Light Regulates Plant Growth and Development
Light is vital for photosynthesis, but
also necessary to direct plant growth and development. Light acts
as a signal to initiate and regulate photoperiodism and photomorphogenesis.
There are two light-sensing systems involved in these responses, the blue
light sensistive system and the red light sensitive or phytochrome
Blue light responses: Many plant
are regulated by blue light, including phototropism, stomatal
and chlorophyll synthesis. The last step of chlorophyll
requires high levels of blue light. The other blue light
are triggered by lower levels of blue light. For more detailed
read [ this
Phytochrome responses: Important
responses regulated by the phytochrome system include photoperiodic induction
of flowering, chloroplast development (not including
synthesis), leaf senescence and leaf abscission.
Characteristics of phytochrome-mediated responses:
The phytochrome molecule is the photoreceptor for red light
It exists in two forms, Pr and Pfr:
- The action spectrum of the light needed for these
a peak in the red at about 660 nm.
- These responses can be reversed by an application of far-red
at 730 nm) soon after the red treatment.
- Sensitive spectrophotometers can measure a decrease in absorbance
nm and in increase in absorbance at 730 nm when sensitive plant tissues
are exposed to red (660 nm) light.
- The change in absorbance is caused by the conversion of a the
from one structural form to another. The red-absorbing form
to the far-red absorbing form when it absorbs red light (660 nm) and
again when it absorb far-red light (730 nm).
The Pr form:
The Pfr form:
- Absorbs at a peak of 666 nm
- Is the form synthesized in dark-grown seedlings.
- When Pr absorbs red light, it is converted to the Pfr form.
Phytochrome is a family of proteins with a small covalently-bound
- Absorbs at a peak of 730 nm
- The Pfr form is the active form that initiates biological
- When Pfr absorbs far red light, it is converted to the Pr form
- Pfr can also spontaneously revert to the Pr form in the dark over
= dark reversion; Pfr is also susceptible to proteinases.
- Pfr absorbs some red light, so in red light, there is a balance
Pfr and 15% Pr
- Pr absorbs very little far red light, so in far red light, there
of 97% Pr to 3% Pfr
Phytochrome allows plants to sense the color of light.
- Phytochrome proteins occur as a dimer of two 124 kDa
with a covalently-attached pigment molecule.
- The pigment is called the chromophore. It is a linear
- When the chromophore absorbs light, there is a slight change in
This causes a change in the conformation of the protein portion to the
form that initiates a response.
- Phytochrome levels are much higher (about 50X) in dark-grown
than in light-grown plants. Its levels are highest near the apex
of the plant.
- Molecular genetics has revealed the existence of several
genes for this protein in a given plant. All of these
use the same chromophore but differ in their sensitivity to light.
- The different phytochromes are involved in different biological
to red light.
- Read more about the phytochrome molecule
Phytochrome and the Circadian Clock in Plants
- Sunlight has a R:FR ratio of 1.2
- Light under a canopy of leaves has a R:FR ratio of 0.13
- Light under 5 mm of soil has a R:FR ratio of 0.88
- A higher proportion of FR light allows plants to detect
- Plants adapted for growth in full sun will display greater stem
when they are transferred to shade. They also develop smaller
and less branching. This change is due to greater proportion of
- Seeds of certain plants require red light for germination;
inhibits germination. Many small seeds with low amounts of
reserves (such as lettuce) show such a red light requirement.
- If these seeds they are buried below the level of light
the soil, they do not germinate.
- If they are shaded by a leaf canopy, causing a high proportion of
is inhibited, Pfr is required for germination.
"Photoreceptors and circadian clocks are universal mechanisms for
sensing and responding to the light environment. In addition to
daily activities, photoreceptors and circadian clocks are also involved
in the seasonal regulation of processes such as flowering.
rhythms govern many plant processes, including movements of organs such
as leaves and petals, stomata opening, stem elongation, sensitivity to
light of floral induction, metabolic processes such as respiration and
photosynthesis and expression of a large number of different genes."
- drawing and quote from Elaine
Tobin's Website, UCLA.
The Phytochrome Molecule
The structure of the linear
is shown below. It is attached to the phytochrome protein
a sulfur linkage.
Phytochrome Genes and Proteins
- There are five phytochrome genes in Arabidopsis, termed
phyC, phyD, phyE.
