in many plants is highly conserved meaning that this system was probably developed early on in the evolutionary history of flowering plants. Ma and dePamphilis (2000) report that the MADS box genes with similar function in unrelated plants are more similar to each other than MADS box genes with different functions in the same species of plant.