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Plant Physiology Forum

Plant Physiology Forum. Co-ordinators: Phillip Joy & Kathy Pickle. Epidermal Cell Death in Rice is Regulated by Ethylene, Gibberellin, and Abscisic Acid. Steffens and Sauter. 2005. Plant Physiology 139:713-721. Programmed Cell Death. Genetically controlled suicide

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Plant Physiology Forum

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  1. Plant Physiology Forum Co-ordinators: Phillip Joy & Kathy Pickle

  2. Epidermal Cell Death in Rice is Regulated by Ethylene, Gibberellin, and Abscisic Acid Steffens and Sauter. 2005. Plant Physiology 139:713-721.

  3. Programmed Cell Death • Genetically controlled suicide • To eliminate undesirable cells • Described in: • Vegetative growth • Reproductive growth • During stress adaptation • Defense mechanism • Hormone meditated

  4. PCD in Rice • Low oxygen stress induces two PCDs • Death of cortical cells in roots • Enhances aerenchyma cell formation • Death of epidermal cells that cover adventitious root primordia • Prevents injury to growing root that emerges under flooding • Both ethylene dependant

  5. Ethylene • Gas • As they approach maturity, many fruits release ethylene. • Promotes the ripening of the fruit. • Affects many other plant functions: • abscission of leaves, fruits, and flower petals; • drooping of leaves; • Sprouting of potato buds; • seed germination; • flower formation in some species.

  6. Gibberellins • Dependent on the type and species • Physiological effects: • Stem elongation - stimulating cell division • Stimulates flowering • Breaks seed dormancy • Stimulates mobilization of seed reserves • Induces maleness in dioecious flowers (sex expression). • Parthenocarpic (seedless) fruit development. • Senescence in leaves and citrus fruits.

  7. Abscisic acid (ABA) • Influences all aspects of plant growth • complex mechanisms rather than linear signaling • Mediates stress tolerance • Regulates stomatal aperture • Pathogens & wounding responses • In seeds, ABA promotes: • seed development, embryo maturation, synthesis of storage products, desiccation tolerance, maintenance of dormancy, apoptosis • Affects plant architecture • Root growth & morphology • Root-to-shoot ratios.

  8. GA in Rice • Ultimate growth-promoting hormone. • Submergence causes: • Accumulation of ethylene, a decrease in ABA, and an increase in bioactive GA. • Ethylene may be responsible • In the youngest internode: • GA has a growth promotive effect • ABA acts as a growth inhibitor • Altered GA to ABA balance promotes growth.

  9. Ethylene in Rice • In submergence-tolerant species • mediates adaptive responses to flooding • accumulates upon submergence through physical entrapment and enhanced biosynthesis • In deepwater rice elevated levels regulate: • accelerated growth of internodes & adventitious roots • cell death processes • underlying enhanced formation of aerenchyma • epidermal cell death at the sites where adventitious roots emerge • Processes need to be tightly coordinated in time and space

  10. Materials & Methods • Deepwater rice plants treated with hormones • Measurements of adventitious root length taken • Cell death measurements taken • 2% Evans blue stain

  11. How is the ethylene signal perceived and interpreted by different organs & tissues to allow for these diverse responses in a timely fashion??

  12. Results - Ethylene • Ethephon (synthetic ethylene releasing cmp) kills 1/4th of the cells within 1.5hrs • Measured by counting the dyed dead cells

  13. Results – GA and ABA • Internodal growth in deepwater rice is mediated through alternating GA and ABA in that tissue. • Is cell death also mediated thru this? • Not according to this study, neither GA or ABA did promote cell death. • How about when combined with ethephon?

  14. Results – Ethylene and GA • Will GA enhance ethylene PCD? • Ethephon alone: 30% in 10hrs • Ethephon and GA: 58% in 10hrs • Will adventitious roots be effected? • Yes, between 10 and 14 hrs root growth was promoted, 2 to 6 hrs after PCD occurred.

  15. Death of epidermal cells treated with 15 µM ethephon or treated with a combination of 15 µM ethephon and 30 µM GA3. Adventitious root growth at the third node of rice stems treated with or without GA3.

  16. Results – PAC, GA and ethylene • What role does GA play in promoting root growth? • Paclobutrazol (PAC) inhibits GA biosynthesis and lowers GA levels in plants • PAC was used to test the effect of GA on induced root growth • No trt: no growth • Ethephon: 1.2mm • Ethephon and PAC: 0.8mm • Ethephon and GA: 6.7mm • Ethephon, GG and PAC: 3.3mm

  17. Results – PAC, GA and ethylene • Must GA be available for ethylene to promote PCD? • Ethephon: 7% PCD • Ethephon and extensive PAC trt: 30% PCD • so, no? • But we know that GA with ethephon increases both PCD and root growth, which is also promoted by PCD……. • Perhaps PAC alters cells sensitivity to GA……

  18. Discussion – PAC, GA and ethylene • Less GA is needed to restore PCD %s in PAC treated plants than controls • Indicates an increases cell sensitivity to GA & ethylene-induced PCD • Assuming linked signaling pathways • PAC may increase ethylene cell sensitivity • This is seen only in epidermis cells and not in adventitious root primordia

  19. Discussion – Ethylene and GA • Ethylene and GA act synergistically • Indicates common signaling pathway • Previous studies show that ethylene is required for PCD signaling. • Primary pathway mediated through ethylene.

  20. Results – ABA, GA and ethylene • What role does ABA play in PCD? • Control: 12% • Ethephon: 26% • GA: no effect • Ethephon and GA: 55% • Ethephon, GA and ABA: PCD was reduced, app 7%

  21. Effect of ethylene, GA3, and ABA on epidermal cell death. Cell death was measured after treatment from 10 h with 0 to 150 µM ethephon (A), 0 to 100 µM GA3 (B), or 0 to 100 µM ABA (C).

  22. Results – ABA, GA and ethylene • Can higher levels of ethylene or GA inhibit the effect of ABA? • Increasing ethephon alone did not reduce the effect of ABA • Increasing ethephon and/or GA did decrease the effect of ABA, but this is very dosage dependent • So, can ethylene or GA inhibit ABA? • ABA seems to be a potent inhibitor of ethylene induced PCD that can be partly overcome by GA.

  23. Results – ABA, ethylene and PAC • What will happen to PCD when supplied with ABA and ethylene, but has its GA levels lowered (with PAC)? • But PAC amplifies ethephon induced PCD • PAC does not inhibit the effect of ABA on PCD • This suggests that changing GA levels will not effect the results of additional ABA

  24. ABA inhibition of ethylene-induced PCD is not dependent on GA. Rice plants were pretreated with or without 2 µM PAC for 8 d. After harvesting of stem sections, incubation with or without PAC was continued for 10 h with addition of ethephon (E) and ABA as indicated.

  25. Discussion - ABA • ABA by itself had no effect on rice PCD • ABA did prevent ethylene induced PCD • May play a protective role in determining the timing and extent of PCD. • Partially overcome by increasing levels of exogenous GA

  26. Inhibitory effect of ABA on PCD bypasses GA production • ABA interferes with ethylene and possibly GA signaling • ABA inhibits not only PCD but adventitious root development

  27. Questions & Comments

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