Roles in Seed Dormancy and Stomatal Closure 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. Ethylene functions: fruit ripening, triple response, abscission, flower senescence, root hair development, etc. Two-component signalling system in ethylene signal transduction.

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

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

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

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.

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

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

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

ABA Metabolism \figures\ch23\pp23023.jpg

Functions of ABA: Seed dormancy Drought stress response

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.

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.

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

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

ABA Closes Stomata in Response to Water Stress

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

ABA Induces the Increase of Cytosolic Calcium

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

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

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+.