The blue light signaling pathways
1. What does blue light do to plants?
1) phototropism: growing towards light
2) Inhibits stem elongation
(overlapping with red light)
3) Stimulate stomatal opening
Dark----blue light
Why stomatal opening depends on light?
2. Blue light receptors
1) The Phototropin family (Phot)
a) How Phot1 was discovered: mutant that is non-phototropic (nph1)
Wild type
mutants
Cloning of NPH1/Phot1
as a protein kinase
b) Functional domains
c) Experiments
--the chromophore as FMN (flavin mononucleotide)
--Kinase autophosphorylation
--spectrum of recombinant receptor
d) Blue light-induced formation of cystein-FMN covalent bonding
e) The second phototropin: mediates chloroplast migration (avoid high
light)
A mutant was isolated that is deficient in light avoidance. The gene
turns out to be Phot2—related to Phot1.
Both Phot1 and Phot2 play a role in phototropism and chloroplast
migration.
f) The double mutant of Phot1 and Phot2 affect stomatal opening and
produce stronger phenotype on both phototropism and chlp;oroplast
migration
2) The cryptochromes
a) discovery: mutants that were defective in blue light
responsive growth inhibition
WT
hy4
Cloning of the gene encoding a protein
with homology to DNA photolyases in
bacteria in charge of UV-dependent DNA
repair. These enzymes bind flavins as
chromophores. The plant blue light
receptor HY4 binds FAD. Later on it is
called cryptochrome 1 (Cry1). Another
similar receptor called Cry2 was
identified more recently that shared
function with Cry1.
b) CRY1 is a nuclear protein and not regulated by light
CRY1 and CRY2 fused to the GFP reporter and found in nucleus
whether grown in the dark or light.
c) What processes Cry1/2 regulate: besides inhibition of stem elongation,
cry1/2 mutants also show altered flowering time and greening (processes
mostly controlled by phytochromes), indicating crosstalk between Cry1/2
and PhyA/B.
d) How does Cry work? Not answered but accumulating results
suggest that they interact with other proteins and protein
phosphorylation may be involved in the signaling pathway. For example,
Cry1 directly interacts with PhyA, and Cry2 directly interacts with
PhyB. Cry1 is also shown to interact with ubiquitin pathway component,
suggesting modification of protein degradation is again involved in light
signaling.