Transcript Plant Responses to Internal and External Signals

Plant Responses to
Internal and External
Signals
Chapter 39
S
Signals
S A signal is something that causes a response.
S The 3 stages of signal transduction pathways:
S 1. Reception: a signal binds to a receptor
S 2. Transduction: The signal is relayed through the cell/body
S 3. Response: Activation of cellular responses
Tropism
S Tropism is when a plant
curves towards or away
from a stimuli.
S Phototropism is when a
shoot grows towards a
light source (the cells on
the dark side elongate)
Plant Hormones
S Hormones are chemical signals that coordinate the different
parts of an organism.
S You need to know 6 plant hormones: auxin, cytokinins,
gibberellins, brassinosteroids, abscisic acid, and ethylene.
Auxin
S Found in seed embryos, meristems of
apical buds, and young leaves
S Simulates stem elongation, root
growth, cell differentiation,
branching; regulates fruit
development, functions in
phototropism, promotes xylem
differentiation.
Cytokinins
S Synthesized in roots, transported to other organs
S Affect root growth and differentiation, stimulates cell
growth and division, germination
Gibberellins
S Found in meristems of
apical buds and roots,
young leaves, and
embryos
S Promotes seed and bud
germination, stem
elongation, leaf growth;
stimulates flower and
fruit development;
affects root growth
Brassinosteroids
S Found in seeds, fruit, shoots,
leaves, and buds
S Inhibits root growth, keeps leaves
on plants, promotes xylem
differentiation
Abscisic acid
S Found in leaves, stems, roots, and green fruit
S Inhibits growth, closes stomata during water stress,
promotes seed dormancy
Ethylene
S Found in tissues of ripening fruit, nodes of stems, aging
leaves and flowers
S Promotes fruit ripening, opposes auxin effects, promotes or
inhibits growth and development of roots, leaves, and
flowers
Responses to light
S Photomorphogenesis
is the effect of light on
plant morphology.
S There are two major
classes of light
receptors: blue-light
photoreceptors and
phytochromes.
Blue-light Photoreceptors
S Blue light causes phototropism, the opening of stomata, and
the growth of plant seedlings.
Phytochromes
S Phytochromes cause:
S Seed germination
S Avoidance of shady
areas
Circadian Rhythms
S Circadian rhythms are 24 hour cycles common to all
eukaryotes. They include:
S Pulse, blood pressure, alertness, urine composition,
metabolic rate, and sex drive
S In plants, it controls opening and closing of flowers and
stomata
Flowering
S Short-day plants flower when daylight is short, such as spring or
late fall.
S Long-day plants flower in late spring or early summer, when
days are 12-14 hours long.
S Day neutral plants are unaffected by photoperiod and flower
whenever they mature.
S Flowering is also affected by length of dark hours and
temperature.
S The flowering hormone in plants is called florigen.
Stimuli other than light
S Gravity: plants grow up towards the sun
S Wind: plants grow out of windy areas
S Drought: plants conserve water in droughts
S Floods: Plant roots undergo apoptosis so they don’t get too much
water
S Salt: Plants absorb less water
S Heat: stomata close
S Cold: increase lipid concentration in cell membranes
Plant Defenses
S Plants defend themselves against herbivores by producing
thorns, chemicals, and distasteful or toxic compounds
S Plants defend themselves from pathogens by having thick
cell walls and developing genetic resistance.