1. Roles in Seed Dormancy and Stomatal Closure
Biol 352 Lecture 9
Abscisic Acid:
Roles in Seed Dormancy and Stomatal Closure
February 5, 2007

2. Review: ethylene Ethylene is a gaseous plant hormone.
Precursor: amino acid methionine; immediate precursor: ACC.
Key enzymes for biosynthesis: ACC synthase, ACC oxidase.
Ethylene functions: fruit ripening, triple response, abscission, flower senescence, root hair development, etc.
Two-component signalling system in ethylene signal transduction.

3. Lecture Outline: Structure of ABA Biosynthesis of ABA
Function of ABA: seed dormancy, water stress response
ABA signalling in stomatal guard cells
Summary

4. Learning Objectives: Reading:
Define the key enzymes in ABA biosynthesis
Determine the roles of ABA in seed germination/dormancy
Explain ABA signalling in stomatal guard cells
Reading:
4th Ed, Plant Physiology, Taiz & Zeiger, Chapter 23: p
(3rd Ed, Plant Physiology, Taiz & Zeiger, Chapter 23: p )

5. ABA: a Dormin Naturally occurring active form
Inactive in stomatal closure
\figures\ch23\pp23010.jpg
Inactive, but interconvertible with active cis form

6. Abscisic acid (ABA): Is not the primary hormone for abscission.
Is the primary hormone responsible for seed dormancy.
Is the major stress hormone.
Is the only member of its class.
Is a 15-carbon compound resembling the terminal portion of some carotenoid molecules.
The (s)-cis-ABA is the naturally occurring active form.

7. ABA Biosynthesis:
Synthesized in almost all cells that contain chloroplasts or amyloplasts.
Key intermediate: Violaxanthin (C40)
Key enzymes:
Zeaxanthin epoxidase (ZEP), ABA1
9-cis-epoxycarotenoid dioxygenase (NCED), VP14
ABA-aldehyde oxidase, ABA3

8. ABA Biosynthesis
\figures\ch23\pp23021.jpg
zeaxanthin epoxidase

9. ABA Biosynthesis 9-cis-epoxycarotenoid dioxygenase
\figures\ch23\pp23022.jpg
ABA-aldehyde oxidase

10. ABA Metabolism
\figures\ch23\pp23023.jpg

11. Functions of ABA:
Seed dormancy
Drought stress response

12. Seed germination: the resumption of growth of the embryo of the mature seed.
…normally requires water, oxygen, suitable temperature, absence of inhibitory substances
Seed dormancy: a viable seed will not germinate even if all the necessary environmental conditions for growth are satisfied.

13. Cause of Dormancy
Coat-imposed dormancy: dormancy imposed in the embryo by the seed coat and other enclosing tissues, such as endosperm, pericarp, or extrafloral organs.
Embryo dormancy: a dormancy that is intrinsic to the embryo and is not due to any influence of the seed coat or other surrounding tissues.

14. Seed Dormancy: Seed dormancy is controlled by the ratio of ABA to GA.
ABA inhibits GA-induced enzyme production.

15. Precocious germination in the ABA-deficient vp14 mutant of maize
VP14 encodes NCED, 9-cis-epoxycarotenoid dioxygenase
\figures\ch23\pp23030.jpg

17. ABA Closes Stomata in Response to Water Stress

18. ABA and Guard Cell: ABA promotes stomatal guard cell closure.
ABA prevents light-induced stomatal opening.

19. ABA Induces the Increase of Cytosolic Calcium

20. ABA Signaling in Stomatal Guard Cells
Cell surface receptor
Ca2+
K+in channel
K+out channel
H+-ATPase
Cl- channel
\figures\ch23\pp23120.jpg

21. ABA Signaling in Stomatal Guard Cells
[Ca2+] pH
K+out
K+in
H+ pump
\figures\ch23\pp23120.jpg
Promote
stomatal closing
Prevent
stomatal opening

22. Summary:
ABA is the major hormone responsible for seed dormancy and water stress response.
Key enzymes for biosynthesis: ZEP (ABA1), NCED (VP14), and ABA-aldehyde oxidase (ABA3).
ABA antagonizes the action of GA in seed germination.
ABA controls stomatal guard cell closure/opening.
ABA signaling in stomatal guard cells: Ca2+, K+, H+.