Light plays an important part in the development of vegetative and floral growth in most plants, A. thaliana (rape seed plant) is often used as a model plant to show the effects of short and long days on plant development. As day length increases in the spring, many plants respond by flowering, these are long day (LD) plants such as A. thaliana, and later when the days shorten some plants flower, or produce tubers. Light is measured by photoperiodism using phytochromes, the genes for blue light receptors are CRY1 and CRY2, far-red receptor genes are PHYA, and red light receptor genes are PHYB. PHYA, CRY1, and CRY2 are clock’ genes which promote the production of Florigen’, which promotes Constans proteins (CO). The red light receptor gene PHYB inhibits the production of Constans, and therefore inhibits flowering. This is a positive feed-back mechanism where PHYA and CRY2 repress TOC1 using LHY/CCA1 proteins, and as TOC1 decreases this increases the amount of LHY/CCA1, whilst inducing florigen. During the morning (red light) flowering is inhibited, and in the evening (far red light, and blue light) flowering is initiated, so during long days there is more red inhibitory light, and during short days there is more far red and blue light which has a positive effect upon flowering. We can see how these processes affect each other by looking at A. thaliana mutants, such as PHYA mutants interfere with the promotion in flowering by long days, causing the plants to flower early, phyB mutants are early flowering, and cry2 mutants are late flowering.
The reproductive phase of Arabidopsis development (flowering) is promoted by long photoperiods. The protein Constance accumulates in the phloem during the light period, this is what promotes flowering. CO expression is controlled by the circadian clock, and it encodes zinc finger transcription factors. FT mRNA is translated at the shoot apical meristem where it interacts with FD proteins, this activates the floral meristem identity genes. However if the flower is a Constans (CO) mutant, it flowers later than wild type (in long days) (Levy, et., al., 2002). Putterill (1995) found that plants with extra CO flowered earlier than the wild type plants, this suggests that CO activity decreases flowering time.
Plant meristem are very sensitive to temperature, and coordinate their developmental phases based upon it, such as seed germination, flowering and breaking dormancy (Ingrouille, and Eddie, 2006). The flowering of A. thaliana is influenced by cold treatment (vernalization), this is where flowering in spring is promoted by low temperatures experienced during the winter months. A. thaliana is a winter annual, flowering locus-C inhibits flowering in the first growing season, and vernalization inhibits flowering locus-C in the warmer conditions (Sung, and Amasino, 2004). The Arabidopsis VRN genes control the vernalization process, a long period of cold induces a mitotically stable state that leads to accelerated flowering during later development (Levy, et, al., 2002). Vernalization induces flowering through changes in gene activity, achieved through reduction in DNA methulation, the flower locus-C gene product blocks the promotion of flowering through GA (Sheldon, et., al., 1999), therefore because flowering locus-C is reduced, flowering is no longer prevented. The Flowering locus-C genes usually inhibit the Agamous-like 20 meristem identity genes, but if it is inhibited then genes such as LEAFY can start the transformation from vegetative to reproductive growth.
Arabidopsis thaliana (Rape) v. Antirrhinum majus (Snap-dragon)
Flowering plants, the Angiosperms have evolved from mostly wind pollinated species, to insect, and animal pollination. Some grasses, and temperate trees have reverted back to wind pollination with exposed anthers in reduced flowers, and feathery stigmas, however most angiosperms have evolved larger, more appealing reproductive structures to attract a wide range of animals and insects. Insect pollinated plants usually have coloured petals, platforms, attractive scents, and sugar rich nectar. The hermaphrodite flowers have also evolved to give rise to better pollination between sexes, this has initiated the evolution of self incompatibility too. The most efficient pollen transferring plants are bilaterally symmetrical which enables insects to visit at certain orientations, platforms are usually made by fusing petals, and stigma and stamen arranged to be touched by insect on underneath (Ennos, and Sheffield, 2000).
Arabidopsis thaliana and Antirrhinum majus appear to have very different morphologies, for example A. majus is bilaterally symmetrical, and A. thaliana is radially symmetrical. A. majus has fused petals forming a platform for insects such as honey bees (Apis andreniformis), and a corella tube for the insects to enter increasing pollen transfer. A. thaliana has four un-fused white petals, their flower arrangements are simple and typical of a highly self-fertile plant, although some wild species have been found to produce olfactory cues for pollinating insects (Chen, et., al., 2003). A. thaliana has exposed stigma and stamens which increases the amount of pollen that can be dispersed by environmental factors such as the wind.
