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Group Schwarz-Sommer
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GENETIK CONTROL OF FLORAL ORGAN IDENTITY
The (A)BC Model
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Antirrhinum flowers are composed of four types of organs, named sepals, petals, stamens and carpels. Organ identity is determined by a set of homeotic genes, called the ABC genes. Based on the morphology of mutants of Class B and Class C genes a combinatorial model has been suggested that later became confirmed by genetic and molecular studies. The role of class A genes in organ identity control as defined in Arabidopsis, could not be corroborated in Antirrhinumand is recently questioned in Arabidopsis as well. However, the role of A-genes in the control of the C expression domain appears to be more universal. Current efforts in the group focus on the genetic and molecular mechanisms underlying this process.
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The overall morphology of wild type flowers is shown in the photographs at the left and is schematically depicted in the floral diagram in the middle. The scheme at the right illustrates the spatial relation between the B- and C-functions. Genes involved in the spatial control of floral homeotic functions are shown with red barred lines.
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Phenotypes of floral homeotic mutants. Intact flowers are shown at the left in front view and the interiour of the flowers is shown at the right. The drawings beneath the photographs indicate the organ identity changes in the formalism shown for the wild type in the figure above.
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MADS-box genes
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Functional domains of MADS-box proteins. |
DEFICIENS, a Class B gene, has been isolated in collaboration with Hans Sommer (MPIZ Köln). Its homology to other transcription factors allowed us to define the MADS-box family of proteins. The role of various family members in diverse species in the control of developmental processes and their evolutionary implications are topics studied worldwide today.
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The B-function
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Genetic and molecular interactions between the two Class B MADS-box proteins. From Tröbner et al., 1992. |
DEFICIENS and GLOBOSA, the two Class B genes together control petal and stamen development - their mutant phenotypes are indistinguishable. Protein-protein interactions between MADS-box proteins have been corroborated for all family members studied in this respect.
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CLASS B GENES in the EPIDERMAL CONTROL OF FLORAL ORGAN IDENTITY
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The epidermal contributions of class B genes to the control of petal and stamen identity is investigated in somatically stable epidermal chimeras produced by transposon excisions as well as in transgenic plants expressing Class B genes under the conrol of the epidermis-specific Antirrhinum FIDDLEHEAD promoter. The transgenic approach allows us to compare epidermis-controlled processes in Antirrhinum and Arabidopsis. Furthermore, it makes studying the mechanism underlying epidermal control feasible by screening for mutants with improved or impaired phenotypes.
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Phenotypes of epidermal chimera. The panel at the top shows Antirrhinum chimera that express the DEFICIENS gene in their epidermis. Somatic chimera can be obtained by stabilising excision events that occurred in the outermost meristematic layer of atransposon-induced deficiens mutant. In the transgenic chimera the wild-type DEFICIENS gene is expressed under the control of an epidermis-specific promoter in the background of a genetically stable deficiens mutation. Using this epidermal transgene the influence of DEF on the morphology of Arabidopsis class B mutants can be studied. From Efremova et al., 2001.
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The C-function
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Two Class C MADS-box genes, PLENA and FARINELLI, control the identity of stamens and carpels. Interestingly, he overall regulatory function of homeotic Class B and C genes in the control of floral organ identity appears to be similar in Antirrhinum and Arabidopsis. Their role in controlling floral determinacy, however, differs. This explains major differences in the phenotypes of homeotic mutants. In this project we demonstrated that it is not safe to predict gene functions based on criteria derived from orthology nor is it possible to generalise molecular mechanisms that control flower development.
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Differences between homeotic mutants of Antirrhinum and Arabidopsis.
Top panel: A comparative schematic representation of the mutants and double mutants of Arabidopsis (right) and Antirrhinum (left) is shown by floral diagrams. The whorls are numbered to highlight the differences between the Arabidopsis and Antirrhinum B-function mutants (pistillata, apetala3, globosa and deficiens) and C-function mutants agamous, plena).
Bottom panel: The two models explain the observed differences in phenotypes in terms of regulatory gene activity in the third and fourth whorls. B-function expression is shown in yellow, SUP (OCT in Antirrhinum) is shown at the boundary of the third whorl in grey and C-function expression (AG in Arabidopsis, PLE and FAR in Antirrhinum) is shown in light blue. The barred lines leading from the C-functions towards the circle indicate that the C-function represses production of a new whorl and causes termination. The barred lines leading from the B-function and from SUP/OCT indicate that they act to prevent C-function-dependent termination. The arrow and barred line between PLE and FAR indicate that PLE activates FAR and FAR represses PLE. Arrows and barred lines are not meant to imply direct interaction. Two key differences in the regulatory control of OCT in Antirrhinum and SUP in Arabidopsis are proposed. Firstly, unlike SUP, OCT expression is wholly dependent on the expression of the B-function. This is shown by an arrow leading from B to OCT in Antirrhinum and a dotted red line leading from B to SUP in Arabidopsis. Secondly, in Antirrhinum, the negative regulation exerted by OCT on the B-function can only occur in the presence of the C-function PLE or FAR. This is indicated by barred lines running from both PLE and FAR and joining lines from SUP to the boundary of the B-function expression domain. These two regulatory changes allow the model to predict the phenotypes of all the single and double mutants shown in A. The thin red barred line leading from AG to the B-function represents a potential minor regulatory interaction. From Davies et al., 1999.
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GENETIC CONTROL OF GENE EXPRESSION BOUNDARIES
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Antirrhinum mutants displaying impaired control of the spatial domains of the B function (left) and the C-function (right). The resulting organ identity changes are indicated schematically in the drawings under the photographs. |
The ABC model demonstrates that controlling the B and C expresion boundaries is pivotal for establishing wild type organ identities. Loss of this control is reveled by expansion of the expression domains resulting in homeotic mutant phenotypes. Genetic, morphological and molecular studies with such mutants promises insights into the mechanism of this control.
We have cloned STYLOSA and showed that it is the orthologue of LEUNIG in Arabidopsis. Both plant proteins are structurally related to the TUP1/GRO group of transcriptional co-repressors, which interact with various DNA-binding proteins and regulate a broad range of processes in yeasts and animals by. In fact, we found that repressing the C-function during flower development is just one aspect of the STY function; the sty mutant also displays fasciation, alteration in leaf venation patterns, impaired phyllotaxis and hypersensitivity towards auxin and polar auxin transport inhibitors. Thus STYLOSA has a general role in plant development controlling both vegetative and reproductive processes. Ongoing research in the group aims at elucidating the precise role of the STYLOSA protein in gene regulation.
FISTULATA genetically interacts with STYLOSA and several other known factors involved in the spatial control of the C-domain. We initiated a map-based approach to molecularly clone this gene and established several genomics tools to achieve this goal.
Current support: DFG/SFB 572
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© 2009, Max Planck Institute for Plant Breeding Research, Cologne |
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