Growth Hormones Ethylene and Abscisic Acid Plant Physiology 751

1. Growth HormonesEthylene and Abscisic Acid Plant Physiology 751 1

2. Ethylene response http://www.youtube.com/watch?v=KCUceQulHdw Pineapple flowering www.pbase.com

3. Ethylene Biosynthesis

6. Some physiological effects of ethylene Triple response Epinasty (downward bending of leaves) Air C2H4 Air C2H4 Air C2H4 Tomato Arabidopsis Pea Promotion of root hair formation Inhibition of flower senescence Lettuce Air C2H4 STS = Silver thiosulfate

7. Ethylene and fruit ripening

11. Role of Auxin and Ethylene during leaf abscission formation and senescence

12. Ethylene signal transduction mechanism Ethylene Response Factor 1 Transcription factors are degraded by F box proteins in the absence of ethylene

13. The pathway summary: The RAN1 protein is essential for assembling the Cu cofactor with the receptor for Ethylene binding. In the absence of Ethylene, the receptor activates the downstream kinase CTR1 that in turn inhibits the response/gene expression. On the contrary in the presence of Ethylene, the receptor is “inhibited” so is the CTR1 kinase. The response is now “on”. The kinase cascade in Ethylene response serve as a negative regulator of the gene expression.

14. Ethylene: Things to Remember • Major hormone regulating fruit ripening • Two types of fruit based on amount of ethylene produced and respiration (definitions and example plant names mentioned in HANDOUT) • Climacteric fruits • Non-climacteric fruits • Induce flowering in Pineapple • Promotes root hair growth • Induces triple response (inhibition and swelling of hypocotyl, inhibition of root elongation and exaggeration of the curvature of the apical hook • S-Adenosyl Methionine derived from Methionine is the precursor for its biosynthesis • ACC-Synthase and ACC-Oxidase are the two rate limiting enzymes in its pathway • The Ethylene response factors contributes submergence tolerance in rice and Tomato • Induce leaf senescence and abscission tissue formation

15. Abscisic Acid (ABA)

16. Abscisic acid biosynthesis a sesquiterpenoid isopentenyl PP (C5) farnesyl PP (C15) geranylgeranyl PP (C20) phytoene vp2, vp5, vp7, vp9 corn mutants zeaxanthin aba1 C40 violaxanthin 9-cis neoxanthin vp14 xanthoxal aba2 ABA aldehyde flacca, sitiens: tomato droopy: potato aba3: At nar2a: barley ABA

17. ABA pool size is regulated by both biosynthesis and inactivation synthesis Developmental and physiological effects of ABA • seed maturation • seed dormancy (ABA/GA ratio) • stomatal closure • promotes root growth/inhibits shoot growth at low water potentials • promotes leaf senescence (but not abscission directly – ethylene) • accumulates in dormant buds of perennials ABA’s principle role is in maintaining water balance and osmotic stress tolerance

18. Roles of ABA in plants • Under Abiotic stress conditions such as drought/water stress it promotes root growth and suppresses shoot growth • Plays role in closing stomata in response to water stress • Role in senescence and Abscission layer formation

19. ABA: Things to Remember • Regulates seed dormancy • Seed dormancy is is of two types namely the primary and secondary (see handout) • Vivipary in seeds • ABA/GA ratio controls seed dormancy • Induces Abscission tissue formation leading to senescence and shattering of leaf and fruits. • Similar to Gibberellic Acid the precursor molecule for Biosynthesis is Gerany-Geranyl-pyrophosphate (GGPP) • Under Abiotic stress conditions such as drought/water stress it promotes root growth and suppresses shoot growth • Inhibits flowering by interacting with Flowering time control gene (FCA) in Arabidopsis