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

Biol 352 Lecture 9 Abscisic Acid: Roles in Seed Dormancy and Stomatal Closure February 5, 2007. Review: ethylene Ethylene is a gaseous plant hormone. Precursor: amino acid methionine; immediate precursor: ACC. Key enzymes for biosynthesis: ACC synthase, ACC oxidase.

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

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  1. 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: • 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: p593-614 • (3rd Ed, Plant Physiology, Taiz & Zeiger, Chapter 23: p539-556)

  5. ABA: a Dormin Inactive in stomatal closure Naturally occurring active form 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 zeaxanthin epoxidase

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

  10. ABA Metabolism

  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

  16. ABA Closes Stomata in Response to Water Stress

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

  18. ABA Induces the Increase of Cytosolic Calcium

  19. ABA Signaling in Stomatal Guard Cells • Cell surface receptor • Ca2+ • K+in channel • K+out channel • H+-ATPase • Cl- channel

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

  21. 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+.

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