Transcript Plant Responses to STRESS

NOTES:
CH 39 - Plant
Responses
to Internal &
External
Signals
Factors Affecting Plant Development:
1) The plant senses and responds to
ENVIRONMENTAL CUES.
2) The plant’s GENOME encodes for enzymes that
take part in development.
3) The plant uses RECEPTORS, such as
photoreceptors, that absorb light.
4) HORMONES (chemical messengers) regulate
the effects of environmental cues on receptors.
PLANT HORMONES:
● HORMONES = regulatory compounds that
act at very low concentrations at sites
distant from where they are produced.
● they mediate / regulate developmental
phenomena, such as:
– Stem growth
– Autumn leaf fall
Plant
hormones &
responses:
Some important plant hormones:
● Abscisic acid: inhibits growth;
maintains seed dormancy &
winter dormancy; closes
stomata during drought stress
● Auxin: promotes stem
elongation & fruit growth;
inhibits lateral bud outgrowth;
gravitropism
● Gibberellins: promote seed
dev. & germination, stem
growth, flowering, & fruit
development
Some important plant hormones:
● Cytokinins: root growth and
differentiation; cell division
and growth; seed
germination; delay leaf
senescence (aging / death)
● Ethylene: (gaseous
hormone) fruit ripening;
promotes leaf abscission
(autumn) & enhances rate of
senescence
Daily and Seasonal Responses
● Circadian
rhythm (24 hour periodicity)
 Transpiration & synthesis of certain
enzymes are plant processes that oscillate
during the course of the day
 Responses to environmental changes in:
light levels, temperature, relative humidity,
however…
 these cyclic processes often continue even
when the environmental cues are
removed (“biological clock”)
Daily and Seasonal Responses
Photoperiodism (a response to a change in
the relative lengths of night and day)
 Seasonal events, such as: flowering, seed
germination, the onset and breaking of bud
dormancy – all occur at specific times of the
year
●
Daily and Seasonal Responses
FROM SEED TO SEEDLING:
● recall: seeds may remain
dormant for weeks,
months, years,
or centuries!
● the mechanisms of dormancy include:
 exclusion of water by an impermeable seed
coat
 mechanical restraint of embryo by tough seed
coat
 chemical / hormonal inhibition of embryo dev.
Seed dormancy can be broken by…
● seed coat is weakened from
tumbling across the ground,
or passing through an
animal’s digestive tract;
● soil microorganisms soften
the seed coat;
● fire (can release a mechanical restraint or
remove the waterproofing of the seed coat)
● leaching (prolonged exposure to water)
FROM SEED TO SEEDLING:
● Germination begins with:
1) IMBIBITION (absorption of water)
-hydration causes the seed
to swell and rupture the
seed coat
-triggers metabolic changes in embryo 
resume growth
-storage nutrients are digested by enzymes
and nutrients are transferred to growing
regions of embryo.
Imbibition of water in peas!
FROM SEED TO SEEDLING:
Germination continues as the:
2) radicle emerges from the seed;
3) shoot tip breaks through soil surface
How does hypocotyl/shoot emerge from the soil?
*in many dicots:
-the hypocotyl is in shape of a
hook (pushed above ground)
-light stimulates the hypocotyl to straighten
-hypocotyl raises the cotyledons and epicotyl
above ground
-epicotyl then spreads the first leaves which
become green and begin photosynthesis
**Many seeds will remain quiescent
(dormant) until suitable environmental
conditions are available; other seeds
await a specific environmental cue (e.g.
heavy rainfall; brush fire; exposure to
cold or sunlight; passage through an
animal’s digestive system) before they
will break dormancy.
Flowering…
● FLOWERING = the formation of reproductive
organs
● May be initiated by:
 Plant reaching an appropriate age / size
 Particular time of year (sensed by the length of
night)…photoreceptors involved!
 It is likely that a “flowering hormone” is sent from
the leaves to where the flowers form
Fruit formation…
● As already studied, fruits form following
FERTILIZATION
● Fruit ripening is under hormonal control
Plant death…
● Some plants are PERENNIALS: continue to
grow year after year
 their buds typically enter a state of winter
dormancy during the cold season
 the hormone ABSCISIC ACID helps
maintain this dormancy
Plant death…
● many plans undergo SENESCENCE of certain
cells / organs / entire plant…during LEAF
ABSCISSION, leaves die & fall off at the end
of the growing season
● both of these processes involve turning on
specific genes leading to apoptosis
(programmed cell death);
 newly formed enzymes break down many
chemical components (DNA, RNA, proteins,
membrane lipids) that the plant will salvage for
a later date…
Plant Responses to STRESS:
ABIOTIC STRESSES:
● DROUGHT: close stomata; wilt / roll up;
promote root growth deeper down
● FLOODING: ethylene production
stimulates apoptosis of root cortex cells,
producing air tubes (“snorkels”) to provide
O2 to roots
● SALT STRESS
● HEAT STRESS / COLD STRESS
Plant Responses to STRESS:
BIOTIC STRESSES:
● HERBIVORY: animals eating plants
 physical defenses: thorns, trichomes
 chemical defenses: distasteful / toxic
compounds
 “recruit” predatory animals to kill the
insect parasites
● PATHOGENS: infection (bacteria, virus)
YUMMY!
Thank you
RUBISCO
for making
this all
possible! 