Download Slide set 8 – Physiology – Hormones and signals

Chapter 14
Physiology – Hormones,
signals, growth and
development
Topics
• What is a hormone? – method of action
• Roles in survival of a sessile organism in the
environment
• Mostly studied five plant hormones
• Other hormones
• Specific growth and developmental effects
• Hormone interactions
• Environmental signaling thru hormones
• Agricultural uses
Hormones - Chemical Messengers
Regulate growth, differentiation and development (among other
roles e.g. stomatal activity, environmental signaling)
Organic molecules produced in one part of the plant and
transported to other parts where they initiate a response – 3
common characteristics
• Act in very low concentrations and in collaboration
• Released into general circulation - not carried specifically to
the target
• At the site of response, receptor molecules, usually on
plasma membrane, bind to them triggering a variety of gene
expressions
Hormones
Hormones
Other compounds show hormone-like activity - much less
studied than the big five
• Brassinosteroids (first isolated from rapeseed (Brassica
napus) = complex chemicals involved in leaf
morphogenesis, root and stem growth, and vascular
differentiation – auxin-like effect – works thru auxins?
• Jasmonic acid - involved in defense against animals and
fungi
• Salicylic acid (related to aspirin) - involved in resistance
to pathogens, especially viruses
Auxin: Activation/Inhibition of
Shoots
• Auxin - produced as
shoot apical meristems
(SAM) grow and
initiate new cells
• Auxin flows mainly
downward, surrounds
all stem cells, causing 3
unique responses:
• Cell elongation
• Apical dominance
• Differentiation of vascular
tissues
Auxin: Apical dominance
Apically produced auxin
induces dormancy in
axillary buds
• Each shoot tip gets only
one active apical
meristem
• In lower buds where
auxin level is below a
threshold – buds grow
branches, flowers etc.
• Spring - buds release
auxins to stimulate
vascular cambium in stem
Hormone Interactions
Auxins and Cytokinins:
• In some species, apical dominance is established
by interplay of 2 or 3 hormones
• Active roots synthesize cytokinins
• Cytokinins are transported to the shoot and
stimulate axillary buds
• Ratio of cytokinin and auxin determines whether
buds become active or remain dormant
Hormone Interactions
• Auxins and Gibberellins:
• Auxin alone activates vascular cambium and
elicits differentiation of xylem
• Gibberellin is also present in a healthy stem and
causes some new cells to differentiate as phloem
Hormones: Environmental signaling
Hormones help
communicate with various
plant parts when one part
has encountered an
environmental change
• If a leaf is damaged by
animal feeding or water
stress, auxin production
and transport drop abscission zone does not
stay quiescent
• Autumn conditions
stimulate production of
ethylene
Hormones: Environmental signaling
• Ethylene suppresses auxin production and
transport in time for abscission before winter
• Premature abscission of fruits is prevented by the
presence and export of auxin through the pedicel
Hormones: Environmental signaling
• Auxin causes the developing fruits to release
ethylene
• At maturity, high concentration of ethylene
stimulates the pedicel abscission zone, overriding
auxin concentration
Hormones: Tropisms
• Blue - most effective wavelength for
phototropism
• Small pigment protein called
phototropin in stem tips or
coleoptile (outermost protective leaf
at the stem tip) absorbs blue light
• Darker side of the stem receives
extra auxin
• That side grows more rapidly and
bends toward the light
Hormones: Tropisms
In gravitropism - root cap acts
as the organ of perception of
gravity by statocytes with their
statoliths
• Lower side of the root is
detected by sinking statoliths
- extra auxin transported to
that lower side of the root cap
and then into the root
• Growth inhibition on lower
side
• Root grows downward
Flowering: Vernalization and
Photoperiodism
• Conversion from juvenile to adult is phase change – flowering
(fruits, seeds – survival or agricultural significance)
• Cold temperatures stimulate phase change in some – vernalin
production – flowering
• Photoperiod (day length – night length really) triggers flowering
in some – phytochrome perceives length of night – next slide
• Some need both critical low T and night length
• Hormones mediate these responses too – e.g. GA can
substitute vernalization and night length requirements in some
Photoperiodic Induction to flower
more common in temperate spp.
Pr - regulates circadian and seasonal phenomena
Day neutral plants - no phytochrome regulation
So many agricultural uses – growth regulators
• Seed germination - GA
• Propagation by cuttings, rooting - Auxin
• Partheniocarpic fruits - Auxin, GA, Cytokinin
• Flower and fruit set or fall - Auxin induced Ethylene, Cytokinin
• Fruit size - Auxin
• Fruit ripening, abscision - Ethylene
• Synchronized flowering - friuting - Ethylene
• Cut flower, vegetable postharvest shelf life - Cytokinin
• Stem/stalk length - GA
• Weedicides - Auxin
• Hybrid plant production e.g. cucumber hybrids - GA
• Antitarnspirants, enhanced color in red grapes - ABA
• Tissue culture - Auxin, cytokinin