Arabidopsis thaliana

FLORAL BIOLOGY OF ARABIDOPSIS THALIANA, ABC MODEL OF FLOWER

1. WELCOME

2. Floral biology of Arabidopsis thaliana PRESENTATION BY Virendra Singh Shekhawat M.Sc. (Agri.) Dept. of Genetics and Plant Breeding

3. Arabidopsis thaliana ( thale cress, mouse-ear cress or arabidopsis) is a small flowering plant native to Eurasia. A . thaliana is considered a weed; it is found by roadsides and in disturbed land. A winter annual with a relatively short life cycle . A . thaliana is a popular model organism in plant biology and genetics. For a complex multicellular eukaryote, A. thaliana has a relatively small genome of approximately 135 megabase pairs (Mbp ). It was the first plant to have its genome sequenced, and is a popular tool for understanding the molecular biology of many plant traits, including flower development and light sensing. INTRODUCTION

4. Kingdom : Plantae Class : Equisetopsida Subclass : Magnoliidae Superorder : Rosanae Order : Brassicales Family : Brassicaceae Genus : Arabidopsis Species : A. thaliana Chromo. No. : 2n=10 TAXONOMY

5. A. thaliana can complete its entire lifecycle in six weeks. The central stem that produces flowers grows after about three weeks . T he flowers naturally self-pollinate. A. thaliana has been extensively studied as a model for flower development. FLORAL BIOLOGY

6. The developing flower has four basic organs: sepals, petals, stamens, and carpels (which go on to form pistils ). These organs are arranged in a series of whorls: F our sepals on the outer whorl, followed by four petals inside this, S ix stamens, and A central carpel region. FLOWER

7. Flower :- Ebracteate , pedicellate , mostly actinomorphic , bisexual, dimerous or tetramerous hypogynous . Calyx :- sepals 4, polysepalous, in two whorls of two each imbricate aestivation. Corolla :- petals 4, arranged in single whorl alternating with sepals, polypetalous, often with long claws and spread out to form a cross. Hence, the name is cruciform corolla. Androecium :- stamen 6, polyandrous, arranged in two whorls of 4 and 2, outer two are short and inner two are long. Gynoecium :- Bicarpellary , syncarpous , short styled, superior ovary.

8. FLOWERS

9. Homeotic mutations in A. thaliana result in the change of one organ to another - in the case of the agamous mutation, for example, stamens become petals and carpels are replaced with a new flower, resulting in a recursively repeated sepal-petal-petal pattern. CONT....

10. Observations of homeotic mutations led to the formulation of the ABC model of flower development by E. Coen and E. Meyerowitz. According to this model, floral organ identity genes are divided into three classes: C lass A genes (which affect sepals and petals), C lass B genes (which affect petals and stamens), and C lass C genes (which affect stamens and carpels). These genes code for transcription factors that combine to cause tissue specification in their respective regions during development. Although developed through study of A. thaliana flowers, this model is generally applicable to other flowering plants. Cont….

11. Floral diagram and Floral formula

12. Three classes of mutations were identified in Arabidopsis .   Class A: F lowers with these mutations have (unfused) carpels instead of sepals in whorl 1, and stamens instead of petals in whorl 2. The pattern of organs (from outside to inside) is carpel, stamen, stamen, carpel. The genes containing these mutations were named APETALA1 (AP1) and APETALA2 (ap2). Below is an image of an apetala2 mutant flower (right) next to a wild type flower (left ).   Class B: flowers with these mutations have sepals in whorl 2 instead of petals, and (unfused) carpels in whorl 3 instead of stamens. The pattern of organs (from outside to inside) is sepal, sepal,carpel, carpel. The genes containing these mutations were named APETALA3 (AP3) and PISTILLATA (PI). Below is an image of a pistillata mutant flower (right) next to a wild type flower (left ).   Class C: flowers with this mutation have petals in whorl 3 instead of stamens, and sepals in whorl 4 instead of carpels. In addition, the floral meristem is not determinate - flowers continue to form within the flowers, so the pattern of organs (from outside to inside) is: sepal, petal, petal; sepal, petal, petal; sepal, petal, petal, etc. The gene containing this mutation was called AGAMOUS (AG). Below is an image of an agamous mutant flower (right) next to a wild type flower (left ) .

13. 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, and class C genes are required in whorls 3 and 4. Both class A and class B genes are required in whorl 2, and both class B and class C genes are required in whorl 3. The ABC model that summarizes these results is shown below. Class A genes control sepals and petals, class B genes petals and stamens, and class C genes regulate stamens and carpels. THE ABC MODEL :

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