Floral Organ Mutants and the Study of Organ Morphogenesis

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Peter Simpson
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1 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?

2 From Botany, M. Neushul, John Wiley and Sons Inc. (eds.), 1974

3 Arabidopsis Floral Development Inflorescence SEM Mature Flower

4 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

5 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?

6 The Apetala2 mutant has flowers that differ dramatically from wild type Wild type Apetala 2

7 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: 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.

8 The Apetala2 mutant has flowers that differ dramatically from wild type Wild type Apetala 2

9 All Floral Organs Have Specific Cell Types

10 Scanning electron micrographs of Apetala 2 flowers and floral organs Wild type floral organ epidermal cell types

11 Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL-like

12 Second whorl organs from Apetala2 mutant flowers

13 Structure of wild type and mutant Arabidopsis flowers Whorl 1 Whorl 2 Whorl 3 Whorl 4 WT SEPAL PETAL STAMEN CARPEL Ap2 CARPEL STAMEN

14 Wild type Apetala2

15 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

16 Structure of an Apetala2 Mutant Flower

17 Perianth organ Reproductive organ (+) AP2 (-) Floral organ primordium

18 Agamous Mutant Phenotype Wild type Agamous

19 Structure of an Agamous Mutant Flower

20 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

21 Perianth organ Reproductive organ (+) AP2 (-) (-) AG (+) Floral organ primordium

22 Apetala3 (and Pistillata) Mutant Phenotype Wild type Apetala3

23 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

24 Structure of a Pistillata Mutant Flower

25 sepal petal AP3 (-) And PI (+) stamen carpel AP3 (+) And PI (-) Floral organ primordium Floral organ primordium

26 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

27 A Model For Control of Organ Type sepal petal stamen carpel B (AP3, PI) A (AP2) C (AG)

28 A Model For Control of Organ Type Mutation in a class B gene sepal sepal petal stamen carpel carpel B (AP3, PI) A (AP2) C (AG)

29 A Model For Control of Organ Type Mutation in a class A gene carpel sepal stamen petal stamen carpel B (AP3, PI) A (AP2) C (AG) C (AG) A function must negatively regulate C function in the outer whorls.

30 A Model For Control of Organ Type Mutation in a class C gene sepal petal stamen petal carpel sepal B (AP3, PI) A (AP2) A (AP2) C (AG) C function must negatively regulate A function in the inner whorls.

31 How can the ABC model of floral organ type be tested?

Testing the ABC floral-organ identity model: expression of A and C function genes

Objectives: Testing the ABC floral-organ identity model: expression of A and C function genes To test the validity of the ABC model for floral organ identity we will: 1. Use the model to make predictions