Phytochrome
Light Regulates Plant Growth and Development
Pr 
Pfr

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 phytochrome molecule is the photoreceptor for red light responses.  It exists in two forms, Pr and Pfr:

The Pr form:

The Pfr form: Phytochrome is a family of proteins with a small covalently-bound pigment molecule: Phytochrome allows plants to sense the color of light. Phytochrome and the Circadian Clock in Plants

Phytochrome and the Circadian Clock
"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 tetrapyrrole is shown below.  It is attached to the phytochrome protein through a sulfur linkage.
 

Phytochrome Genes and Proteins

The Elusive Phytochrome "Receptor"

The mechanism by which the phytochrome (phy) photoreceptor family transduces informational light signals to photoresponsive genes is still unclear, although progress has been made. 

Several candidates for a phytochrom receptor are being investigated.  For example:

"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