Regulation of Floral Organ Identity. Dr. Chloe Diamond Mara

Transcription

1 Regulation of Floral Organ Identity Dr. Chloe Diamond Mara

2 Flower Development Angiosperms (flowering plants) are the most widespread group of land plants Flowers are the reproductive organs that consist of variations of the same fundamental pattern Genetic analysis in model plant species has identified many of the genes responsible for patterning

3 Arabidopsis thaliana Arabidopsis is a small flowering plant that is widely used as a model organism in plant biology. Arabidopsis is a member of the mustard (Brassicaceae) family, which includes cultivated species such as cabbage and radish. Arabidopsis is not of major agronomic significance, but it offers important advantages for basic research in genetics and molecular biology.

4 Why Arabidopsis? Small genome (114.5 Mb/125 Mb total) has been sequenced in the year Extensive genetic and physical maps of all 5 chromosomes. A rapid life cycle (about 6 weeks from germination to mature seed). Prolific seed production (10,000-40,000/plant) and easy cultivation in restricted space. Self fertilization Efficient transformation methods utilizing Agrobacterium tumefaciens. A large number of mutant lines and genomic resources many of which are available from Stock Centers. Such advantages have made Arabidopsis a model organism for studies of the cellular and molecular biology of flowering plants.

5 Arabidopsis Genome

6 Arabidopsis thaliana genome sequence Nature 408, 796 (2000)

7 Genetic Nomenclature in Arabidopsis

8 Fig. B.5

9 Inflorescence 4 2 IM shoots 3 Inflorescence Meristem (IM) Rosette leaf Arabidopsis plant

10 Pattern Formation: How to make a flower? Flower organs form in concentric rings, called whorls. Sepals form first, followed by petals, and then by stamen, and finally by carpels. (

11 Sepal Morphology Enclose developing reproductive organs Green chloroplasts Trichomes

12 Petal Morphology Simple, laminar structure Two distinct regions: Blade and Claw with distinct cell types

13 Stamen Morphology Simple filament Anther composed of more than 10 cell types involved in pollen production and dispersal

14 Carpel Morphology Many distinct cell types and tissues involved in fertilization, seed development and seed dispersal Figure 1. Diagram of a gynoecium structure including a cross section view (right ) (Dinneny & Yanofsky, 2004)

15 Molecular Basis of Differentiation Different transcription factors are required to turn on the correct genetic pathways to specify each distinct floral organ Downstream target genes function to coordinate cell division, cell expansion and cell signaling required to promote differentiation

16 How do the proper flower parts form the proper arrangement? Scientists identified homeotic mutations that resulted in misplacement of floral organs Three classes of mutations were identified

17 The ABC Model Each class of genes is required in two adjacent whorls. Class A genes are required in whorls 1 and 2 Class B genes are required in whorls 2 and 3 Class C genes are required in whorls 3 and 4 Class A and C genes act antagonistically B A C Whorl: Sepals Petals Stamens Carpels

18 ABC Class genes APETALA1-2 APETALA1, 2 APETALA3 PISTILLATA AGAMOUS A class genes alone sepals A + B class genes petals B + C class genes stamens C class alone carpels

19 ag-1 C class: AGAMOUS (AG) MADS box transcription factor Specifies stamen and carpel identity Represses sepal and petal identities Controls floral meristem determinacy Expressed in whorls 3 and 4 B A SEPAL PETAL PETAL NEW FLOWER whorl 1 whorl 2 whorl 3 whorl 4

20 B class: PISTILL!ATA (PI) APETALA3 (AP3) MADS box transcription factors Specifies petal and stamen Expressed in whorls 2 and 3 Heterodimerize with each other in vitro ap3-3 A C SEPAL SEPAL CARPEL CARPEL whorl 1 whorl 2 whorl 3 whorl 4

21 B class: APETALA3 (AP3) and DNA binding PISTILLATA (PI) MADS I K C CArG Box CC(A/T) 6 GG Dimerization Loss-of-function Loss of function Ectopic co-expression (Weigel world and Jack et al., 2003)

22 Ectopic B B A C Whorl: Petals Petals Stamens Stamens

23 A class: APETALA1 (AP1) MADS box protein Meristem identity specification: activate floral homeotic gene expression Organ identity specification: specifies sepal and petal identity ap1-1 Early expression: in the entire floral meristem Later expression: in whorls 1 and 2 B C Bract (fl) STAMEN CARPEL whorl 1 whorl 2 whorl 3 whorl 4

24 A class: APETALA2 (AP2) AP2 encodes a novel type transcription factor with two 68 aa. AP2 domains Specifies sepal and petal development AP2 negatively regulates AG ap2-2 B C CARPEL STAMEN STAMEN CARPEL whorl 1 whorl 2 whorl 3 whorl 4

25 ac bc Leaf-like organs in 1, 4, mosaic petal/stamen in 2, 3 abc Endless whorls of sepals Leaf-like organs in all whorls

26 A, B, C gene mrna expression pattern revealed by in situ hybridization AP1 AP3 AG

27 35S::PI 35S::AP3 35S::B c mutant 35S::B a mutant Petal, petal, stamen, stamen All petals B A Mosaic stamen/petal, stamen, stamen, stamen B C

28 Revisionist ABC Model B sepal A E C petall stamen carpel

29 SEP1, SEP2, SEP3, SEP4!= E class MADS box proteins (most similar to AP1) Have redundant function Single mutants show subtle phenotype Triple mutant show flower phenotype similar to bc double mutant SEP1,2,3, SEP4 :expressed in whorls 2-3 (SEP 1,2 also in whorl1 in young flowers) Interact with B and C proteins based on yeast two-hybrid assay Pelaz et al., Nature 405, , 2000

30 A+B+E: B+C+E: C+E: A+E: Petal Stamen Carpel Sepal

31 Questions?