Floral Organ Mutants and the Study of Organ Morphogenesis Objectives: 1. How does one use mutants to understand floral organ morphogenesis? 2. What are the phenotypes of some floral organ mutants? 3. What is the genetic basis for the phenotypes? 4. What are the similarities between the different mutant phenotypes described? 5. On the basis of the mutants, what hypothesis can we develop concerning the control of floral organ type in Arabidopsis?
From Botany, M. Neushul, John Wiley and Sons Inc. (eds.), 1974
Arabidopsis Floral Development Inflorescence SEM Mature Flower
The Arabidopsis Flower is complete with four concentric whorls of four organ types Organs of a similar type arranged in a circle is called a whorl
If we are going to use mutants to study floralorgan identity, what kind of mutants do we screen for? What would the role of the genes involved in morphogenesis be?
The Apetala2 mutant has flowers that differ dramatically from wild type Wild type Apetala 2
Apetala2 selfed All Apetala2 mutants Conclusion = heritable Genetic Analysis Apetala2 x Columbia F1 phenotype = is wild type Conclusion = mutant phenotype is recessive to wild type F2 phenotypes 406 wild type, 128 Apetala2 mutants 3:1 expected: 400.5 133.5 X 2 = 0.3 probability = 0.6 (0.05 cutoff) Accept 3:1 Conclusion Apetala2 is a single nuclear locus = APETALA2 All floral defects are due to a single mutation.
The Apetala2 mutant has flowers that differ dramatically from wild type Wild type Apetala 2
All Floral Organs Have Specific Cell Types
Scanning electron micrographs of Apetala 2 flowers and floral organs Wild type floral organ epidermal cell types
Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL-like
Second whorl organs from Apetala2 mutant flowers
Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL STAMEN
Wild type Apetala2
Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL STAMEN STAMEN CARPEL
Structure of an Apetala2 Mutant Flower
Perianth organ Reproductive organ (+) AP2 (-) Floral organ primordium
Agamous Mutant Phenotype Wild type Agamous
Structure of an Agamous Mutant Flower
Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL STAMEN STAMEN CARPEL Ag SEPAL PETAL PETAL SEPAL
Perianth organ Reproductive organ (+) AP2 (-) (-) AG (+) Floral organ primordium
Apetala3 (and Pistillata) Mutant Phenotype Wild type Apetala3
Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL STAMEN STAMEN CARPEL Ag SEPAL PETAL PETAL SEPAL Pi SEPAL SEPAL CARPEL CARPEL Ap3 SEPAL SEPAL CARPEL CARPEL
Structure of a Pistillata Mutant Flower
sepal petal AP3 (-) And PI (+) stamen carpel AP3 (+) And PI (-) Floral organ primordium Floral organ primordium
Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL STAMEN STAMEN CARPEL Ag SEPAL PETAL PETAL SEPAL Pi SEPAL SEPAL CARPEL CARPEL Ap3 SEPAL SEPAL CARPEL CARPEL
A Model For Control of Organ Type sepal petal stamen carpel 1 2 3 4 B (AP3, PI) A (AP2) C (AG)
A Model For Control of Organ Type Mutation in a class B gene sepal sepal petal stamen carpel carpel 1 2 3 4 B (AP3, PI) A (AP2) C (AG)
A Model For Control of Organ Type Mutation in a class A gene carpel sepal stamen petal stamen carpel 1 2 3 4 B (AP3, PI) A (AP2) C (AG) C (AG) A function must negatively regulate C function in the outer whorls.
A Model For Control of Organ Type Mutation in a class C gene sepal petal stamen petal carpel sepal 1 2 3 4 B (AP3, PI) A (AP2) A (AP2) C (AG) C function must negatively regulate A function in the inner whorls.
How can the ABC model of floral organ type be tested?