Transcript Control Systems in Plants

Plant Responses to Internal &
External Signals
For the readings, pay special
attention to the diagrams
Review of Signal Transduction
Pathways
Reception: signal is detected by some
sort of receptor
Transduction: signal is “carried” from the
receptor to the nucleus
This is where you have secondary
messengers transfer the signal
Protein kinase, or
The cGMP pathway (like the cAMP pathway)
Response: activates different
transcriptional factors or enzymes
Example: Potatoes
Potatoes grow beneath the soil in a dark
environment (they produce many short
stems that lack leaves in the hope that the
stem will break through the soil surface)
The adaptations to grow in the dark is known as
etiolation
When a stem breaks through the surface,
leaves expand, roots elongate, and the
plant produces chlorphyll
This process is called de-etiolation
Reception
A photoreceptor, phytochrome, detects
that light after the shoot breaks through
the soil
Phytochromes are not located on the
cell surface, but are instead located in
the cytoplasm
When light strikes the phytochrome, it
causes a conformational change that
causes a transduction pathway to begin
Transduction
Reception can be from a VERY weak light
source, therefore you need to “amplify” the
signal through transduction
The phytochrome, when activated, causes an
increase in the concentration of cGMP (like
cAMP) and calcium ion.
cGMP activates protein kinases (through
phosphorylation)
The increased calcium concentration with the
activation of kinases leads to a response
Response
 Many transcriptional factors are activated in deetiolation
Some are activated by phosphorylation
Some are activated by cGMP
Some are activated by calcium
 In addition to transcriptional factors, posttranslational enzymes are also activated to modify
the created proteins
 Most of the proteins created are associated with
photosynthesis and chlorophyll production
PLANT HORMONES
How hormones coordinate growth,
development and response to
environment
Plant Hormones
Hormones are chemical signals that
coordinate the various parts of an
organism
A hormone is a compound produced in one part
of the organism which is then transported to
other parts of the organism, where it triggers
responses in target cells and tissues
Many hormones are effective in VERY small
concentrations
Many times, hormone concentrations are
dependent on environmental stimuli
Example with Light
 Example of the action of hormones
 Auxin is a hormone that induces a plant to move
towards or away from a stimuli, tropism
Stimulus: Light
Response: the plants growth pattern will
cause the growth shoot to move towards or
away from the light
Phototropism: growth towards the light
Negative phototropism: growth away
from the light
Plant Hormones
There are 5 major classes of plant
hormones, each with specific functions:
Auxin
Cytokinins
Gibberellins
Ethylene
Abscisic acid
NOTE: Many hormones interact with each
other to enhance or inhibit their activities
Auxin
 Found:
In the embryo of seeds, meristems of apical
buds and young leaves
 Function:
Stimulates stem elongation and root growth
(causes the root cells to elongate)
Stimulates development of fruit
Involved in phototropism and gravitropism,
response of a plant to the effects of gravity
Cytokinins
 Found:
Made in the roots and transported to other
organs of the plant
 Function:
Affect the growth and differentiation of roots
Stimulates cell division and growth (in
conjunction with auxins)
Stimulates germination, growth from a seed
Delay senescence, or the aging of the plant
Gibberellins
 Found: in meristems of
apical buds and roots,
young leaves and
embryos
 Function:
Promote seed and bud
germination, stem
elongation and leaf
growth
Stimulate flowering
and fruit development
Affect root growth and
differentiation
Ethylene
 Found: in tissues of ripe
fruit, nodes of stems,
and aging leaves and
flowers
 Function:
Opposes some of the
effects of auxin
(feedback)
Promotes fruit
ripening
Senescence (aging)
is at least party
caused by ethylene
“One bad apple spoils
the whole bunch”
Abscisic Acid
 Found: in leaves, stems,
roots, and green fruit
 Function:
 Induces seed dormancy
 Anti-gibberellin
 Inhibits cell growth
 Anti-cytokinin
 Inhibits fruit ripening
 Anti-ethylene
 Closes stomata during
water stress, allowing
many plants to survive
droughts
PLANT RESPONSES
How plants respond to various
factors
Tropisms
Tropisms are growth responses that
result in curvatures of whole plant organs
toward or away from a stimuli
There are three major stimuli that induce
