*******************************************************************************************
There are a couple posts with a lot of technical/scientific reading...so you can skip down four posts and read the outcome
*******************************************************************************************
From Wikipedia, the free encyclopedia
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.
Isoforms or states
Phytochromes are characterised by a red/far-red photochromicity. Photochromic pigments change their "colour" (spectral absorbance properties) upon light absorption. In the case of phytochrome the ground state is Pr, the r indicating that it absorbs red light particularly strongly. The absorbance maximum is a sharp peak 650–670 nm, so concentrated phytochrome solutions look turquoise-blue to the human eye. But once a red photon has been absorbed, the pigment undergoes a rapid conformational change to form the Pfr state. Here fr indicates that now not red but far-red (also called near infra-red; 705–740 nm) is preferentially absorbed. This shift in absorbance is apparent to the human eye as a slightly more greenish colour. When Pfr absorbs far-red light it is converted back to Pr.
Hence, red light makes Pfr, far-red light makes Pr. In plants at least Pfr is the physiologically active or "signalling" state.
Summary of the characteristics of plant phytochromes:
Purified Cph1 phytochrome in the Pr state (left) and the Pr/Pfr mixture (right) that is formed by irradiation with red light. Since daylight contains a lot of red light, during the day phytochrome is mostly converted to Pfr. At night, phytochrome will slowly convert back to the Pr form. Treatment with far-red light will also convert Pfr back to Pr. Since plants use red light for photosynthesis, and reflect and transmit far-red light, the shade of other plants also can make Pfr into Pr, triggering a response called
shade avoidance. In most plants, a suitable concentration of Pfr stimulates or inhibits physiological processes, such as those mentioned in these examples.
Since both the ground state Pr and excited state Pfr are unusually stable (Pfr has a half-life of hours or days) the quantum nature of this transition was not immediately recognized. These two forms are therefore commonly (though technically incorrectly) referred to as isoforms.
Biochemistry
(deleted some info about the cell from here)
The Pfr state 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.
Discovery
Using a
spectrograph built from borrowed and war-surplus parts, they discovered that red light was very effective for promoting germination or triggering flowering responses. The red light responses were reversible by far-red light, indicating the presence of a photoreversible pigment.
In 1983 the laboratories of Peter Quail and Clark Lagarias reported the chemical purification of the intact phytochrome molecule, and in 1985 the first phytochrome
gene sequence was published by Howard Hershey and Peter Quail. By 1989, molecular genetics and work with
monoclonal antibodies that more than one type of phytochrome existed; for example, the
pea plant was shown to have at least two phytochrome types (then called type I (found predominantly in dark-grown seedlings) and type II (predominant in green plants)). It is now known by
genome sequencing that
OpenDNS has five phytochrome genes (PHYA - E) but that rice has only three (PHYA - C). While this probably represents the condition in several di- and monocotyledonous plants, many plants are
polyploid. Hence
maize, for example, has six phytochromes - phyA1, phyA2, phyB1, phyB2, phyC1 and phyC2. While all these phytochromes have significantly different protein components, they all use phytochromobilin as their light-absorbing chromophore. In the late 1980s, the Vierstra lab showed that phyA is degraded by the ubiquitin system, the first identified natural target of the system to be identified in eukaryotes.
In 1996 a gene in the newly sequenced genome of the
cyanobacterium OpenDNS was noticed to have a weak similarity to those of plant phytochromes. Jon Hughes in Berlin and Clark Lagarias at UC Davis subsequently showed that this gene indeed encoded a
bona fide phytochrome (named Cph1) in the sense that it is a red/far-red reversible chromoprotein. Presumably plant phytochromes are derived from an ancestral cyanobacterial phytochrome, perhaps by gene migration from the chloroplast to the nucleus. Subsequently phytochromes have been found in other
prokaryotes including
Deinococcus radiodurans and
Agrobacterium tumefaciens. In
Deinococcus phytochrome regulates the production of light-protective pigments, however in
Synechocystis and
Agrobacterium the biological function of these pigments is still unknown.
In 2005, the Vierstra and Forest labs at the University of Wisconsin published a three-dimensional structure of the photosensory domain of
OpenDNSphytochrome. This breakthrough paper revealed that the protein chain forms a knot - a highly unusual structure for a protein.
Genetic engineering
Around 1989 several laboratories were successful in producing (
transgenic plants) which produced elevated amounts of different phytochromes (
overexpression). In all cases the resulting plants had conspicuously short stems and dark green leaves. Harry Smith and coworkers at Leicester University in England showed that by increasing the expression level of phytochrome A (which responds to far-red light)
shade avoidance responses can be altered.
As a result, plants can expend less energy on growing as tall as possible and have more resources for growing seeds and expanding their root systems. This could have many practical benefits: for example, grass blades that would grow more slowly than regular grass would not require mowing as frequently, or
crop plants might transfer more energy to the grain instead of growing taller.
