BB30129: Blue Light Signalling
Blue light responses
Blue light responses are diverse and occur throughout the life cycle of the
plant, from germination to flowering.
De-etiolation & cotyledon expansion (cotyledons are embryonic
structures, the first connections generated by the seedling during
germination)
Anthocyanin production
Stomatal opening (required for gas exchange in photosynthesis)
Sun tracking
Chloroplast movement
Shade avoidance
Positive phototropism: curvature towards unidirectional light (plants
maximise light absorption for photosynthesis by bending towards
the light source)
Phototropism
Early experiments on phototropism elucidating auxin as the first
plant hormone:
1. Charles Darwin (1809-1882) – directional growth of maize
Upon exposure to unidirectional light, plant stem exhibited curved
growth towards the light source.
Where in the plant is this light-sensing taking place?
Foil cap placed on the shoot tip, blocking light access to tip. No
growth curvature, implying that the tip is the point of light sensing.
Separation of the tip from the rest of the shoot using a barrier which
does not permit chemical passage also blocks phototropism.
Separation using a gelatin block which does allow chemical diffusion
results in a phototropic response to light.
Implies that phototropic growth curvature is due to diffusion of a
chemical from the tip and down the shoot.
2. Fritz Went – discovery of auxin
Removed shoot tip and placed it on agar cube, left for an hour to
allow diffusion of chemicals produced by tip into the agar.
This agar cube alone was added to the remainder of the shoot
(shoot ‘stump’), and was able to confer growth curvature.
This chemical was identified as the growth hormone auxin.
Two families of blue-light photoreceptors: cryptochromes and
phototropins
, The roles of these photoreceptors in phototropism were revealed
through exposing cryptochrome and phototropin Arabidopsis
mutants to blue light.
Non-directional blue light:
WT plants do not exhibit growth curvature
Single/double cry mutants exhibit elongated growth, but no
growth curvature
phot1 mutant resembles WT
Overall implies that cryptochromes inhibit hypocotyl elongation,
and phototropins are not involved in hypocotyl elongation.
Uni-directional blue light:
WT plants exhibit growth curvature towards light source
Single/double cry mutants also exhibit phototropic response
phot1 mutant does not exhibit any growth curvature
Overall implies that cryptochromes are not involved in growth
curvature, and phototropins promote this growth curvature.
Implies cryptochromes inhibit hypocotyl elongation, and
phototropins promote hypocotyl curvature!
Cryptochromes: inhibit hypocotyl elongation
Similar to DNA photolyases:
These are blue light
activated enzymes that
repair UV-B damaged DNA.
However, cryptochromes
lack photolyase activity.
Two domains: light
perception domain and
output domain
Light perception domain is
conserved between
cryptochromes and photolyases – 30% similarity in amino acid
sequence.
Output domain is present in cryptochromes but not photolyases
Cryptochromes:
Light perception domain of cryptochromes binds to 2 chromophores:
pterin and FAD (flavin adenine dinucleotide).
Transfer of light energy occurs from pterin and FAD to light
perception domain, ultimately influencing the output domain
Overexpression of C-terminal region affects light signalling
Cryptochromes are found in all plant tissues, in the cytoplasm and
nucleus. They do not undergo nuclear translocation in response to
light – this means that the nuclear and cytoplasmic populations of
cryptochomes have different functions.
Blue light responses
Blue light responses are diverse and occur throughout the life cycle of the
plant, from germination to flowering.
De-etiolation & cotyledon expansion (cotyledons are embryonic
structures, the first connections generated by the seedling during
germination)
Anthocyanin production
Stomatal opening (required for gas exchange in photosynthesis)
Sun tracking
Chloroplast movement
Shade avoidance
Positive phototropism: curvature towards unidirectional light (plants
maximise light absorption for photosynthesis by bending towards
the light source)
Phototropism
Early experiments on phototropism elucidating auxin as the first
plant hormone:
1. Charles Darwin (1809-1882) – directional growth of maize
Upon exposure to unidirectional light, plant stem exhibited curved
growth towards the light source.
Where in the plant is this light-sensing taking place?
Foil cap placed on the shoot tip, blocking light access to tip. No
growth curvature, implying that the tip is the point of light sensing.
Separation of the tip from the rest of the shoot using a barrier which
does not permit chemical passage also blocks phototropism.
Separation using a gelatin block which does allow chemical diffusion
results in a phototropic response to light.
Implies that phototropic growth curvature is due to diffusion of a
chemical from the tip and down the shoot.
2. Fritz Went – discovery of auxin
Removed shoot tip and placed it on agar cube, left for an hour to
allow diffusion of chemicals produced by tip into the agar.
This agar cube alone was added to the remainder of the shoot
(shoot ‘stump’), and was able to confer growth curvature.
This chemical was identified as the growth hormone auxin.
Two families of blue-light photoreceptors: cryptochromes and
phototropins
, The roles of these photoreceptors in phototropism were revealed
through exposing cryptochrome and phototropin Arabidopsis
mutants to blue light.
Non-directional blue light:
WT plants do not exhibit growth curvature
Single/double cry mutants exhibit elongated growth, but no
growth curvature
phot1 mutant resembles WT
Overall implies that cryptochromes inhibit hypocotyl elongation,
and phototropins are not involved in hypocotyl elongation.
Uni-directional blue light:
WT plants exhibit growth curvature towards light source
Single/double cry mutants also exhibit phototropic response
phot1 mutant does not exhibit any growth curvature
Overall implies that cryptochromes are not involved in growth
curvature, and phototropins promote this growth curvature.
Implies cryptochromes inhibit hypocotyl elongation, and
phototropins promote hypocotyl curvature!
Cryptochromes: inhibit hypocotyl elongation
Similar to DNA photolyases:
These are blue light
activated enzymes that
repair UV-B damaged DNA.
However, cryptochromes
lack photolyase activity.
Two domains: light
perception domain and
output domain
Light perception domain is
conserved between
cryptochromes and photolyases – 30% similarity in amino acid
sequence.
Output domain is present in cryptochromes but not photolyases
Cryptochromes:
Light perception domain of cryptochromes binds to 2 chromophores:
pterin and FAD (flavin adenine dinucleotide).
Transfer of light energy occurs from pterin and FAD to light
perception domain, ultimately influencing the output domain
Overexpression of C-terminal region affects light signalling
Cryptochromes are found in all plant tissues, in the cytoplasm and
nucleus. They do not undergo nuclear translocation in response to
light – this means that the nuclear and cytoplasmic populations of
cryptochomes have different functions.