tropisms
Light (Phototropism)
Gravity (Gravitropism)
Touch (Thigmotropism)
Phototropism
Phototropism is
the growth of a
shoot towards light
This is primarily
due to the action
of auxin
Auxin elongates
the cells on the
non-light side
Light Receptors:
There are 2 main types of photoreceptors
 Blue-light
 Phytochromes:
photoreceptor:
These receptors
These receptors
absorb mostly red
absorb mostly blue
light
light
 Responsible for de May be responsible for
etiolation, seed
opening stomata, and
germination, and
inhibit hypocotyl
“avoid” shade
elongation in seedlings
breaking ground
Circadian Rhythms
 The production of enzymes, hormones and
other processes oscillate during the day
This is due to many environmental factors
Light levels, temperature, humidity
 There are other processes that occur with a
frequency of every 24 hours that are not
dependent on environment: circadian
rhythms
Biological Clocks/Circadian Rhythms
 A physiological cycle with a frequency of about
24 hours is called a circadian rhythm
 Even without external, environmental cues,
circadian rhythms persist in humans and in all
eukaryotes
 Example: jet lag in humans, leaf position in bean
plants
 It is believed these are due to some internal
biological clock that regulate these processes
(these work independent of the day/night cycle)
Photoperiodism
 A physiological response to day length (differs in winter,
summer, spring, and fall) is known as photoperiodism
 Short-day plants
 Require a shorter light period
 Flower in later summer/fall/winter
 Example: poinsettias
 Long-day plants
 Require a longer light period
 Flower in late spring/early summer
 Example: spinach
 Day-neutral plants
 Are unaffected by photoperiod
 Example: tomatoes
 But it’s actually the night that matters!! (if there is even a
little sunlight during the “night” the flowers will not bloom)
Other Factors that Affect Flowering
In addition to photoperiod, some plants
need additional environmental cues to
induce flowering
Example: Some plants need to be exposed to
critical temperature ranges
Vernalization: the need to be exposed to long
periods of “cold” temperatures to induce
flowering (this occurs in winter wheat)
Missing Flower Hormone
It is believed that the photoperiod is
detected by some chemical signal
located in the leaves, florigen (not
yet found)
If all of the leaves are removed
from the plant, it is no longer
affected by photoperiod
Plant Defenses
Plants defend themselves against
herbivores in several ways
Physical defenses, such as thorns
Chemical defenses, such as
producing distasteful/toxic
compounds
Can use chemicals to attract
insects to help defend the plant
• Wasps
Plant Defenses
Chemical warning systems:
When there is an infestation by
insects, plants can release a
chemical signal that causes other
plants to activate “defense” genes
to counteract the infesting
organisms
Plant Defenses
 Defense against pathogens is also important for plant
survival
First line of defense is the plant’s “skin”
Plant dermis, cuticle, bark
If a plant becomes infected, they release a series of
chemicals that destroy the pathogen (much like our
immune system)
If a pathogen is able to “avoid” or “suppress” a
plants defenses, the pathogen is said to be
virulent
Many times the pathogen weakens, but does not
kill, the plant so that the pathogen may survive
• This condition is called avirulent
Pathogen Detection
 At the genetic level, plant disease resistance can begin
with gene-for-gene recognition
The plant is able to “recognize” the protein products
of the pathogen and able to mount a defense against
the disease
 Another “detection” method are molecules called
elicitors
Example: oligosaccharins are molecules that derive
from damage cell walls
They can also mount a defense against disease
Plant Response to Pathogen
 Once warned, plants can release chemicals that can
fight an invader
Phytoalexins are a group of compounds that are
antimicrobial (the equivalent of our B and Tcells)
There are also a general group of proteins (PR,
or pathogenesis-related, proteins) that are
antimicrobial or act as messengers to activate
further defense
If the pathogen is avirulent, then there may be a
more aggressive, localized, response called
hypersensitive response
General Defense
A hypersensitive response may produce a
chemical signal that alerts the rest of the
plant
 As a result, more phytoalexins and PR
proteins can be released to produce a
nonspecific defense, system acquired
resistance (SAR)
A hormone that is thought to produce
this resistance is salicylic acid