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

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Presentation on theme: "Biol 352 Lecture 9 Abscisic Acid: Roles in Seed Dormancy and Stomatal Closure February 5, 2007."— Presentation transcript:

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

2 Review: ethylene 1.Ethylene is a gaseous plant hormone. 1.Precursor: amino acid methionine; immediate precursor: ACC. 2.Key enzymes for biosynthesis: ACC synthase, ACC oxidase. 3.Ethylene functions: fruit ripening, triple response, abscission, flower senescence, root hair development, etc. 4.Two-component signalling system in ethylene signal transduction.

3 Lecture Outline: 1.Structure of ABA 1.Biosynthesis of ABA 2.Function of ABA: seed dormancy, water stress response 1.ABA signalling in stomatal guard cells 2.Summary

4 Learning Objectives: 1)Define the key enzymes in ABA biosynthesis 2)Determine the roles of ABA in seed germination/dormancy 3)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 Inactive, but interconvertible with active cis form

6 Abscisic acid (ABA): 1)Is not the primary hormone for abscission. 2)Is the primary hormone responsible for seed dormancy. 3)Is the major stress hormone. 4)Is the only member of its class. 5)Is a 15-carbon compound resembling the terminal portion of some carotenoid molecules. 6)The (s)-cis-ABA is the naturally occurring active form.

7 ABA Biosynthesis: 1)Synthesized in almost all cells that contain chloroplasts or amyloplasts. 1)Key intermediate: Violaxanthin (C 40 ) 1)Key enzymes: Zeaxanthin epoxidase (ZEP), ABA1 9-cis-epoxycarotenoid dioxygenase (NCED), VP14 ABA-aldehyde oxidase, ABA3

8 ABA Biosynthesis zeaxanthin epoxidase

9 ABA Biosynthesis 9-cis-epoxycarotenoid dioxygenase ABA-aldehyde oxidase

10 ABA Metabolism

11 Functions of ABA: 1. Seed dormancy 1. 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: 1)Seed dormancy is controlled by the ratio of ABA to GA. 2)ABA inhibits GA-induced enzyme production.

15 Precocious germination in the ABA-deficient vp14 mutant of maize VP14 encodes NCED, 9-cis- epoxycarotenoid dioxygenase

16

17 ABA Closes Stomata in Response to Water Stress

18 ABA and Guard Cell: 1)ABA promotes stomatal guard cell closure. 1)ABA prevents light-induced stomatal opening.

19 ABA Induces the Increase of Cytosolic Calcium

20 ABA Signaling in Stomatal Guard Cells 1)Cell surface receptor 2)Ca 2+ 3)K + in channel 4)K + out channel 5)H + -ATPase 6)Cl - channel

21 ABA Signaling in Stomatal Guard Cells [Ca2+] pH K + out K + in H + pump ABA Prevent stomatal opening Promote stomatal closing

22 Summary: 1.ABA is the major hormone responsible for seed dormancy and water stress response. 1.Key enzymes for biosynthesis: ZEP (ABA1), NCED (VP14), and ABA-aldehyde oxidase (ABA3). 2.ABA antagonizes the action of GA in seed germination. 3.ABA controls stomatal guard cell closure/opening. 4.ABA signaling in stomatal guard cells: Ca 2+, K +, H +.


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