Plant Responses to Signals Phytohormones (cont.)

It might seem unfair to reward a person for having so much pleasure over the years, asking the maize plant to solve specific problems and then watching its responses.

Barbara McClintock

Plant Responses to Signals III

Cytokinin“cell division”

Cell Division Factorsdiffusable

Haberlandt (1913) showed that vascular tissue could induce cell division in quiescent tissue.

potato explants

vascular tissue

control

cell division

expl

ant

Cytokinins• Van Overbeek discovered that

coconut milk stimulated the growth (cell division) of embryos,

• Carlos Miller, working in Folke Skoog’s lab in Wisconsin, systematically tried every random chemical he could get his hands on,

• Autoclaved DNA (degraded) did the trick. Zeatin

– cytokinin is synthesized in the root, and transported acropetally via the xylem,

– bacteria infect plants, make cytokinin and cause tumors..

Cytokinin Biosynthesis

• Plants and bacteria make cytokinins,

Don’t memorize this pathway.

Cytokinin / Auxin Balance

Undifferentiated plant tissue (callus) can be induced to make shoots and roots

when given proper levels of cytokinin and auxin.

[ auxin ]

[ cy

toki

nin

]

0

shoots

roots

Control of Organogenesissimplification

Shoot / Root BalancedShoot (auxin production) is reduced in relation to root

(cytokinin production)

Root (cytokinin production) is reduced in relation to

Shoot (auxin production)

What happens?

What happens?

Cytokinins …retard senescence ,

overexpressor wt

• Senescence,

– an active, genetically controlled, developmental process,

– in which cellular structures and macromolecules are broken down

– and transported to growing organs.

high cytokinin

normal cytokinin

Concept Map

Cytokinin

DiscoveryReceptor/Sygnal Transduction

TransportFunction(s)

auxin interaction

Synthesis (where)

Plant Responses to Signals III

Giberellins“Mendel’s dwarf”

Bakanae“foolish seedlings”

• Rice farmers in Asia have long known of a disease which makes the rice plant grow tall, but eliminates seed production,

– plants are prone to “lodging” (falling over),

• Shotaro Hori (1898) demonstrated that the symptoms were induced by infection with a fungus belonging to the genus Fusarium,

• Eiichi Kurosawa (1926) showed that filtrate from culture of Gibberellin fugikuroi, was an active agent,

• Margaret Radley (1950s) demonstrated that

Giberellin was synthesized by plant tissues. > 100 forms of

gibberellin acid (GA),

… 30 are active.

Gibberellin …contributes to mobilization of storage products,

embryo,

...absorbs H2O,

gibberellins,

…diffuse to the aleurone layer,

GA signal transduction,

…results in the expression of -amylase,

-amylase,

…hydrolyzes starch, resulting sugars nourish growing seedling.

Synthesis and Transport

• Gibberelins and intermediates are synthesized in young, actively growing buds and leaves,

– long distance transport via the phloem,

– short distance via symplastic routes,

• Some synthesis in the roots,

– transported acropetally via the xylem sap.

reporter gene,

…firefly luciferase.

Meristem and young leaves

GA gene promoter, expressing firefly “glow” gene.

Gibberellin …stimulates cell elongation and cell division.

rice leaf sheath bioassay

[ GA ]dwarf pea(mutant)

dwarf pea(mutant)

+ GA

Gibberellin …contibutes to “phase changes”,

Cabbage induced to flower by GA alone.

GA

... activates vegetative growth of the embryo,

• weakens structures in the seed,

• mobilizes stored food reserves.

...stimulates “bolting” in rosette habit plants,

…involved in juvenile to adult, and adult to reproductive phase changes,

• may be “florigen” the flowering hormone in some species,

Gibberellin …contributes to fruit development,

Thompson seedless grapes

untreated treated

…larger grapes,

... longer stems,

- healthier bunches.

animal

Concept Map

Gibberellin

Discovery

TransportFunction(s)

Plant Responses to Signals III

Abscisic Acid“stress hormone”

Dormin, Abscisin II• In 1963, abscisic acid was first

identified and characterized by Frederick Addicott

– through biochemical analysis of cotton fruit abscission (Abscisin II),

• Concurrently, a group headed by Philip Wareing was studying bud dormancy in Sycamore trees (Dormin),

… Plant physiologists agreed to call the compound abscisic acid.

Abscisic acid (ABA)

…isomers and enantiomers occur naturally and synthetically.

Synthesis and Transport

• ABA is synthesized in the chloroplasts,

• ABA is transported in the vascular tissue,

– from leaves to roots, through the phloem,

– from roots to leaves through xylem.dry roots,

...send stress signals to the leaves (ABA). Result: well hydrated leaves, closed stomates.

Abscisic Acid…induces guard cell closure,

• ABA concentration in well-watered xylem sap from sunflowers is ~ 1 - 5 nM,

• ABA concentration in water stress xylem sap is as much as 3000 nM,

– ABA is synthesized (or accumulates) in the roots, is mobilized for transport to the leaves.

Abscisic Acid…inhibits germination,

• ABA (at high concentrations) in seeds inhibits germination,

• ABA / GA balance often determines “internal” dormancy status,

• ABA is water soluble, imbibition may serve to leach ABA from dormant seeds,

– imbibition: the uptake of water by germinating seeds.

ABA deficient seed

Concept Map

Abscisic Acid

Discovery

TransportFunction(s)

GA interaction

Synthesis

Plant Responses to Signals III

Ethylene“the gaseous hormone”

Gas light• Egyptians gassed figs in order to stimulate

ripening,

• The ancient Chinese burned incense in closed rooms to enhance the ripening of pears.

• In 1864, gas leaks from street lights were observed to stunt plant growth, twist plants, and abnormally thicken stems.

• Dimitry Neljubow (1901) showed that the active component was ethylene.

• R. Gane (1934) reported that plants synthesize ethylene.

ethylene

Ethylene…promotes fruit ripening,

• Ethylene signals the transition from unripe to ripe fruits,

– cell wall components are broken down,

– starches and acids are broken down resulting in “sweetening” and aromatic compounds ,

– pigmentation may also be induced.

Ethylene…promotes the “triple response”,

…in etiolated seedlings,

– reduced stem elongation,

– thicker stem,

– horizontal growth,

• May provide the plant with “behavior” that will provide escape from soil impediments.

Ethylene…mutant analysis,

wild type ein

ein (ethylene present),

…ethylene insensitive.

wild type

ctr (ethylene absent),

…constitutive triple response.

ctr

Ethylene…contributes to apoptosis and abscission,

twig petiole

cork

...suberized protective layer

abscision layer,

…large, thin walled cells.

apoptosis,

…programmed cell death

Concept Map

Ethylene

Discovery

Triple ResponseFunction(s)

Plant Responses to Signals III

Brassinosteroids“steroid hormone”

Steroid Hormones

plantsmammalian

sex hormones

insects

Brassinosteroids…de-etiolation factors and auxin-like functions,

dark

wt det2

de-etiolation,

...constitutive,

light

wt det2

dwarfs,

…in light.

Concept Map

BrassinosteroidsTable 39.1

the bare essentials...