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A quartet for a flower

​We are working on MADS transcription factors which, in plants, form DNA in the form of heterotetrameric complexes involved in the formation of flowers and in the specification of floral organs.

Published on 30 April 2018
The growth and development of all organisms requires appropriate gene regulation. DNA binding proteins, called transcription factors, play a central role in this process by orchestrating the activation and repression of their target genes in a tightly controlled manner. Under-standing how transcription factors act at the molecular level to find their binding sites and activate gene transcription are major challenges in biology.

In plants, MADS transcription factors (TFs) act as the main regulators of flower organ development. These factors form heterotetrameric complexes, called quartets, and each quartet determines what type of floral organ will be produced: sepals, petals, stamens, or carpels. The MADS transcription factor, SEPALLATA3, is at the heart of the interaction network between proteins: without a SEPALLATA protein, the MADS floral quartets do not form.

In the study carried out by researchers from the Integrated structural biology and plant development team of the Cell & Plant Physiology laboratory, a splice variant of the SEP3 transcription factor, SEP3Δtet, defective in its ability to form tetramers but still capable of binding to DNA in the form of dimers, has been used to understand the role of MADS quartets in flower development. Unexpectedly, plants expressing this variant have been able to launch all flower organ development programs. However, whereas in the wild type flower, the production of stem cells stops once all the organs are produced, it has continued in the center of the floral meristem expressing the variant SEP3Δtet, giving rise to the growth of flowers inside a flower due to the continuous production of floral organs. This phenotype is caused by the inability of SEP3Δtet to activate a subset of its target genes responsible for the termination of stem cells and which are located in closed chromatin regions nucleosome-rich and poorly accessible regions of the genome for the binding of transcription factors.

These results suggest that the formation of quartets by MADS transcription factors produces a significant cooperative effect to increase DNA binding affinity and is only necessary for the activation of target genes that are in a strongly repressed state. These results provide new information not only about the underlying molecular mechanisms of flower development, but also suggest a general mechanism used by transcription factors to overcome poor accessibility of their binding sites..

Diagram showing the effect of the dimerization and tetramerisation of SEP3 with respect to flower development. The dimers and tetramers of MADS transcription factors are represented by red and blue ovals with their DNA binding site in yellow. Medallion on the left: The formation of tetramers is necessary to produce a fertile flower with a finite number of organs. Medallion on the right: The absence of tetramerisation produces indeterminate flowers with floral organs produced from the center of the flower, instead of a normal carpel (the insert shows a close-up of the organ opened with additional stamens visible).
The name MADS name comes from the acronym formed by the first letter of the 4 founding members MCM1, AGAMOUS, DEFICIENS and SRF.

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