Phytochrome

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Phytochromes are a class of common photoreceptor proteins found in plants , algae , bacteria , cyanobacteria, and fungi . They measure the ratio of light red to dark red light and control a broad spectrum of responses to light stimuli, such as the greening of parts of plants, the escape of shadows or the germination of seeds in plants. Besides the cryptochromes and the phototropins , they are the most important class of photoreceptors.

It was discovered in the 1950s by a team led by Harry Borthwick .

meaning

In higher plants, phytochromes control a multitude of processes, including germination , photomorphogenesis of the seedlings, flower formation, photoperiodism, and the avoidance of green shadows, which can be found under leaf covers. In lower plants such as mosses, phytochromes are responsible for phototropism and polarotropism.

In cyanobacteria , phytochromes are responsible for the chromatic adaptation of the photosynthetic apparatus: the composition of the accessory photosynthetic pigments is adapted to the spectral properties of the light. This also determines the composition of the photosynthetic apparatus in bacteria that carry out anoxygenic photosynthesis.

In some mushrooms, the phytochrome signal decides whether the plant enters the vegetative or generative development phase .

Response to light: photoconversion

Phytochromes can exist in two different conformations : the Pr form ( r for red, English red ) has an absorption maximum at 660  nm , the Pfr form ( fr for dark red, English far red ) at 730 nm. These spectral properties are both by the chromophore , as well as the surrounding protein.

It was recognized early on that the absorption properties of the molecule and also the plant responses can be changed by irradiation with certain light: tissues grown in the dark contain only the physiologically inactive Pr form of phytochrome, which after irradiation with bright red light is converted into the active Pfr- Form passes. The plant then shows typical reactions such as photomorphogenesis . This effect can be reversed by irradiation with dark red light - Pfr returns to the inactive Pr form. Due to its thermodynamic instability, Pfr returns to its Pr form independent of light. This process is called dark reversal or dark conversion.

The biological responses are thus determined by the Pr / Pfr ratio and the extent of dark reversal.

This process is caused by a change in the conformation of the chromophore when irradiated with appropriate light, which results in a structural change in the entire protein. This changes the kinase activity of the protein. In plants, the widely separated domains P3 / GAF and PAS-A and PAS-B are exposed. They carry signals for transport to the nucleus and possibly interaction surfaces for partner proteins.

Protein building

Phytochromes are in large parts strongly conserved, all have an N-terminal sensor region and a C-terminal regulator or dimerization region in common.

The sensor region is composed of the three domains P2 / PAS, P3 / GAF and P4 / PHY, wherein the chromophore in bacteria at the P2 / PAS and in plants at the P3 / GAF domain with the A-ring covalently linked via a thioester bond to a cysteine ​​of the protein is bound and sunk into a deep pocket. In plants there is also an additional N-terminal P1 domain that has different functions in the various species. P2 / PAS and P3 / GAF are essential for light perception and signal transduction have bilin-lyase activity, which is necessary for the incorporation of the chromophore. P4 / PHY is responsible for the spectral properties, the kinase activity and the fine-tuning of the activity of the phytochrome and induces its transport into the nucleus in plants.

In its original form, the regulatory region has a domain with histidine kinase activity, with the help of which proteins can be phosphorylated . This function has been lost in plants, where the kinase activity is brought about by a serine-threonine kinase , which, however, was found in the N-terminus of the protein. Plants have additional PAS-A and PAS-B domains that are responsible for dimerization and transport into the nucleus.

In eukaryotes, phytochromes always appear as dimers , in prokaryotes only as monomers .

Structure of the chromophore in plants

Phytochromobilin in the C15- Z, anti- conformation (Pr) and in the C15- E, anti- conformation (Pfr)

Depending on the group of organisms, different chromophores can be found that are derived from heme . Bacteria and fungi use Biliverdin IX α , higher plants use phytochromobilin , cyanobacteria and algae phycocyanobilin .

The chromophore is a linear tetrapyrrole that is attached to rings A, B and C by its surroundings. As a result of the photoconversion, the D-ring can rotate after receiving a bright red light quantum. The molecule changes into a cis-trans isomer form at the double bond between C15 and C16. The Pr form of the molecule is C15-Z, anti -configured, the Pfr form has C15- E, anti -configuration. The change in the conformation of the chromophore results in a change in the conformation of the entire protein, which then changes from the Pr to the Pfr form. This process is reversed by irradiation with dark red light.

Plant phytochrome gene family

So far, five genes for phytochrome have been found in Arabidopsis thaliana , three in rice , four in pine and three in ginkgo biloba . Phylogenetic analyzes have shown that the phytochromes split into the two main groups Phy A and Phy B before the divergence of the seed plants, into which all phytochromes can be classified. In Ceratodon purpureus , a moss, 4 phytochromes are known to date, one of which has a gene sequence that is unique for phytochromes, but it has not yet been clarified whether this version is a functional protein or a pseudogene.

Phy A occurs almost exclusively in large quantities in etiolated plants because its Pfr form is unstable and inhibits its own gene transcription . Phy B is found in plants grown under normal light conditions and in small amounts in etiolated plants. In contrast to Phy A, the Pfr form of Phy B is stable and is constitutively expressed .

Many of the phytochromes have similar properties and share certain tasks, and many of them have specialized roles in their biological function.

Signal transduction

Intracellular signal transduction is largely unknown. However, it can be assumed that the histokinase domain in prokaryotes plays an important role in the transfer of phosphate residues . Histokinase activity is no longer present in eukaryotes, but serine-threonine kinase activity is present.

A translocation of the protein into the nucleus has been demonstrated in eukaryotes, where it interacts with transcription factors and can directly influence gene expression there. However, cytosolic responses are also generated.

literature

  • Peter Schopfer, Axel Brennicke : Plant Physiology . 6th edition Elsevier Spektrum Akademischer Verlag, Munich 2006, ISBN 978-3-8274-1561-5 .
  • Nathan C. Rockwell et al. a .: Phytochrome Structure and Signaling Mechanisms . In: Annual Review of Plant Biology , Vol. 57 (2006), pp. 837-58, ISSN  1543-5008 .
  • Kurt Schaffner: On the photophysics and photochemistry of phytochrome, a photomorphogenic regulator in green plants , in: Rheinisch-Westfälische Akademie der Wissenschaften (natural, engineering and economic sciences, lectures), vol 362, Opladen: Westdeutscher Verlag 1988, ISBN 978-3 -531-08362-9 .

Individual evidence

  1. ^ Phytochrome . In: Spectrum of Science , Spectrum Academic Publishing House, Heidelberg, 2001.