Product Code Database
Phytochromes are a class of photoreceptor proteins found in , bacteria and Fungus. They respond to light in the red and far-red regions of the visible spectrum and can be classed as either Type I, which are activated by far-red light, or Type II that are activated by red light. Recent advances have suggested that phytochromes also act as temperature sensors, as warmer temperatures enhance their de-activation. All of these factors contribute to the plant's ability to Germination.
Phytochromes control many aspects of plant development. They regulate the germination of seeds (photoblasty), the synthesis of chlorophyll, the elongation of seedlings, the size, shape and number and movement of leaf and the timing of flowering in adult plants. Phytochromes are widely expressed across many tissues and developmental stages.
Other plant photoreceptors include and , which respond to blue and ultraviolet-A light and UVR8, which is sensitive to ultraviolet-B light.
The experiment consisted in the Wild type form of Arabidopsis, phyA-101(phytochrome A (phyA) null mutant), phyB-1 (phytochrome B deficient mutant). They were then exposed to white light as a control blue and red light at different fluences of light, the curvature was measured. It was determined that in order to achieve a phenotype of that of the wild-type phyA-101 must be exposed to four orders of higher magnitude or about 100umol m−2 fluence. However, the fluence that causes phyB-1 to exhibit the same curvature as the wild-type is identical to that of the wild-type. The phytochrome that expressed more than normal amounts of phytochrome A it was found that as the fluence increased the curvature also increased up to 10umol-m−2 the curvature was similar to the wild-type. The phytochrome expressing more than normal amounts of phytochrome B exhibited curvatures similar to that of the wild type at different fluences of red light up until the fluence of 100umol-m−2 at fluences higher than this curvature was much higher than the wild-type.
Thus, the experiment resulted in the finding that another phytochrome than just phytochrome A acts in influencing the curvature since the mutant is not that far off from the wild-type, and phyA is not expressed at all. Thus leading to the conclusion that two phases must be responsible for phototropism. They determined that the response occurs at low fluences, and at high fluences. This is because for phyA-101 the threshold for curvature occurred at higher fluences, but curvature also occurs at low fluence values. Since the threshold of the mutant occurs at high fluence values it has been determined that phytochrome A is not responsible for curvature at high fluence values. Since the mutant for phytochrome B exhibited a response similar to that of the wild-type, it had been concluded that phytochrome B is not needed for low or high fluence exposure enhancement. It was predicted that the mutants that over expressed phytochrome A and B would be more sensitive. However, it is shown that an over expression of phy A does not really effect the curvature, thus there is enough of the phytochrome in the wild-type to achieve maximum curvature. For the phytochrome B over expression mutant higher curvature than normal at higher fluences of light indicated that phy B controls curvature at high fluences. Overall, they concluded that phytochrome A controls curvature at low fluences of light.
The phytochrome chromophore is usually phytochromobilin, and is closely related to phycocyanobilin (the chromophore of the used by cyanobacteria and red algae to capture light for photosynthesis) and to the bile pigment bilirubin (whose structure is also affected by light exposure, a fact exploited in the phototherapy of newborns). The term "bili" in all these names refers to bile. Bilins are derived from the closed tetrapyrrole ring of haem by an oxidative reaction catalyzed by haem oxygenase to yield their characteristic open chain. Chlorophyll and haem (Heme) share a common precursor in the form of Protoporphyrin IX, and share the same characteristic closed tetrapyrrole ring structure. In contrast to bilins, haem and chlorophyll carry a metal atom in the center of the ring, iron or magnesium, respectively.
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.
The phytochrome pigment was identified using a spectrophotometer in 1959 by biophysicist Warren Butler and biochemist Harold Siegelman. Butler was also responsible for the name, phytochrome.
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 Arabidopsis 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. Phytochrome A or phyA is rapidly degraded in the Pfr form - much more so than the other members of the family. In the late 1980s, the Vierstra lab showed that phyA is degraded by the ubiquitin system, the first natural target of the system to be identified in eukaryotes.
In 1996 David Kehoe and Arthur Grossman at the Carnegie Institution at Stanford University identified the proteins, in the filamentous cyanobacteria Fremyella diplosiphon called RcaE with similarly to plant phytochrome that controlled a red-green photoreversible response called chromatic acclimation and identified a gene in the sequenced, published genome of the cyanobacteria Synechocystis with closer similarity to those of plant phytochrome. This was the first evidence of phytochromes outside the plant kingdom. Jon Hughes in Berlin and Clark Lagarias at UC Davis subsequently showed that this Synechocystis 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 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 a truncated Deinococcus phytochrome (PAS/GAF domains). This paper revealed that the protein chain forms a knot - a highly unusual structure for a protein. In 2008, two groups around Essen and Hughes in Germany and Yang and Moffat in the US published the three-dimensional structures of the entire photosensory domain. One structures was for the Synechocystis sp. (strain PCC 6803) phytochrome in Pr and the other one for the Pseudomonas aeruginosa phytochrome in the Pfr state. The structures showed that a conserved part of the PHY domain, the so-called PHY tongue, adopts different folds. In 2014 it was confirmed by Takala et al that the refolding occurs even for the same phytochrome (from Deinococcus) as a function of illumination conditions.
In 2002, the light-induced interaction between a plant phytochrome and phytochrome-interacting factor (PIF) was used to control gene transcription in yeast. This was the first example of using photoproteins from another organism for controlling a biochemical pathway.
|
|
