Structure, function and mechanism of phytochrome was discovered by Borth wick et al in 1952. Phytochromes are amorphous photoreceptors chromoprotein and are sensitive to red light. It is found in almost all eukaryotic plants and exists in two forms mainly cytosol associated Pr or P660 and membrane associated Pfr or P730. Pr absorbs light at 660nm (red light) and Pfr absorbs light at 730nm in the far red light region of spectrum. Pr and Pfr forms are interconvertible. On absorption of far red light it changes back to Pr or P660nm. Pfr can also spontaneously revert to the Pr form in the dark.
Phytochrome proteins are the dimmers of two 124 kDa polypeptides. Both the polypeptides have pigment molecules that are covalently attached. It is a linear tetrapyrrole, which is attached to the phytochrome protein through a sulfur linkage. After the chromophore has absorbed light the structure changes a little bit. That in turn causes slight changes in the conformation of the protein to trigger it to initiate a response.
Functions of Phytochromes
Phytochrome plays role in germination of some seeds.
Phytochrome plays role in developmental phenomenon like photomorphogenesis, photoperiodism and cleistogamy.
Phytochrome also helps in formation of rhizomes and bulbs.
It plays role in seed dormancy, leaf abscission and synthesis of gibberellins, anthocyanins, carotenoids etc.
Phytochrome A and B have distinct roles in the regulation of plant growth and development.
Phy B predominantly perceives the red and far-red light.
Mechanism of action
Phytochrome has serine/threonine kinase property so the phytochromes possess serine/threonine kinase activity; parts of their structure resemble to the histidine kinases involved in bacterial chemotaxis. The phytochrome is thought to phosphorylate itself and then to phosphorylate one or more other proteins in the cell when activated by red light. The activated phytochrome migrates into the nucleus in some light responses, where it interacts with gene regulatory proteins to alter gene transcription. In other cases, in the cytoplasm the activated phytochrome activates a gene regulatory protein, which then regulates the gene transcription.
Structure, function and mechanism of action of cryptochromes
Cryptochromes are photoreceptors that are found in plants, animals and humans as well. These photoreceptors sense blue light. They are important in regulating the circulation clock that is present in the organisms. It might be possible that the cryptochromes are the evolutionary descendents of DNA photolyases (DNA repair enzymes). The homology between these two is that they both are receptors for blue and ultraviolet (UV-A) light. Cryptochromes can be subdivided into subfamilies according to their sequence similarities: plant cryptochromes, animal cryptochromes and cryptochrome-DASH proteins, which are found iin photosynthetic cyanobacteria as well as in non-photosynthetic bacteria, fungi, plants and animals, including Arabidopsis, Neurospora, zebrafish and Xenopus. The first cryptochrome gene to be identified was Arabidopsis CRY1.
Structure of cryptochromes
Cryptochromes possess no photolyase activity. Most cryptochromes are composed of two domains- an amino-terminal photolyase-related region and a carboxy-terminal domain of varying size. The PHR region of cryptochromes binds to two chromophores; one chromophore is flavin adenine dinucleotide (FAD) and the other 5, 10-methenyl tetrahydrofolate (pterin or MTHF). The carboxy-terminal domain of cryptochromes is generally less conserved than the PHR region and CRY-DASH proteins lack this domain
Functions of cryptochromes
Cryptochromes are one of the clock transcription factors that control the circadian behavior of the whole organism. Cryptochromes is generally a nuclear proteins that regulate gene expression.
Drosophila cryptochromes is a nuclear protein that mediates regulation of the circadian clock by light by interacting directly with the protein Tim to suppress the clock’s negative feedback loop. Light stimulates the Cry-Tim interaction, which promotes ubiquitination and proteosome-dependent degradation of Tim and represses formation of the Per-Tim heterodimer. In addition to its role as a photoreceptor for the entrainment of the central oscillator of Drosophila, Cry also has a light-independent role in the function of the peripheral circadian oscillator.
Mammalian cryptochromes has the same features as that of drosophila cryptochromes. Mammalian cryptochromes are predominantly nuclear proteins but they can also be found in the cytosol. Cry proteins are components of the negative-feedback loop of the circadian clock and perform both light-dependent and light-independent functions in the regulation of the circadian clock.
Structure, function and mechanism of action of phototropins
The phenomenon of growth or turgor-driven movement of a plant organ toward or away from a light source is known as phototropism. Phototropism is a photomorphogenetic response. Phototropins are plant specific blue light receptorsand they play role in phototropism, chloroplast movement, leaf expansion and stomatal opening. In short, phototropins optimize photosynthesis and control plant growth by capturing light efficiently and by reducing photo damage. Plants are able to perceive light through various photoreceptors and downstream genes. PIF3, PHYE and CRY3 genes are involved in light perception.
The phototropins contain two FMN binding LOV domains and Ser/Thr protein kinase domain at the C terminus. FMN stands for flavin mononucleotide and LOV stands for light, oxygen and voltage domain. FMN domain tightly bound to LOV domains allows light sensing. Two LOV domains are present in phototropins – LOV1 and LOV2. It has been studied that LOV2 domain is more important for biological activity than the LOV1 domain as it is predominant photosensing domain of phototropins. LOV domains are encountered in numerous photoreceptors from plants, fungi and bacteria. They are coupled with a wide variety of signaling domains. LOV domains are structurally related to PAS (Per, Arnt, Sim) domains.
Function of Phototropism
Phototropins phot1 and phot2 plays role in phototropism. It was seen that Phot 1 mutants in response to low intensity blue light shows lack of both hypocotyl and root phototropism but retain phototropic responsiveness to high-intensity blue light. This shows that there is second photoreceptor other that phot 1 which plays role in phototropism. This was confirmed by experimenting with phot1 phot2 double mutants