Ab206 hormones 3 cytokinins & florigen

28/08/14 BIOL2156/2334 Plant Structure & Function

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PLANT HORMONES 3CYTOKININS + FLORIGEN

Nitin Mantri

School of Applied Sciences

RMIT University

Room 223.1.28

Tel. 03 9925 7152

Email: [email protected]


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PLANT HORMONES 3CYTOKININS (PHYTOCHROME) & FLORIGEN� auxins 1880 Darwin

� ethylene 1924/70 Osborne

� gibberellins (GA) 1926 Kurosawa/Brien

� abscisic acid (ABA) 1965 Wareing/Aldicott

� cytokinins 1956 Skoog, Miller

� ‘florigen’?/phytochromes 1940s


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CYTOKININS -HISTORY OF DISCOVERY� 1941 van Overbeek (Holland)

� tissue culture� cells would expand but not divide� coconut milk -> cell division

� 1950s Skoog (USA)� 1000-fold pure compound (growth factor)� callus culture (undifferentiated cells)� no differentiation (morphogenesis) into shoots,

roots, etc.


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CYTOKININS -HISTORY OF DISCOVERY

� 1956 Miller, Skoog et al. (USA)� herring sperm DNA (old – denatured)� active ingredient = kinetin; clue from adenine� (not natural cytokinin)

� 1964 Miller isolated ‘zeatin’� natural cytokinin from maize – Zea mays� also in production of disease symptoms

• e.g. root and leaf galls; root nodules on legumes


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STRUCTURE OF CYTOKININS

• all substituted adenine derivatives

• adenine not active

• >100 known

• activity depends on group at 6-position on ring

� greater activity for N-containing group


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STRUCTURE OF CYTOKININS

zeatin(naturally occurring)


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MOLECULAR ACTIVITY� Production

� mostly in root tips in plants� but found in leaves, fruits, seeds; bleeding sap

caused by wounds

� Transport� Xylem; in direction of high auxin concentration

� Mechanisms� synthesis from adenine� binds to ribosomes� alteration of translation of RNA


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PHYSIOLOGICAL ACTIVITIES 1. Cell division

� main use in tissue culture

� important in agriculture, horticulture, biotechnology

� e.g. tobacco, clover, kangaroo paw

� Undifferentiated plant cell can:

� Auxin – rapid cell expansion (giant cells)

� Kinetin alone – little/no effect

� Auxin + Kinetin – rapid cell division (meristematic)

� High Auxin – roots

� High Kinetin - shoots


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PHYSIOLOGICAL ACTIVITIES 2. Organ/tissue differentiation


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2. Organ/tissue differentiation


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PHYSIOLOGICAL ACTIVITIES2. Organ/tissue differentiation

� tobacco callus differentiation

Auxin:cytokinin ratio

Product

low shoots intermediate callus

high roots


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PHYSIOLOGICAL ACTIVITIES3. Release from dormancy

� anti-ABA action� e.g. lettuce seeds + cytokinin ->

germination� e.g. buds of winter twigs + cytokinin -> bud

burst


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PHYSIOLOGICAL ACTIVITIES4. Preventing senescence/abscission

� anti-ABA and anti-ethylene action, e.g.

• leaves + cytokinins• chlorophyll retained• senescence delayed• abscission prevented

� potential uses - delay in deterioration of leafy cropsin shops

agritech.tnau.ac.in


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PHYSIOLOGICAL ACTIVITIES4. Release from apical dominance

� anti-auxin action� intact lateral buds� + cytokinin -> growth


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INTERACTIONS BETWEEN HORMONES

� Difficult to isolate effects of individual hormones due to interactions with others

� control of growth depends on� balance of hormones at the site� receptiveness of tissue at the site


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INTERACTIONS BETWEEN HORMONES

� Main interactions are� auxins and ethylene� GA and ABA� cytokinins and all others

� Also important interactions with environmental conditions

� e.g. light, cold, daylength (photoperiod)


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HORMONE INTERACTIONExample: leaf abscission

� Young leaf still has active meristems� auxins secreted� auxins attract cytokinins from roots� prevention of ethylene induced abscission or

senescence� prevents ABA accumulation� prevention of senescence and abscission


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INTERACTIONS BETWEEN HORMONES� Old leaf has few active meristems

� auxin production decreases� cytokinins no longer attracted� meristem activity decreases� ethylene produced� abscission layer begins to form� ABA formed instead of GA� abscission accelerated + senescence begins


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PLANT HORMONES 3CYTOKININS (PHYTOCHROME) & FLORIGEN� auxins 1880 Darwin




� cytokinins 1956 Skoog & Miller

� ‘florigen’?/phytochromes 1940s


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LIGHT, PHYTOCHROME AND FLORIGEN - Topics

