Signals from Sunlight and Tropism - 13 Flashcards
What 3 categories make up photoreceptors
- phytochromes
- cryptochromes
- phototropins
Define photoreceptors
receptor molecules that plants use to detect sunlight
What is the function of photoreceptors
they absorb photons of a given wavelength and use that energy as a signal to initiate a photoresponse
What do photoreceptors consist of
consist of a protein and a light absorbing pigment molecule called a chromophore
What photoreceptors are the most important
ones that absorb red and blue light
Define phytochromes
photoreceptors that absorb red and far-red light most strongly, but also blue and UVA radiation
What 3 main classes of photoreceptors mediate the effects of UV-A/blue light
- cryptochromes
- phototropins
- ZEITLUPE
(all use flavin molecules as chromophores)
What are action spectrum used to determine
they determine which wavelengths of light are necessary to bring a particular plant response
action spectrum are commonly compared to what
absorption spectrums because they show all the light absorbed while the action spectrum shows the light actually used to emit a response
What kind of light is seed germination stimulated by and is inhibited by
stimulated by red light and inhibited by far-red light
The action spectrum for blue light stimulates what responses and has what kind of peak characteristic
phototropism, stomal movements
three finger shape
what absorption spectrum is almost identical to the action spectrum of phototropism (orientation of plant in response to light)
phototropin
explain how phytochrome is photoreversible
phytochrome can exist in two forms and can be changed from one form to another by the influence of light and is responsible for both red and far-red activities
What does phytochrome interconvert between
Pr (inactive form using far-red light)
Pfr (active form using red light)
How can phytochrome be degraded
after photoconversion occurs into active form Pfr, it can be degraded by the ubiquitin pathway (molecules broken down into smaller molecules)
Explain what causes Pfr to slowly or rapidly convert back to Pr
slowly in the darkness
or rapidly converted back due to far-red light
where is red light abundant and where is far-red light abundant
red = direct sunlight
far-red = foliage canopies
What is the R:FR ratio and what is it used for
the ratio of red to far-red light that are used for plants growing under a canopy to regulate seed germination processes and shade avoidance
the ratio decreases as the canopy increases because of less red light
What are some examples of typical photoreversible responses that are induced by phytochrome
in a seed it promotes germination, in an etiolated seedling it promotes leaf unrolling
Explain how long red-light induced events are reversible for
can be reversible by removing pfr… for a limited period of time after which they reach a point of no return and proceeds irreversibly
What are the functions of phytochrome (A)
mediates responses to continuous far-red light, change membrane potentials, and regulate gene expression
what does phytochrome-induced photomorphogenesis involve
protein degradation
Define cryptochromes
photoreceptors that interact with phytochromes to regulate photomorphogenesis (light-mediated development)
What functions are phototropins involved in
light-dependent processes that optimize photosynthetic efficiency (directing organs, chloroplast, and nuclear movements, solar-tracking, and stomatal opening)
What is the function of ZEITLUPE
participates in control of circadian clocks and flowering
Explain the chloroplast distribution patterns in response to low, high, or darkness light
low light the chloroplasts have accumulation response directly in the lights path, in high light the chloroplasts have avoidance response on the outside, and in darkness they gather under the vacuole in dark position
how does blue-light responses differ compared to normal light responses
they have a lag time after irradiation, and the effect of the response declines gradually after the disappearance of the light signal
whereas with normal light, the response turns on as soon as light is present and stops as soon as light goes off
Define UVR8
the photoreceptor that is involved in responding to UV-B irradiation that lacks a prosthetic chromophore
What is the function of UVR8
it mediates photomorphogenic processes by regulating transcription of UVR8-induced genes
What 3 abiotic factors influence the initial growth habits of young plants during seedling establishment
gravity, touch, and light influence
Define tropisms
directional growth responses of plants in response to particular external stimuli caused by the asymmetric growth (turning/rotating) of the stem and roots
Explain how can tropisms be positive or negative
positive = growth towards the stimulus
negative = growth away from the stimulus
Define gravitropism
plant growth in response to gravity
What does gravitropism allow in plants
roots grow downward for water and nutrients
shoots grow upward toward sunlight for photosynthesis
Define phototropism
change in plant growth patterns in response to the direction of incident radiation, especially blue-light
What does phototropism allow
shoots to grow toward sunlight maximizing photosynthesis while roots grow away from sunlight
Define thigmotropism
differential plant growth in response to touch (turning and bending in response to touch stimulus)
What does thigmotropism allow/enable
enables roots to grow around roots and shoots to wrap around support structures
What is the Cholodny-Went Hypothesis
mechanism proposed for tropisms that involves the stimulation of the bending of the plant axis due to lateral transport of auxin in response to a stimulus
auxin transport is ____ and independent of ____
polar
gravity
Explain when auxin moves basipetally and when it moves acropetally
basipetally = to the root-shoot junction (down from shoot, up from roots)
acropetally = from the root-shoot junction (up to shoot, down to roots)
What facilitates gravitropism in coleoptiles
lateral redistribution of auxin in coleoptile tips
Define statoliths
dense amyloplasts that synthesize and store starch involved in the perception of gravity contained in the root cap
What are the 2 secondary messengers for signaling that occurs during gravitropism
pH and calcium ions
Explain what occurs with auxin in gravitropic bending
auxin from the root tip is redirected to the lower side of the root inhibits cell elongation causing the root to bend downward
Explain what occurs with auxin in phototropic bending
auxin for the shoot tip is redirected to the shaded side and stimulates cell elongation which causes the shoot to bend toward the light
Define photoperiodism
ability of an organism to detect day length to ensure that events occur at the right time of year, which allows seasonal response
What are examples of plant responses that are controlled by day length
initiation of flowering, asexual reproduction, formation of storage organs and onset of dormancy
What is the determining factor in flowering
day length rather than the accumulation of photosynthate
What are the 2 main categories that flowering plants fall into and define each
short day plants = flower only in short days or flowering is accelerated by short days
long day plants = flower only in long days or flowering is accelerated by long days
Define critical day length
the certain duration when plants exceed this day length, they promote flowering in long-day plants and less than this length they promote flowering in short-day plants
What are the 3 adaptations that plants exhibit to avoid the ambiguity of the day-length signal
- juvenile phase that prevents plants form responding to a day length during spring
- coupling of a temperature requirement to a photoperiodic response
- some plants distinguish between shortening and lengthening days
what are the 2 categories of dual day-length plants
long-short-day plants = flower after long days followed by short, respond to shortening (flower in late summer and fall)
short-long-day plants = short days followed by long days, respond to lengthening days (flower in spring in response to lengthening days)
Define day neutral plants
are insensitive to day length, they flower under any photoperiodic condition
ex. desert plants
Define endogenous oscillator
an internal pacemaker mechanism that circadian rhythms are based on, not based on light or darkness! endogenous oscillators are regulated by four sets of genes expressed in dawn, morning, afternoon and evening
what photoreceptors influence circadian rhythms
phytochromes primarily, then cryptochromes
flowering of short day and long day plants are primarily determined by what
duration of darkness
(because long day plants could flower in short days as long as the following night/darkness was shorter to pretend it was a long day in comparison)
Define night break
a short exposure to light that can make dark periods ineffective by this short interruption
the inhibition of flowering in short day plants by night breaks is a physiological process under the control of what photoreceptor
phytochromes
a night break can become effective in short day plants if…
the light is sufficient to convert the pigment back to inactive Pr form and restores the flowering response (if Pfr is present it inhibits flowering)
