An Introduction For Swie Biology Essay

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Plants as organisms with indeterminate growth, grow as a result of the continuous proliferation and differentiation of cells in meristematic regions. The consequence of this indeterminate growth, is that plants are able to produce a large variation in size and architecture.1 The angiosperm or flowering plants also show a great diversity in the colour, size, number and position of their flowers.2 In general, the way flowers are positioned on the plant (the inflorescence) can take one or a combination of three forms apart from the solitary flower: the raceme, the cyme or the panicle.2 During the development of a raceme, the shoot apical meristem (SAM) (from which the above ground vegetative parts of the plant have grown) transforms into an inflorescence meristem (IM). The IM continues to grow indeterminately and generates along the way floral meristems (FM) in an alternating pattern2-5. The floral meristems eventually terminate by the production of a flower. In contrast to what occurs in plants that develop racemes, in plants that develop cymes the SAM transforms into a FM, which produces a flower and forms a lateral IM. The IM in like manner transforms into a FM, repeating the process, thereby causing the typical zigzag appearance of the cyme to arise2-4. In plants that develop panicles, the inflorescence results from a combination of the processes that form the raceme and cyme2.

The transition from the vegetative phase to the reproductive phase of plants, which ultimately leads to the formation of flowers which are fundamental for reproduction, can be induced by environmental signals like temperature, day length and/or endogenous signals like lifetime and hormone balance.1,4,6-8 Signals that are able to inducible the switch to the reproductive phase, affect the expression of numerous genes in specific pathways. In the racemose model plant Arabidopsis thaliana for example, it has been demonstrated that a number of genes in what is known as the photoperiodic pathway, are either activated or repressed.9,10 At the end of this pathway, key genes such as LEAFY (LFY), AGAMOUS-LIKE 24 (AGL24) and APETELA 1 (AP1) actuate the floral identity of a meristem.4,11

Although crucial for the initiation of flower development, research has shown that LFY alone is not sufficient to start the process.1,4 In order to achieve this, it has to interact with the F-box protein UNUSUAL FLORAL ORGANS (UFO). 1,4 The limiting factor for this interaction however, is not UFO, which is expressed even throughout the vegetative phase of the plant in substantial amounts, but LFY, whose expression level rises during the lifetime of the plant.8

In other plants, such as the cyme producing Petunia, it has been shown that similar processes induce flower development. In Petunia, the LFY homologue ABERANT LEAF AND FLOWER (ALF) acts together with the UFO homologue DOUBLE TOP (DOT).4 Both homologues are functionally interchangeable; however, their expression varies markedly between the plants.12 In contrast to A. thaliana, in Petunia ALF is not the limiting factor, but DOT is. During the vegetative phase of the plant, ALF is continuously expressed, the expression of DOT however only starts with the onset of flowering.4 According to Souer et al. (2008), the functional exchangeability and difference in expression may be the result of changes in the cis regulatory regions and the transcription factors that regulate the genes.

Recently, our group which investigates the DOT promoter and its upstream transcription factors, has identified two Arabidopsis proteins (AGL24 and SPL3) that are able to activate the Petunia DOT gene.

While this is interesting, it does not necessarily explain which and how the endogenous Petunia proteins regulate the expression of DOT. The aim of this paper therefore, is to identify and characterise the homologous Petunia proteins (and their genomic sequences) that are able to bind to the promoter region of the DOT gene and regulate its expression. In the long run, knowledge of how DOT is regulated may prove helpful in breeding and improving food crops belonging to the nightshade family, of which Petunia is a member.

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2 Castel, R., Kusters, E. & Koes, R. Inflorescence development in petunia: through the maze of botanical terminology. Journal of experimental botany 61, 2235-2246, doi:10.1093/jxb/erq061 (2010).

3 Rebocho, A. B. et al. Role of EVERGREEN in the development of the cymose petunia inflorescence. Developmental cell 15, 437-447, doi:10.1016/j.devcel.2008.08.007 (2008).

4 Souer, E. et al. Patterning of inflorescences and flowers by the F-Box protein DOUBLE TOP and the LEAFY homolog ABERRANT LEAF AND FLOWER of petunia. The Plant cell 20, 2033-2048, doi:10.1105/tpc.108.060871 (2008).

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7 Liu, Z. & Mara, C. Regulatory mechanisms for floral homeotic gene expression. Seminars in cell & developmental biology 21, 80-86, doi:10.1016/j.semcdb.2009.11.012 (2010).

8 Blazquez, M. A., Soowal, L. N., Lee, I. & Weigel, D. LEAFY expression and flower initiation in Arabidopsis. Development 124, 3835-3844 (1997).

9 Ehrenreich, I. M. et al. Candidate gene association mapping of Arabidopsis flowering time. Genetics 183, 325-335, doi:10.1534/genetics.109.105189 (2009).

10 Fornara, F., de Montaigu, A. & Coupland, G. SnapShot: Control of flowering in Arabidopsis. Cell 141, 550, 550 e551-552, doi:10.1016/j.cell.2010.04.024 (2010).

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12 Kusters, E. Genetic control of meristem identity in petunia Ph.D. thesis, Vrije Universiteit Amsterdam, (2011).