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 Factors
diffusable
potato explants
control
vascular tissue
cell division
Haberlandt (1913) showed that vascular tissue could induce cell
division in quiescent tissue.
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
Biosynthesis
• Plants and bacteria make
cytokinins,
– cytokinin is synthesized in
the root, and transported
acropetally via the xylem,
– bacteria infect plants,
make cytokinin and
cause tumors..
Don’t memorize this pathway.
Cytokinin / Auxin Balance
[ auxin ]
[ cytokinin ]
0
Undifferentiated plant tissue (callus) can
be induced to make shoots and roots
when given proper levels of cytokinin
and auxin.
shoots
roots
Control of Organogenesis
simplification
What happens?
What happens?
Shoot / Root Balanced
Shoot (auxin production) is
reduced in relation to root
(cytokinin production)
Root (cytokinin production) is
reduced in relation to
Shoot (auxin production)
high
cytokinin
normal
cytokinin
Cytokinins
…retard senescence ,
• Senescence,
– an active, genetically controlled,
developmental process,
– in which cellular structures and
macromolecules are broken down
– and transported to growing organs.
overexpressor
wt
Concept Map
Receptor/Sygnal Transduction
Discovery
Synthesis
(where)
Cytokinin
Function(s)
Transport
auxin interaction
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.
a-amylase,
Gibberellin
…contributes to mobilization of storage products,
embryo,
gibberellins,
...absorbs H2O, …diffuse to the
aleurone layer,
…hydrolyzes starch,
resulting sugars nourish
growing seedling.
GA signal transduction,
…results in the
expression of a-amylase,
Synthesis and Transport
Meristem and young leaves
• 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.
GA gene promoter,
expressing firefly “glow”
gene.
Gibberellin
…stimulates cell elongation and cell division.
[ GA ]
rice leaf sheath bioassay
dwarf pea
(mutant)
dwarf pea
(mutant)
+ GA
Gibberellin
…contibutes to “phase changes”,
...stimulates “bolting” in rosette habit plants,
Cabbage induced to
flower by GA alone.
…involved in juvenile to adult, and adult to
reproductive phase changes,
• may be “florigen” the flowering hormone
in some species,
... activates vegetative growth of the embryo,
• weakens structures in the seed,
• mobilizes stored food reserves.
Gibberellin
…contributes to fruit development,
untreated
animal
treated
…larger grapes,
... longer stems,
- healthier bunches.
Thompson seedless grapes
Concept Map
Discovery
Gibberellin
Function(s)
Transport
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,
dry roots,
...send stress signals to the
leaves (ABA).
– from roots to leaves through
xylem.
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
Discovery
Synthesis
Abscisic Acid
Function(s)
Transport
GA interaction
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
wild type
ctr
ein (ethylene present),
ctr (ethylene absent),
…ethylene insensitive.
…constitutive triple response.
twig
petiole
Ethylene
…contributes to apoptosis and abscission,
apoptosis,
…programmed cell death
cork
...suberized
protective layer
abscision layer,
…large, thin walled cells.
Concept Map
Discovery
Ethylene
Function(s)
Triple Response
Plant Responses to Signals III
Brassinosteroids
“steroid hormone”
Steroid Hormones
insects
plants
mammalian
sex hormones
Brassinosteroids
…de-etiolation factors and auxin-like functions,
dark
wt
light
det2
wt
de-etiolation,
dwarfs,
...constitutive,
…in light.
det2
Concept Map
the bare essentials...
Table 39.1
Brassinosteroids