Plant Physiology Forum

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Plant Physiology Forum

• Co-ordinators Phillip Joy
• Kathy Pickle

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Epidermal Cell Death in Rice is Regulated by
Ethylene, Gibberellin, and Abscisic Acid

• Steffens and Sauter. 2005. Plant Physiology
  139713-721.

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Programmed Cell Death

• Genetically controlled suicide
• To eliminate undesirable cells
• Described in
• Vegetative growth
• Reproductive growth
• During stress adaptation
• Defense mechanism
• Hormone meditated

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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

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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.

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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.

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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.

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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.

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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

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Materials Methods

• Deepwater rice plants treated with hormones
• Measurements of adventitious root length taken
• Cell death measurements taken
• 2 Evans blue stain

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How is the ethylene signal perceived and
interpreted by different organs tissues to
allow for these diverse responses in a timely
fashion??
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Results - Ethylene

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

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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?

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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.

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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.
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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

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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

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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

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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.

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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

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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).
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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.

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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

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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.
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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

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• 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

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Questions Comments