� Discovery of photoperiodism and phytochrome

� Light spectrum in plant growth

� Molecular activity of ‘florigen’

� Physiological activities and uses

� Evidence for ‘florigen’


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DISCOVERY OF PHOTOPERIODISM AND PHYTOCHROME

� 1920s Garner and Allard� Maryland Mammoth tobacco� breeding experiments� would not flower in summer (long days)

day night


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� short days -> flowering (short-day plant)

day night


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� if long nights

� interrupted by short flashes of light� white/� far-red

� no flowering

day night night


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� all plants vary in photoperiod requirements

� main factor is habitat

� flowering in good conditions for seed maturation

� some require cold (vernalisation) too


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� Photoperiod (length of time in light in hours/day) important

� now know that dark length is what is important

� Hypothesis: a hormone ‘florigen’ formed in the ‘right’ photoperiod causes flower induction


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LIGHT SPECTRUM IN PLANT GROWTH� Photomorphogenesis

� irreversible change of form due to photoperiod• e.g. seed germination• e.g. induction of flowering• e.g. leaf abscission (some plants, not all)

� active wavelengths• red light (660 nm)• far-red light (730 nm)

� receptors = phytochromes (2 forms)


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LIGHT SPECTRUM IN PLANT GROWTH

phytochrome (PR) phytochrome (PFR)

blue yellow

inactive active

low energy state high energy state

stable unstable

transformed to highenergy form by red light

decays to low energyform in dark


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Phytochrome and light

slow, dark or light

fast, 660 nm (red)

fast, 730 nm (far-red)

destruction

t ½ 8-12 h

destruction

t ½ 1-2 h

PR PFR


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LIGHT SPECTRUM IN PLANT GROWTH

� in light, PR ↓ and PFR ↑

� in dark, PR ↑ and PFR ↓

� in light, ratio PFR: PR = 2:1

� ratio (balance) PFR: PR determines:� direction and rate of growth� development (photomorphogenesis)


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MOLECULAR ACTIVITY� Production

� only receptive tissues (leaves)� on plasmalemma or ER

� Transport� probably phloem

� Mechanisms� deprotonated PR -> PFR

� protein binding� affects repression of enzymes


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PHYSIOLOGICAL ACTIVITIESPhotomorphogenesis-phytochrome/hormones

Activity auxins C2H4 GA ABA cytokininsSeed germination ↑ ↓Cessation of etiolation(dark -> light form)

↓ ↓ ↑

Leaf senescence andabscission

↓ ↑ ↓ ↑ ↓

Induction of dormancy inwinter twigs

↑ ↓

Induction of flowering –‘florigen’

? ? ? ? ?


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Activity auxins C2H4 GA ABA cytokininsInduction of flowering– ‘florigen’

? + + + ?

Long-day plants• PFR: PR ↑• e.g. chrysanthemum

↑

Short-day plants• PFR: PR ↓• e.g. poinsettia

↑


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� ‘Florigen’ common between long-day and short-day plants

� shown by grafting experiments

� transmission of chemical stimulus


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LONG DAYS LONG DAYS LONG DAYS

long-day plant short-day plant short-day plant

↓ ↓ ↓

leaf grafted leaf leaf

↓ ↓ ↓

flowering flowering no flowering


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SHORT DAYS SHORT DAYS SHORT DAYS

long-day plant long-day plant short-day plant

↓ ↓ ↓

shoot grafted on short-day plant

↓ ↓ ↓

no flowering flowering in both plants

flowering

‘Florigen’•transmissible•common to SD + LD plants

Grafting experiments

Ridge, I. (1991). Plant Physiology. The Open University, Milton Keynes, UK


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Grafting experiments

A permanently inductive state can be demonstrated for leaves of some photoperiodic species. After 28 d of short days (SD) a leaf of Perilla returned to long days (LD) will continue to produce graft-transmissible flowering stimulus for at least 97 d, involving its successive grafts of the same leaf to vegetative, long-day-grown receptor plants. (Zeevaart 1958).

Do we know FLORIGEN?� Huang et al. (2005) The mRNA of the

Arabidopsis FLOWERING LOCUS T (FT) Gene Moves from Leaf to Shoot Apex and Induces Flowering

� Huang et al. article retracted from Science in 2007

� Notaguchi et al. (2008) Long-Distance, Graft-Transmissible Action of Arabidopsis FLOWERING LOCUS T Protein to Promote Flowering


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PLANT HORMONES 3PHYTOCHROME + ‘FLORIGEN’

� auxins 1880 Darwin




� cytokinins 1956 Skoog & Miller

� ‘florigen’? 1940/2008 Notaguchi


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Who knows the limits of plants in future?

Documents

Ab206 hormones 3 cytokinins & florigen