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Fitokrom: Perbedaan antara revisi

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{{bedakan|sitokrom}}
'''Fitokrom''' adalah suatu [[reseptor cahaya]], suatu [[pigmen]] yang digunakan oleh [[tumbuhan]] untuk men[[pencerapan|cerap]] (menyerap/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.
 
Fitokrom ditemukan pada semua tumbuhan. Senyawa[[Molekul]] yang serupa juga ditemukan pada [[bakteri]]. Tumbuhan menggunakan fitokrom untuk mengatur beberapa aspek [[fisiologi tumbuhan|fisiologi]] [[adaptasi]] terhadap lingkungan, seperti [[fotoperiodisme]] (pengaturan saat berbunga pada tumbuhan), [[perkecambahan]], pemanjangan dan pertumbuhan [[kecambah]] (khususnya pada [[dikotil]]), [[morfologi tumbuhan|morfologi]] [[daun]], pemanjangan ruas [[batang]], serta pembuatan (sintesis) [[klorofil]].
Secara struktur kimia, bagian sensor fitokrom adalah suatu [[kromofor]] dari kelompok [[bilin]] (jadi disebut ''fitokromobilin''), yang masih sekeluarga dengan [[klorofil]] atau [[hemoglobin]] (kesemuanya memiliki kerangka [[heme]]). Kromofor ini dilindungi atau diikat oleh apo[[proteinapoprotein]], yang juga berpengaruh terhadap kinerja bagian sensor. Kromofor dan apoprotein inilah yang bersama-sama disebut sebagai fitokrom.
 
<!-- Other plant photoreceptors include [[cryptochrome]]s and [[phototropin]]s, which are sensitive to light in the [[blue]] and [[ultra-violet]] regions of the spectrum. -->
 
== Penemuan ==
Dalam seriPenelitian penelitianrintisan terhadap pengaruh cahaya merah dan merah jauh terhadap pertumbuhan tumbuhan antara 1940-1960, dilakukan oleh Sterling Hendricks dan Harry Borthwick dari Pusat Penelitian Pertanian Beltsville di [[Maryland]], dengan menggunakan [[spektrografi|spektrograf]] dari bahan-bahan sisa [[Perang Dunia Kedua,]]. menemukanDari hasilnya diketahui bahwa cahaya merah mendorongmemacu [[perkecambahan]] dan memicu tanggap untuk [[pembungaan]]. TanggapanSelain terhadapitu, cahaya merah inijauh dapatberpengaruh dibalikkansebaliknya olehterhadap pengaruh cahaya merah jauh. IniPenelitian lanjutan menunjukkan adanyabahwa "sesuatu" pada tumbuhanbagian yang berkaitanpeka denganterhadap rangsang cahaya merahini danberada merahdi jauh[[daun]].
 
Baru pada tahun 1959, Warren Butler, ahli [[biofisika]], dan Harold Siegemman, ahli [[biokimia]], berhasil mengidentifikasi [[pigmen]] yang bertanggung jawab untuk gejala ini menggunakan teknik [[spektrofotometri]]. Butler menamakanpigmenmenamakan pigmen itu sebagai '''fitokrom.''' (secara harafiah berarti "zat warna tumbuhan").
 
Diperlukan waktu 23 tahun sebelum Peter Quail dan Clark Lagarias melaporkan pemurnian kimiawi fitokrom dari tumbuhan (1983). Selanjutnya, perhatian diarahkan pada struktur dan aspek [[genetika molekular]]nya. [[Sekuens gen]] fitokrom pertama kali diterbitkandiumumkan pada tahun 1985 oleh Howard Hershey and Peter Quail. Berturut-turut dilaporkan bahwa terdapat bermacam-macam tipe fitokrom, yang dibuatdikendalikan oleh [[gen]]-gen yang berbeda. [[Kapri]], misalnya, hingga sekarang diketahui memiliki paling sedikit dua tipe, ''[[Arabidopsis thaliana]]'' memiliki lima gen fitokrom, sementara [[padi]] hanya tiga. [[Jagung]] memiliki enam gen. Namun Perbedaan-perbedaan itu semua terletak pada bagian apoprotein, sementara senyawa sensor cahayanya tetap sama: fitokromobilin.
 
Pada tahun 1996, diketahui adanya gen (disebut ''Cph1'') dari [[sianobakteria|bakteri biru hijau]] ''[[Synechocystis]]'' yang agak memiliki kemiripan sekuens dengan gen-gen fitokrom dari tumbuhan. Jon Hughes dari [[Berlin]] dan Clark Lagarias dari Universitas California di Davis secara berturut-turut melaporkan bahwa gen ini mengkode "fitokrom", dalam artipengertian sebagai kromoprotein yang sensitif terhadap perubahan rangsang cahaya merah/merah jauh. Dari penelitian terhadap ''Cph1'' selanjutnya orang mengetahui bagaimana mekanisme kerja fitokrom. Penggunaan ''Cph1'' lebih ekonomis karena sianobakteriabakteri biru hijau relatif mudah dikerjakan di laboratorium daripada tumbuhan, dan juga lebih produktif. Dalam perkembangan selanjutnya, gen-gen fitokrom ditemukan pula pada [[prokariota|prokariot]] ''Deinococcus radiodurans'' dan ''[[Agrobacterium tumefaciens]]''. Peran biologi fitokrom pada ''Synechocystis'' dan ''Agrobacterium'' belum diketahui, sementara bagi ''Deinococcus'' fitokrom mengatur produksi pigmen pelindung dari cahaya.
 
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 ==
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While all these phytochromes have significantly different protein components, they all use phytochromobilin as their light-absorbing chromophore. In the late 1980s, the 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 [[cyanobacteria|cyanobacterium]] ''[[Synechocystis]]'' 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 [[prokaryote]]s 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.
 
 
== Isoforms or states ==
[[Image: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.
 
== Discovery ==
 
 
== 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 ==
<references/>
 
== Other sources ==
* http://www.ars.usda.gov/is/timeline/light.htm
* http://www.mobot.org/jwcross/duckweed/phytochrome.htm#tetrapyrrole
Baris 62 ⟶ 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
-->
 
== Lihat pula ==
* [[Kriptokrom]], reseptor cahaya [[biru]]
* [[Fototropin]], reseptor cahaya [[ultraungu]]
 
[[Kategori:Biokimia]]
[[Kategori:Fisiologi tumbuhan]]
[[Kategori:Senyawa hemetetrapirol]]
 
[[en:Phytochrome]]
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