The Elusive Phytochrome "Receptor"
- Phytochrome A (PhyA), present only in angiosperms, is
for early events in germination and seedling de-etiolation. It is
powerfully down-regulated in light both at the transcriptional and
levels. In darkness it accumulates to (comparatively) high
levels. [ reference
Expression of the other phytochrome types (B to E in
is independent of light and both Pr and Pfr forms are stable.
- Phytochrome B (PhyB) is probably the photoreceptor
involved in shade
detection and avoidance. This response allows many species to
increase their stem extension rate when they become shaded by
The relative amount of Pfr is reduced strongly by the
of chlorophyll-bearing leaves that filter-out red light but not
The absolute irradiance is irrelevant. Through this red/far-red
phytochrome provides the plant with a degree of color perception.
PhyB also is considered responsible for daylength detection in
and for tuberization in the potato, though the mechanisms are not
understood. [ reference
- Phytochrome C (phyC) is a low-abundance member of
phytochrome family of photoreceptors in Arabidopsis. Experimental
data indicate that phyC may have some physiological roles that are
to those of phyA and phyB in the control of seedling responses to light
signals. [ reference
The mechanism by which the phytochrome (phy) photoreceptor family transduces
informational light signals to photoresponsive genes is still unclear,
although progress has been made.
- Phytochrome-GFP fusion proteins migrate to the cell nucleus
after they are activated by red light.
- In the case of PhyB, both photoactivation and nuclear
translocation combined are necessary
and sufficient for biological function. Conversely, neither
artificial nuclear translocation of non-photoactivated phyB nor
artificial retention of photoactivated phyB in the cytosol provides
detectable biological activity.
Several candidates for a phytochrom receptor are being
investigated. For example:
- PIF3, a phytochrome-interacting factor necessary for normal
signal transduction, is a novel basic helix-loop-helix protein.
Ni M, Tepperman JM, Quail PH. Cell 1998 Nov
The mechanism by which the phytochrome (phy) photoreceptor family
transduces informational light signals to photoresponsive genes is
a yeast two-hybrid screen, we have identified a hytochrome-interacting
factor, PIF3, a basic helix-loop-helix protein containing a PAS domain.
PIF3 binds to wild-type C-terminal domains of both phyA and phyB, but
less strongly to signaling-defective, missense mutant-containing
Expression of sense or antisense PIF3 sequences in transgenic
photoresponsiveness in a manner indicating that PIF3 functions in both
phyA and phyB signaling pathways in vivo. PIF3 localized to the nucleus
in transient transfection experiments, indicating a potential role in
controlling gene expression. Together, the data suggest that
to photoregulated genes includes a direct pathway involving physical
interaction between the photoreceptor and a transcriptional regulator.
- Phytochrome B protein binds to a clock protein, ADO1
"An Arabidopsis circadian clock component interacts with
both CRY1 and phyB"
JOSE A. JARILLO, JUAN CAPEL, RU-HANG TANG, HONG-QUAN YANG, JOSE M.
ALONSO, JOSEPH R. ECKER & ANTHONY R. CASHMORE. Nature 410, 487 -
Most organisms, from cyanobacteria to mammals, use circadian
to coordinate their activities with the natural 24-h light/dark cycle.
The clock proteins of Drosophila and mammals exhibit striking homology
but do not show similarity with clock proteins found so far from either
cyanobacteria or Neurospora. Each of these organisms uses a
transcriptionally regulated negative feedback loop in which the
messenger RNA levels of
the clock components cycle over a 24-h period. Proteins containing PAS
domains are invariably found in at least one component of the
eukaryotic clocks. Here we describe ADAGIO1 (ADO1), a gene of
that encodes a protein containing a PAS domain. We found that a
loss-of-function ado1 mutant is altered in both gene expression and
in circadian rhythmicity. Under constant white or blue light, the ado1
mutant exhibits a longer period than that of wild-type Arabidopsis
whereas under red light cotyledon movement and stem elongation are
arrhythmic. Both yeast two-hybrid and in vitro binding studies show
that there is a
physical interaction between ADO1 and the photoreceptors CRY1 and phyB.
We propose that ADO1 is an important component of the Arabidopsis
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Last revised: April 16, 2006