Phytochrome pigment.?

What is phytochrome pigment?


How does light affect germination?


Does it have to do with the absorption spectra

Phytochrome pigment.?
The extensive researches of scientists at the plant industry station at Beltsville,yale and Duke university and at several research institutes have culminated in the isolation of a blue or yellow green proteinaceous pigment, called phytochrome which is the primary receptor of the visible radiation. These are photosensitive photoreceptor found in almost all flowering plants.It was discovered by Borthwick et al (1952) and isolated by Butler et al (1959) from etiolated seedlings of maize.It exists in two interconvertible states, blue and yellow green,Pr and Pfr. Pfr is associated with cellmembrane while proccurs diffused in cytosol.It causes germination of seeds, inhibit flowering in SDP and in LDP plants.Pr absorbs light at 660nm and is changed to Pfr or P730. The latter absorbs light of 730nm and is changed back to Pr quickly.It is required in seed germination of some olants, bud dormancy, leaf abscisson,synthesis of gibberellins and etylene,prevention of photooxidation,photoperiodism and morphogenesis.Pfr is essential for seed germination.Pr is inactive form whil Pfr is active form.
Reply:ok, phytochrome far red (Pfr) reacts to red light and converts phytochrome red (Pr), it reacts to fr light, etc.
Reply:Phytochrome a red light photopigment that plays a major role in virtually all aspects of plant growth and development.


Phytochrome exists as two forms – a red absorbing form (Pr) and a far-red absorbing form (Pfr).


Pfr is the form that mediates photomorphogenic effects.





Absorption of red light by Pr converts pigment to active form (Pfr) which mediates effects on plants.





Absorption of far-red light by Pfr drives the pigment back to the Pr (inactive) form
Reply:Phytochrome is a photoreceptor, a pigment that plants use to detect light. It is sensitive to light in the red and far-red region of the visible spectrum. Many flowering plants use it to regulate the time of flowering based on the length of day and night (photoperiodism) and to set circadian rhythms. It also regulates other responses including the germination of seeds, elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.





Phytochrome has been found in most plants including all higher plants; very similar molecules, now also called phytochromes, have been found in several bacteria.





Other plant photoreceptors include cryptochromes and phototropins, which are sensitive to light in the blue and ultra-violet regions of the spectrum.


The Pfr isoform passes on a signal to other biological systems in the cell, such as the mechanisms responsible for gene expression. Although this mechanism is almost certainly a biochemical process, it is still the subject of much debate. It is known that although phytochromes are synthesized in the cytosol and the Pr form is localized there, the Pfr form, when generated by light illumination, is translocated to the cell nucleus. This implies a role of phytochrome in controlling gene expression, and many genes are known to be regulated by phytochrome, but the exact mechanism has still to be fully discovered. It has been proposed that phytochrome, in the Pfr form, may act as a kinase, and it has been demonstrated that phytochrome in the Pfr form can interact directly with transcription factors.


A thin seed coat is so thin that it is no barrier to water. Some other kind of dormancy mechanism is needed. Knowing that light can penetrate thin layers of plant tissue (leaves for example) should give you the idea that light might be a signal. That plants can absorb light and respond biochemically is a fact you know from your study of photosynthesis. All we need is a pigment molecule that can absorb light and cause a change in the behavior of the embryo.





The pigment is phytochrome. Like chlorophyll, it is made of a chromophore with tetrapyrole structure and is associated with proteins. This pigment is different from chlorophyll, however, in one critical way. It exists in two inter-convertible forms.





One form of phytochrome, named Pfr, is the form of the phytochome found in plant cells that are exposed to red (660 nm) or common white light. This form of phytochrome is biologically very active and plays a role in all systems when a plant needs to know if the lights are "on" or "off." In lettuce (Lactuca sativa) seeds, Pfr causes the seeds to begin to germinate as we will soon see. Thus lettuce seeds germinate only when placed in white or red light. Buried in deep soil, they will not germinate. Given that lettuce has a small seed, I think you can figure out why evolution arrived at this solution.





The other form of phytochrome, named Pr, is formed when phytochrome is exposed to far-red (730 nm) light. This form is biologically inactive or inhibits responses. Thus if lettuce seeds are placed in far-red light they do not germinate.





Large seeds have lots of storage material. If their seed coat is very thin, their evolution may have arrived at a completely different response. Think about pea seeds. They are large and have very thin seed coats.