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Research: Project

Transcriptional networks underlying Arabidopsis floral development

Alice Pajoro, Plant Research International, Angenent group

The transition from vegetative to reproductive growth is a major developmental switch in the life cycle of plants. During the vegetative phase the shoot apical meristem produces leaves, while during the reproductive phase the floral meristem produces flowers. An Arabidopsis flower contain four types of floral organ (sepals, petals, stamens and carpels) arranged in four concentric rings, known as whorls. To generate all the different floral organs a fine orchestration of different developmental processes is needed. Early in the 1990s an elegant model, known as ABC model, was proposed based on a series of homeotic mutants. According to the classical ABC model three functions (A, B and C) specify the different floral organs in the four whorls. The specification of floral organs is a result of jointly expressed MADS-box transcription factors that form organ-specific protein complexes. However, the transcriptional network downstream of these master regulators controlling floral organ development remains largely unknown.

The aim of this project is to identify and characterize specific targets of two MADS-domain proteins, SEPALLATA3 (SEP3) and APETALA1 (AP1), at early stages of flower development.

In order to identify target genes for AP1 and SEP3, chromatin immunoprecipitation experiments followed by deep sequencing (ChIP-SEQ) have been performed on homogenous populations of young floral meristems. These homogenous meristems are produced by simultaneous induction of flower development in apetala1 cauliflower mutant plants (Fig1).

Fig 1. AP1:GR ap1 cal 8 days after dexamethasome treatment

Figure 1

Comparison between ChIP-SEQ data and expression data allows the identification of genes dynamically regulated during flower development (Fig2A). The in planta expression dynamics of direct target genes is investigated by creating promoter-gene fusions to a GREEN FLUORESCENT PROTEIN (GFP) reporter gene. Reporter gene expression in transgenic plants is studied using Confocal Laser Scanning Microscopy (CLSM) (Fig 2B). The role of these genes in floral development is analysed by (flower-specific) knock-down mutants and by overexpression.

We expect that the results of this project will lead to a deeper understanding of the transcriptional network underlying flower development in Arabidopsis.

Fig 2. Direct target gene expression profile and DNA binding profile. (A) AP1 and SEP3 binding profiles at different stages in promoter region of a target gene. (B) In planta expression dynamics of direct target gene at early stages of flower development have been investigated by creating promoter-gene fusions to a GREEN FLUORESCENT PROTEIN (GFP) reporter gene. Reporter gene expression in transgenic plants has been studied using Confocal Laser Scanning Microscopy.

Figure 2

References:

[1] Causier, B., Schwarz-Sommer, Z. & Davies, B. Floral organ identity: 20 years of ABCs. Seminars in Cell & Developmental Biology 21, 73-79 (2010).

[2] Goto, K., Kyozuka, J. & Bowman, J.L. Turning floral organs into leaves, leaves into floral organs. Current Opinion in Genetics & Development 11, 449-456 (2001).

[3] Immink, R.G. et al. SEPALLATA3: the 'glue' for MADS box transcription factor complex formation. Genome Biol 10, R24 (2009).

[4] Kaufmann, K. et al. Target genes of the MADS transcription factor SEPALLATA3: integration of developmental and hormonal pathways in the Arabidopsis flower. PLoS Biol 7, e1000090 (2009).

[5] Kaufmann, K. et al. Orchestration of floral initiation by APETALA1. Science 328, 85-9 (2010)

[6] Krizek B.A. and Fletcher J.C. Molecular mechanisms of flower development: an armchair guide. Nature 6:688-698 (2005)

[7] Pelaz, S., Ditta, G.S., Baumann, E., Wisman, E. & Yanofsky, M.F. B and C floral organ identity functions require SEPALLATA MADS-box genes. Nature 405, 200-3 (2000)

[8] Pelaz S., et al. APETALA1 and SEPALLATA3 interact to promote flower development. The Plant Journal 26(4): 385-394. (2001)

[9] Pelaz S., et al. Conversion of leaves into petals in Arabidopsis. Curr. Biol. 6: 182-184 (2001)

[10] Thei├čen G. and Saedler H. Floral quartets. Nature 409 (2001)

[11] Urbanus, S.L. et al. In planta localisation patterns of MADS domain proteins during floral development in Arabidopsis thaliana. Bmc Plant Biology 9, - (2009).

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