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Plant Responses to Internal & External Signals
For the readings, pay special attention to the diagrams
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Review of Signal Transduction PathwaysReception: signal is detected by some sort of
receptorTransduction: signal is “carried” from the
receptor to the nucleusThis is where you have secondary messengers
transfer the signalProtein kinase, orThe cGMP pathway (like the cAMP pathway)
Response: activates different transcriptional factors or enzymes
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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 chlorphyllThis process is called de-etiolation
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Reception
A photoreceptor, phytochrome, detects that light after the shoot breaks through the soilPhytochromes 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
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Transduction
Reception can be from a VERY weak light source, therefore you need to “amplify” the signal through transductionThe 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
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Response
Many transcriptional factors are activated in de-etiolationSome are activated by phosphorylationSome are activated by cGMPSome are activated by calcium
In addition to transcriptional factors, post-translational enzymes are also activated to modify the created proteins
Most of the proteins created are associated with photosynthesis and chlorophyll production
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PLANT HORMONES
How hormones coordinate growth, development and response to environment
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Plant Hormones
Hormones are chemical signals that coordinate the various parts of an organismA 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
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Example with Light
Example of the action of hormonesAuxin is a hormone that induces a plant to move
towards or away from a stimuli, tropismStimulus: LightResponse: the plants growth pattern will cause
the growth shoot to move towards or away from the lightPhototropism: growth towards the lightNegative phototropism: growth away from
the light
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Plant Hormones
There are 5 major classes of plant hormones, each with specific functions:AuxinCytokininsGibberellinsEthylene Abscisic acid
NOTE: Many hormones interact with each other to enhance or inhibit their activities
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Auxin
Found:In the embryo of seeds, meristems of apical
buds and young leavesFunction:
Stimulates stem elongation and root growth (causes the root cells to elongate)
Stimulates development of fruitInvolved in phototropism and gravitropism,
response of a plant to the effects of gravity
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Cytokinins
Found:Made in the roots and transported to other
organs of the plantFunction:
Affect the growth and differentiation of rootsStimulates cell division and growth (in
conjunction with auxins)Stimulates germination, growth from a seedDelay senescence, or the aging of the plant
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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
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EthyleneFound: in tissues of ripe
fruit, nodes of stems, and aging leaves and flowers
Function:Opposes some of the
effects of auxin (feedback)
Promotes fruit ripeningSenescence (aging) is
at least party caused by ethylene
“One bad apple spoils the whole bunch”
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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
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PLANT RESPONSES
How plants respond to various factors
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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 tropismsLight (Phototropism)Gravity (Gravitropism)Touch (Thigmotropism)
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Phototropism
Phototropism is the growth of a shoot towards lightThis is primarily
due to the action of auxin
Auxin elongates the cells on the non-light side
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Light Receptors: There are 2 main types of photoreceptors
Blue-light photoreceptor: These receptors absorb mostly blue light
May be responsible for opening stomata, and inhibit hypocotyl elongation in seedlings breaking ground
Phytochromes: These receptors absorb mostly red light
Responsible for de-etiolation, seed germination, and “avoid” shade
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Circadian Rhythms
The production of enzymes, hormones and other processes oscillate during the dayThis is due to many environmental factors
Light levels, temperature, humidityThere are other processes that occur with a
frequency of every 24 hours that are not dependent on environment: circadian rhythms
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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)
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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 periodFlower in later summer/fall/winterExample: poinsettias
Long-day plantsRequire a longer light periodFlower in late spring/early summerExample: spinach
Day-neutral plantsAre unaffected by photoperiodExample: tomatoes
But it’s actually the night that matters!! (if there is even a little sunlight during the “night” the flowers will not bloom)
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Other Factors that Affect Flowering
In addition to photoperiod, some plants need additional environmental cues to induce floweringExample: Some plants need to be exposed to
critical temperature rangesVernalization: the need to be exposed to long
periods of “cold” temperatures to induce flowering (this occurs in winter wheat)
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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
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Plant Defenses
Plants defend themselves against herbivores in several waysPhysical defenses, such as thornsChemical defenses, such as
producing distasteful/toxic
compoundsCan use chemicals to attract
insects to help defend the plant
• Wasps
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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
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Plant Defenses
Defense against pathogens is also important for plant survivalFirst line of defense is the plant’s “skin”
Plant dermis, cuticle, barkIf 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 virulentMany times the pathogen weakens, but does not kill,
the plant so that the pathogen may survive• This condition is called avirulent
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Pathogen Detection
At the genetic level, plant disease resistance can begin with gene-for-gene recognitionThe 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 elicitorsExample: oligosaccharins are molecules that derive
from damage cell wallsThey can also mount a defense against disease
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Plant Response to Pathogen
Once warned, plants can release chemicals that can fight an invaderPhytoalexins 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
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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