Phytochrome in plants is a soluble protein pigment that carry out photomorphogenic growth. It is present almost in all eukaryotic plants and was first discovered by a scientist named Sterling Hendricks and Herry Borthwick in the year 1940-1960. A term phytochrome was also given by Warren Butler.
In the year 1983, Peter and Clark were the two scientists who introduced chemical purification of the phytochrome, and the gene sequence of phytochrome was reported in the year 1985 by the two scientists Howard and Peter.
Content: Phytochrome in Plants
Definition of Phytochrome
Phytochrome can define as the photoreceptor, which is a photomorphogenic pigment that absorbs red and far-red light wavelengths to induce photomorphogenesis or a light-induced growth and development in various plants, bacteria etc. It can also refer as a “Light-activated switch” which turns on by the absorption of red light and turns off by the absorption of far-red light. Phytochrome is the photosensitive proteinaceous pigments having a protein component and prosthetic group.
Structure: Phytochrome consists of two elements; a protein part and a prosthetic group. A protein part refers as apoprotein, and the prosthetic group refers as phytochromobilin or chromophore. Neither apoprotein nor chromophore can absorb the light alone, i.e. both should be present in combination to form a complete protein or holoprotein.
Synthesis: In plants, plastid synthesizes the chromophore component of phytochrome, while nuclear genome synthesizes the protein complex (apoprotein).
Structural conformation: It exists in two disparate structures like PR and PFR. As the phytochromes are of two kinds, one of its type (PR) is cytosol associated, while the other form (PFR) is membrane-associated. PR and PFR are the two forms of phytochromes that are photo interchangeable.
Absorption spectra: Phytochrome absorbing red light represents as PR, but when absorbing far-red light, then it represents as PFR. These are photosensitive in nature, which can absorb red light of wavelength ranging between 600-650 nm and far-red light of wavelength ranging between 700-750 nm.
Occurrence: In plants, phytochrome pigments can be found in different parts like roots, cotyledons, hypocotyls, epicotyls, coleoptile, stems etc.
Types: There are two classes of phytochrome protein, namely type-I and type-II. PHY-A, B, C, D and E are the phytochrome genes found in dicots, whereas PHY-A, B, C are common in monocots.
Phytochrome has a “dimeric” existence. It consists of homodimers, each of molecular weight 125 kDa. Its conformation of phytochrome comprises two primary structural elements.
Phytochromobilin: It is a photopigment that appears as a linear tetrapyrrole structure. Phytochromobilin mainly absorbs red or far-red light at different wavelengths.
Apoprotein: An apoprotein comprises of two domains, one showing the photoreceptor activity and the other showing the kinase activity.
Amino-terminal domain: It is the core photosensory domain that holds the chromophore and includes PAS, GAF and PHY subdomain. The N-terminal sometimes includes an additional variable, NTE (Amino-terminal extension). The PAS gene stabilizes the PFR form, GAF subdomain contains a cysteine residue to which a chromophore group binds via thioether linkage, and PHY gene participates in the synthesis of phytochrome receptor proteins from mRNA.
Carboxy-terminal domain: It is the regulatory domain, which primarily includes PAS repeats and HKRD subdomains. HKRD stands for a histidine-kinase related domain, which acts as a serine-threonine kinase and participates in the phytochrome signalling. PAS subdomain is responsible for the phytochrome dimerization and also contains nucleolar organization signals.
Classes of Phytochrome
“PHY gene family” encodes a phytochrome protein that comprises of two types, namely type-I and type-II.
Type-I phytochrome is encoded by PHY-A gene. These are light-sensitive pigment, wherein the presence of light the transcription of Phy is inhibited. It only functions in the dark or activates by absorbing the far-red light. Type-I phytochromes occur in a majority of etiolated seedlings.
Type-II phytochrome is encoded by PHY-B, C, D and E genes. These are light stable pigment, which is present in both light and dark-grown plants. It mainly functions in light or activates by absorbing the red light. Type-II phytochromes occur in a majority of green plants and seeds.
Biosynthesis of Phytochrome in Plants
Apoprotein component singly cannot absorb light wavelength for which a phytochromobilin must be joined covalently to the apoprotein. Phytochromobilin pigment is synthesized within the plastid and then liberated into the cytosol. A protein part (Apoprotein) synthesizes by the transcription of the nuclear genome into mRNA.
Then, mRNA is translated by the help of 80-S ribosome and synthesizes PR phytochrome in the cytoplasm. A chromophore pigment is synthesized within the plastid and released out in the cytosol, which then attaches to the protein via bilin lyase activity of GAF-domain through thioester linkage. Thus, it makes a holoprotein.
|Colour||Blue green in colour||Light green in colour|
|light mediated response||It does not show photo-mediated responses||It mediates photomorphogenic responses|
|Maximum light absorption||In red region approximately at 680 nm||In far red region approximately at 730 nm|
|Conformational change||In red light, it transforms into PFR.||In far-red light, it transforms into PR.|
In the inactive state, phytochrome exists as PR and activated state it exists as PFR. Activated phytochrome works as a serine-threonine kinase, which phosphorylates serine residue towards the hinge region. Then on the phosphorylated region, the nucleolar organization signals get exposed by which the PFR enters the nucleus and converts into more stable phytochrome through protein phosphatase that removes the phosphate group.
Functional Role of Phytochrome in Plants
Phytochrome in plants perform the following significant tasks:
Initiation of Transcription: Phytochrome triggers the exit of cop-1 or constitutive photomorphogenic 1, by activating three transcriptional factors like HF-1, LAF-1 and HY-5 in the presence of red light when the PR converts into active PFR. Cop-1 is basically an E-3 ubiquitin ligase. Oppositely, in darker conditions, the cop-1 remains inside the nucleus as an inactive form and target HY-5 transcriptional factor for the degradation by the 26S proteasome or inhibits the transcription.
Photomorphogenesis: Phytochrome carry out the light-mediated growth and developmental processes like seed germination, leaf growth, etiolation, photoperiod, shade avoidance, flowering etc.