Light Regulates Plant Growth and Development
Light is vital for photosynthesis, but is 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 system.
Blue light responses: Many plant responses are regulated by blue light, including phototropism, stomatal opnening and chlorophyll synthesis. The last step of chlorophyll synthesis requires high levels of blue light. The other blue light responses are triggered by lower levels of blue light. For more detailed information, read [ this link ].
Phytochrome responses: Important plant responses regulated by the phytochrome system include photoperiodic induction of flowering, chloroplast development (not including chlorophyll synthesis), leaf senescence and leaf abscission.
Characteristics of phytochrome-mediated responses:
The Pr form:
"Photoreceptors and circadian clocks are universal mechanisms for sensing and responding to the light environment. In addition to regulating daily activities, photoreceptors and circadian clocks are also involved in the seasonal regulation of processes such as flowering. Circadian 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
a sulfur linkage.
Phytochrome Genes and Proteins
Expression of the other phytochrome types (B to E in
is independent of light and both Pr and Pfr forms are stable.
The mechanism by which the phytochrome (phy) photoreceptor family
informational light signals to photoresponsive genes is still
although progress has been made.
Several candidates for a phytochrom receptor are being investigated. For example:
Ni M, Tepperman JM, Quail PH. Cell 1998 Nov 25;95(5):657-67
The mechanism by which the phytochrome (phy) photoreceptor
transduces informational light signals to photoresponsive genes
a yeast two-hybrid screen, we have identified a
factor, PIF3, a basic helix-loop-helix protein containing a PAS
PIF3 binds to wild-type C-terminal domains of both phyA and
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
phyA and phyB signaling pathways in vivo. PIF3 localized to the
in transient transfection experiments, indicating a potential
controlling gene expression. Together, the data suggest that
to photoregulated genes includes a direct pathway involving
interaction between the photoreceptor and a transcriptional
"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 - 490 (2001)
Most organisms, from cyanobacteria to mammals, use circadian clocks 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 characterized eukaryotic clocks. Here we describe ADAGIO1 (ADO1), a gene of Arabidopsis thaliana that encodes a protein containing a PAS domain. We found that a loss-of-function ado1 mutant is altered in both gene expression and cotyledon movement in circadian rhythmicity. Under constant white or blue light, the ado1 mutant exhibits a longer period than that of wild-type Arabidopsis seedlings, 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 circadian system.
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Last revised: August 3, 2013