phototropin-1
Although plants do not have eyes, their dependence upon light as a source of energy means that they must have some mechanism to sense light and respond accordingly. For example, germinating seeds use light as a signal to grow upward, while roots grow away from light. Through a disproportionate growth of cells on one side of a stem, it can lean towards light. This effect is especially pronounced in seedlings, which must quickly reach a light source in order to survive. Directed plant growth in response to a light signal is termed phototropism. This effect was first studied in depth by none other than Charles Darwin in 1880 (Kutschera 442). It would take more than a century for the molecular players in this process to be discovered.
There are five classes of plant photoreceptors currently known (Kutschera 446): phytochromes, cryptochromes, phototropins, a UV-B photoreceptor, and F-box photoreceptors. Phototropins mediate phototropism, stomatal opening (Kinoshita), leaf expansion, rapid retardation of stem elongation, chloroplast movements, and several other blue light-activated responses (Kutschera 443).
Chloroplast avoidance under intense light (so as to protect them from damage) is mediated by Phot-2, while chloroplast accumulation under low light is largely mediated by Phot-1 (Briggs & Christie 207). Positive shoot phototropism in response to low light is mediated by Phot-1. Stomatal opening is mediated by both Phot-1 and Phot-2.
Autophosphorylation of Ser-849 and Ser-851 is essential for all phot 1-mediated responses investigated, but that of other phosphorylation sites is not (Inoue 589). The substrates of phototropin kinase, which are probably expressed in a tissue-specific or cell-specific manner, may generate the diverse physiological responses, but such substrate proteins are yet to be identified.
Phototropin [1 and 2] is a dual chromophoric photoreceptor in which both chromophores are FMN, [flavin mononucleotide] (Briggs & Christie 205). They contain two Light Oxygen or Voltage (LOV) domains and a kinase domain.
Although Phot-1 does not have a published crystal structure, some facts on its operation have been discerned. In its inactive state, a LOV2 domain binds FMN, which obstructs the kinase active site (Inoue 590). When FMN absorbs light, it causes a change in a LOV2 domain and the LOV2-FMN domain to move away, freeing the kinase active site. The kinase then autophosphorylates, becoming fully activated. Phosphatases act to turn Phot-1 off by dephosphorylating it in the dark, and the LOV2-FMN domain can again block the kinase active site.
There are five classes of plant photoreceptors currently known (Kutschera 446): phytochromes, cryptochromes, phototropins, a UV-B photoreceptor, and F-box photoreceptors. Phototropins mediate phototropism, stomatal opening (Kinoshita), leaf expansion, rapid retardation of stem elongation, chloroplast movements, and several other blue light-activated responses (Kutschera 443).
Chloroplast avoidance under intense light (so as to protect them from damage) is mediated by Phot-2, while chloroplast accumulation under low light is largely mediated by Phot-1 (Briggs & Christie 207). Positive shoot phototropism in response to low light is mediated by Phot-1. Stomatal opening is mediated by both Phot-1 and Phot-2.
Autophosphorylation of Ser-849 and Ser-851 is essential for all phot 1-mediated responses investigated, but that of other phosphorylation sites is not (Inoue 589). The substrates of phototropin kinase, which are probably expressed in a tissue-specific or cell-specific manner, may generate the diverse physiological responses, but such substrate proteins are yet to be identified.
Phototropin [1 and 2] is a dual chromophoric photoreceptor in which both chromophores are FMN, [flavin mononucleotide] (Briggs & Christie 205). They contain two Light Oxygen or Voltage (LOV) domains and a kinase domain.
Although Phot-1 does not have a published crystal structure, some facts on its operation have been discerned. In its inactive state, a LOV2 domain binds FMN, which obstructs the kinase active site (Inoue 590). When FMN absorbs light, it causes a change in a LOV2 domain and the LOV2-FMN domain to move away, freeing the kinase active site. The kinase then autophosphorylates, becoming fully activated. Phosphatases act to turn Phot-1 off by dephosphorylating it in the dark, and the LOV2-FMN domain can again block the kinase active site.
references
- Baum, Gideon, Joanne Long, Gareth Jenkins, and Anthony Trewavas. “Stimulation of the blue light phototropic receptor NPH1 causes a transient increase in cytosolic Ca++.” Proceedings of the National Academy of Science 96:23 (1999): 13554-13559. Print.
- Briggs, Winslow & John Christie. “Phototropins 1 and 2: versatile plant blue-light receptors.” TRENDS in Plant Science 7:5 (2002): 1360-1385. Print.
- A good general review.
- Briggs, Windslow & Margaret Olney. “Photoreceptors in Plant Photomorphogenesis to Date. Five Phytochromes, Two Cryptochromes, One Phototropin, and One Superchrome.” Plant Physiology 125 (2001): 85-88. Print.
- Christie, John, Philippe Reymond, Gary Powell, Paul Bernasconi, Andrei Raibekas, Emmanuel Liscum, and Winslow Briggs. “Arabidopsis NPH1: A Flavoprotein with the Properties of a Photoreceptor for Phototropism.” Science 282 (1998): 1698-1701. Print.
- Huala, Eva, Paul Oeller, Emmanuel Liscum, In-Seob Han, Elise Larsen, Winslow Briggs. “Arabidopsis NPH1: A Protein Kinase with a Putative Redox-Sensing Domain.” Science 278 (1997): 2120-2123. Print.
- Original cloning paper.
- Jaedicke, Katharina, Anna Lichtenhaeler, Rabea Meyberg, Mathias Zeidler, and John Hughes. “A phytochrome-phototropin light signaling complex at the plasma membrane.” Proceedings of the National Academy of Science 109:30 (2012):12231-12236. Print.
- Kinoshita, Toshinori, Michio Doi, Noriyuki Suetsugu, Takatoshi Kagawa, Masamitsu Wada, and Ken-ichiro Shimazaki. “phot1 and phot2 mediate blue light regulation of stomatal opening.” Nature 414 (2001): 656-660. Print.
- Kutshcera, Ulrich & Winslow Briggs. “Root phototropism: from dogma to the mechanism of blue light perception.” Planta 235 (2012): 443-452. Print.
- This is a good review of Phot-1 , with root phototropism as a model case.
- Inoue, Shin-ichiro, Atsushi Takemiya, and Ken-ichiro Shimazaki. “Phototropin signaling and stomatal opening as a model case.” Current Opinion in Plant Biology 13 (2012): 587-593. Print.
- Another good review of Phot-1, but with stomatal opening discussed. Contains a great summary table.