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Baris 1: {{bedakan\|sitokrom}} ~~''Tidak untuk dikacaukan dengan '''[[sitokrom]]'''.''~~ '''Fitokrom''' adalah [[reseptor cahaya]], suatu [[pigmen]] yang digunakan oleh [[tumbuhan]] untuk men[[pencerapan\|cerap]] (mendeteksi) [[cahaya]]. Sebagai sensor, ia terangsang oleh cahaya [[merah]] dan [[infra merah]]<ref>. Dalam konteks fitokrom, infra merah sering kali disebut "merah jauh" atau ''far-red''.</ref> Infra merah bukanlah bagian dari [[cahaya tampak]] oleh mata manusia namun memiliki panjang gelombang yang lebih besar daripada merah. Baris 18 ⟶ 17: Pada tahun 2005 tim dari laboratorium Vierstra berhasil membuat model tiga dimensi fitokrom dari [[bakteri]] ''Deinococcus''. Bentuk protein fitokrom sangat tidak lazim karena memiliki simpul ("knot"). == Referensi == ~~<!--~~ {{reflist}}<!--▼ ~~== Isoforms or states ==~~ [[Berkas:Phytochrome str.png\|thumb\|250px\|Two hypothesis, explaining the light - induced phytochrome conversions (P<sub>R</sub> - red form, P<sub>IR</sub> - far red form, B - protein). Left - <ref name='BritzGalston1983'>Britz SJ, Galston AW.. Physiology of Movements in the Stems of Seedling Pisum sativum L. cv Alaska : III. Phototropism in Relation to Gravitropism, Nutation, and Growth, Plant Physiol. 1983 Feb;71(2):313-318</ref>. Right - <ref name = 'WalkerBailey1968'>Walker TS, Bailey JL. Two spectrally different forms of the phytochrome chromophore extracted from etiolated oat seedlings. Biochem J. 1968 Apr;107(4):603–605.</ref>.]] 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 P<sub>r</sub>, the <sub>r</sub> 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 P<sub>fr</sub> state. Here <sub>fr</sub> 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<!-- Unsourced image reference removed: "(see image below)" . When P<sub>fr</sub> absorbs far-red light it is converted back to P<sub>r</sub>. Hence, red light makes P<sub>fr</sub>, far-red light makes P<sub>r</sub>. In plants at least P<sub>fr</sub> 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 P<sub>fr</sub>. At night, phytochrome will slowly convert back to the P<sub>r</sub> form. Treatment with far-red light will also convert P<sub>fr</sub> back to P<sub>r</sub>. Since plants use red light for photosynthesis, and reflect and transmit far-red light, the shade of other plants also can make P<sub>fr</sub> into P<sub>r</sub>, triggering a response called [[shade avoidance]]. In most plants, a suitable concentration of P<sub>fr</sub> stimulates or inhibits physiological processes, such as those mentioned in these examples. Since both the ground state P<sub>r</sub> and excited state P<sub>fr</sub> are unusually stable (P<sub>fr</sub> 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. For some time it was believed that infra red isoform has the cyclic (chlorophyll-like) configuration when the red form is linear <ref name = 'WalkerBailey1968'>. Later other hypothesis were also proposed. ~~== Biochemistry ==~~ Chemically, phytochrome consists of a ''[[chromophore]]'', a single bilin molecule consisting of an open chain of four [[pyrrole]] rings, bonded to the [[protein]] moiety. It is the chromophore that absorbs light, and as a result changes the conformation of bilin and subsequently that of the attached protein, changing it from one state or isoform to the other. ~~The phytochrome chromophore is usually '''phytochromobilin''', and is closely related to [[phycocyanobilin]] (the chromophore of the [[phycobiliprotein]]s 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 [[jaundice]]d 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 catalysed by haem oxygenase to yield their characteristic open chain. [[Chlorophyll]] too is derived from haem. In contrast to bilins, haem and chlorophyll carry a metal atom in the center of the ring, iron or magnesium, respectively. The P<sub>fr</sub> 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 P<sub>r</sub> form is localized there, the P<sub>fr</sub> 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 P<sub>fr</sub> form, may act as a [[kinase]], and it has been demonstrated that phytochrome in the P<sub>fr</sub> form can interact directly with [[transcription factor]]s. ~~== 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. ~~== References ==~~ ▲{{reflist}} ~~== Other sources ==~~ * http://www.ars.usda.gov/is/timeline/light.htm * http://www.mobot.org/jwcross/duckweed/phytochrome.htm#tetrapyrrole Baris 55 ⟶ 24: * Terry and Gerry Audesirk. ''Biology: Life on Earth.'' * Linda C Sage. ''A pigment of the imagination: a history of phytochrome research.'' Academic Press 1992. ISBN 0-12-614445-1 --> ~~== Catatan kaki ==~~ ~~{{reflist}}~~ == Lihat pula ==

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