Plant photoreceptors are able to respond to visible light. There are three main classes of photoreceptors; phytochromes are able to measure red to far-red light ratios; cryptochromes respond to blue light; the third class are phototropins.
The capability to to detect light helps the plant to control developmental processes. The ability to measure red to far red ratios enables the plant to access its growing conditions and determine if light is significant for photosynthesis and/or a shade avoidance response should be carried out. All known flowering plants have phytochromes.
In addition to flowering plants phytochromes are found in many other organisms these include cryptophytes, cyanobacteria, bacteria and fungus. Therefore even though phytochromes are known to have a role in light detection, they are present in many light insensitive organisms; this suggests that phytochromes have a role in addition to that of being a photoreceptor. It is known that their are at least 120 phytochrome related proteins. In non-photosynthetic organisms the role of phytochromes ranges from the regulation of pigmentation to sexual development.
Phytochromes are reversible photo conversion switches
There are two main forms of plant phytochromes Pr and Pfr.If plants are grown in dark light then subjected to red light the Pr form will absorb the light and will convert to the far red light absorbing Pfr form and trigger photomorphogenesis. Vice versa if the Pfr form of phytochrome receive far red light then it will convert to the red light absorbing Pr form of phytochrome. The reaction of phytochromes to light is therefore reversible. A large amount of light is required to maintain a phytochrome Pr/Pfrratio in plants as reversion of phytochromes from Pfr to Pr occurs in the dark; this suggests that organisms such as fungi which make use of Phytochromes in sexual activity do not use phytochromes in the same way as flowering plants. Additionally the natural conversion of Pfr to Pr occuring in plants when grown in the dark proceeds much slower than that seen in non-photosynthetic organisms.
Classification of phytochromes
Using DNA sequence to classify phytochromes
As more and more phytochrome and phytochrome like sequence information has become available from different organisms it has enabled them to be classified based upon common motifs. The structure of plant phytochromes has been conserved throughout the evolution of plants. Phytochromes from plants (plant phy) have three conserved domains on their N-terminal region; these are the PAS / P2 domain, the GAF / P3 domain and the PHY / P4 domain, and a C-terminal HKRD region. These domains are also present in the phytochromes of cyanobacteria and many bacteriophytochromes. Plant phytochromes can be distinguished from the organisms in that they have a domain known as P1; this domain is thought to be involved in the inhibition of dark reversion of Pfr to Pr. Additionally Plant phytochromes have a further two PAS domains (PAS1 and PAS2). The phytochromes from other organisms such as fungi keep the core Pas / GAF / PHY domain but have a C-terminal response regulator RR / REC motif. The PAS and GAF domains of phytochromes are synonymous with other signalling molecules, whereas the PHY domain seems to be more specific to phytochromes themselves.
Further information on phytochromes
Phytochromes make use of bilin chromophores, these chromophores photoisomerise when Pr is converted to Pfr.
Regulation of the process takes place through conserved N-terminal phytochrome photosensors that can fuse to regulatory domains. Phytochromone's are still evolving.
Reference and Recommended Reading
Rockwell et al,. 2006. Phytochrome Structure and Signaling Mechanisms. Annual Review of Plant Biology. 57: 837 - 858