Most flowers are actinomorphic, they are radially symmetrical like A. thaliana and the petals are usually similar in shape, size, and colour. Zygomorphic are bilaterally symmetrical like A. majus, they usually have petals of different shapes, sizes, and colours. In most cases, these different kinds of floral symmetry are linked to particular pollinators. Peloric flowers are mutated, where the wild type flower would usually produce zygomorphic flowers, the mutated flowers are actinomorphic, this can be developmental, or it can have a genetic basis (Endress, 2001).
A. majus, shows all five characteristic trends of flower evolution in attracting insects, the petals have increased in size compared to the smaller carpels, the flowers are grouped together into a large inflorescence, the flower shape has been changed to allow for more efficient pollination, the petals are fused at the base creating a tubular structure, which forces visiting insects to touch both stigma and stamen as they gather nectar from the base, and they are bilaterally symmetrical, which encourages the insects to enter in a certain direction, with a landing platform (Ennos, and Sheffield, 2000). The snapdragon is a typical insect pollinated flower with its purple pigmentation, and its bilaterally symmetrical arrangement, unlike beetle pollinated flowers which prefer white or yellow open flowers.
Mutations in the ABC model’
Mutants in flowers usually involve four organ types, sepals, petals, stamens, and carpels, which are transformed into their neighbouring organ types. The homeotic selector genes, are used as transcription factors that are expressed in overlapping patterns called MADS- box proteins. Homeotic agamous mutants show extra petals and sepals at the expense of stamens and carpels (Klug, and Cummings, 2000). Most dicotyledonous wild type A. majus (Snapdragon) flowers have 4 whorls, these arise from an indeterminate inflorescence meristem in the axils of bracts. The fist whorl has 5 sepals, the second whorl has the petals with has 5 distinct lobes, two dorsal, two lateral, and one ventral. The lobes are fused along most of their length to form the corolla tube. The third whorl contains four stamens, and the fourth whorl has two fused carpels with a style, and stigma.
These flower organs are specified by floral organ identity genes, and mutations in these genes result in homeotic transformations. There are three classes of identity genes, A, B and C (A- Squamosa, B- Deficiens and Globosa, C- Agamous). In regions of the meristem the expression of genes overlaps, combinations of these genes are important for the identity of the second and third floral whorls (petals and stamens). The A, B, C genes directly influence the organisation of the cells in the shoot apical meristem, and decide the positioning and organ types of the flowers, additional genes decide the shapes, pigmentation, and function, such as the cycloidea (CYC) gene which controls floral symmetry in A. majus (Fukuda, et., al., 2003).
Floral organ identity homeotic genes are called MADS-box genes, regulation of these genes influences the structural evolution of flowers. MADS-box genes are conserved sequences, that code for the homeobox domain that makes contact with DNA (Ingrouille, and Eddie, 2006). There are four members of the family, MCM1, Agamous (AG), Deficiens (DEF), and Serum Response Factor (SRF), these genes include Globosa (GLO), Squamosa (SQUA), Plena (PLE), and Fari- Nelli (FAR) (Saedler, et. al., 2001).
The MADS-box genes have been evolving by gene duplication, the change in sequences over time has allowed reconstruction of plant phylogeny based on sequences.
Dorsoventral asymmetry in flowers of A. majus depends on expression of the cycloidea gene in dorsal regions of floral meristems. Cycloidea is activated shortly after floral induction. Shoots expressing cycloidea include secondary branches lying just below the inflorescence, and shoots of the mutants. Cycloidea mutants of A. majus seem to have a semi-peloric phenotype, and the expression of cycloidea within flowers can be modified by mutations in organ identity genes, the results suggest that cycloidea can respond to a common dorsoventral pre-pattern in the apex and that the specific effects of cycloidea on the flower depend on interactions with floral-specific genes (Clark, and Coen, 2002).
In A. majus two very similar MADS-box genes are known Plena and Farinelli. Ple mutants are filled flowers, with the sexual organs are replaced by petals. Far mutants do not feature a homeotic phenotype, but rather are only male sterile, this gene seems not to classify as a C-function gene, whereas the Plena gene seems to link directly to the loss of the gamete producing organs Overall the MADS-box proteins are needed for floral meristem identity (SQUA), organ identity (DEF/GLO, PLE), cell proliferation (DEF), blocking cell proliferation (PLE), male fertility (FAR), and floral architecture (SQUA,DEF/GLO) (Saedler, et., al., 2001).
In Antirrhinum majus, petal and stamen organ numbers and positioning are controlled by Deficiens and Globosa. Mutations in either of these genes result in the replacement of petals by sepaloid organs and stamens by carpelloid organs (Perbal, et., al., 1996).
These B-function genes result in a complex regulatory system of cell proliferation, B-mutants do not form whorl organs, and C-mutants show excessive number of whorls. This is shown in the deficiens mutant, where only three whorls were present, with a reduced number of organs, and in the plena mutant which had an extra whorl than the wild type A. majus.
