Light/shade responses Flashcards
in daylight what is the light environment?
much blue light
canopies transmit and reflect FR and green
describe the shade tolerance strategy
decreased: growth rate,
increased: Specific Leaf area, defence from herbivores, suppresses shade avoidance
when is phototropism enhanced?
in low R:FR
stems bend towards blue light
asymmetric auxin production
what is a photoreceptor
made from chromophore and protein eg opsin
4 types of photoreceptor
Phytochromes - detect r:FR ratio, have a phytochromobillin chromophore. PHY A - E
cryptochromes - blue
phototropins - blue
UVR8 - UV light
describe active and inactive phytochromes
Pr - inactive
Pfr -active, suppresses stem elongation, as not in shade so no shade avoidance.
Pr is activated by more red light to form Pfr. addition of FR inactivates it to Pr again
describe shade avoidance strategy
plant channels energy into stem elongation
reduced crop yield
if extreme, may attempt to flower and set seed before death.
describe light environment in the shade
lower R:FR ratio
threat of shade - plenty of light for ph still,
true shade - no blue, low UV, low R:FR, reduced photosynthesis, auxin synthesis upregulated.
sequence of phytochrome signalling
Pfr active in high R:FR
Pfr enters nucleus
Pfr binds to PIF and degrades PIF.
Low R:FR, PIFs not degraded by Pfr
Increase YUCCA gene exp which produces auxin.
Auxin carried through stem in eflux carriers - promotes stem elongation and growth.
Acid growth hypotheses - auxin increases activity of proton pumps in cell wall, increasing acidity, acivates enzymes to loosen cell wall to allow expansion of stem-cells. (elongation is not driven by cell division).
in Low R:FR also more giberellin hormone, GA.
GA binds to GIDI. GIDI cannot interract with DELLA (growth supressor), and allows growth.
how do DELLAS function
DELLA is a growth supressor.
interacts with GIDI to supress growth.
when GIDI is bound to GA, della cannot supress growth.
DELLA degrades PIFs so there is no hypocotyl elongation.
what are PIFs
identified by Peter Quail
Phytochrome interacting factors - transcription factors
regulate many responses eg high temp, shade av, de-etiolation, seedling dev.
PIF 4 and 5 - degraded by Pfr in high R:FR, so no expression of shade av genes.
altered expression of PIF4 and 5 affects elongation growth.
PIF 7 - regulated by phosphorylation/dephosphorylation. gives 2 bands on western blot. in lor R:FR, lose dephosphorylation bands.
what is ethylene
gaseous hormone
signals canopy shade and submergence.
types of UV
UVA - longest wl, sensed by blue light photoreceptors.
UVB - damaging at high levels
UVC - shortest, attenuated by atmosphere.
how is UVB damaging
damaging to membranes, produces ROS in chloroplasts, mit and peroxisomes when understress
DNA absorbs UVB to form pyrimidine dimers - these are accelerated repair byphotolyase in blue light
what does UVB regulate
flavenoid production sotmatal closing entrains circadian clock reduction in plant growth increased pathogen resistance inhibits shade avoidance - dwarfed compact plants.
protection against UVB
photolyase - efficient DNA repair
waxy leaves and hairs reflect UVB
flavenoid synthesis - sunscreen
antioxidant responses
how is uvb detected?
UVR8 photoreceptor
dimeric (when no uvb present), tryptophan AA connect monomers.
tryptophan residues absorb uvb, causing monomerisation. re dimerization attenuates UVR8 signalling, using COP1 and RUP1/2.
what are RUP1and 2, and what regulates them?
cause redimerisation of UVR8s, to attenuate UVB signalling.
regulated by HY5.
what is HY5?
3 interactions of HY5
transcription factor
1. involved in light regulated development.
degraded in dark by COP1.
in light, phytochromes bind to cop 1, preventing HY5 degradation.
2. also regulates RUP1 and 2, which help redimerise UVR8 to stop UVB signal.
3. Depletes GA.
what does the HFR1 gene do
upregulated in low R:FR
limits shade av
binds to PIFs, preventing excessive elongation, in a negative feedback loop.
HFR1 mutants have long hypocotyls and petioles.
how does UVB affect soybeans and powdery mildew
soybeans - sig less stinkbugs on those with UVB than on those without. flavenoids act as repellent, it is bitter and interrupts herbivore digestion.
Powdery mildew - supressed by UVB, blue light reduces this suppression. however, UVB + Blue = more mildew than just uvb. suggested light enhanced DNA repair occures in pathogen
how does UVB inhibit auxin activity
low R:FR - high auxin activity, drives shade avoidance.
in UVB, no shade avoidance. UVB degrades PIFS, which are v active in low R:FR.
also UVB stabilises DELLAS, so PIFs are further inhibited.
how can lighting be modifed in greenhouses
more LED tubes to increase vit C in tomatoes
photoselective filters to reduce FR, lowers sh av.
