Phytochrome

Details about Phytochrome

1. DEPARTMENT OF BOTANY DR. RASIKA N. PATIL ASSISTANT PROFESSOR DEPARTMENT OF BOTANY SHRI SHIVAJI COLLEGE OF ARTS COMMERCE AND SCIENCE AKOLA PHYTOCHROME

2. What is Phytochrome? Phytochromes  are a class of  photoreceptor  in  plants , bacteria and  fungi  used to detect light. They are sensitive to light in the  red  and  far-red  region of the  visible spectrum  and can be classed as either Type I, which are activated by far-red light, or Type II that are activated by red light .  Recent advances have suggested that phytochromes also act as temperature sensors, as warmer temperatures enhance their de-activation .  All of these factors contribute to the plant's ability to  germinate . Phytochromes control many aspects of plant development. They regulate the  germination  of  seeds  ( photoblasty ), the synthesis of  chlorophyll , the elongation of seedlings, the size, shape and number and movement of  leaves  and the timing of  flowering  in adult plants. Phytochromes are widely expressed across many tissues and developmental stages . Other plant photoreceptors include  cryptochromes  and  phototropins , which respond to  blue  and  ultraviolet -A light and  UVR8 , which is sensitive to  ultraviolet -B light. In addition to their roles in nature, the light-induced interaction between a plant phytochrome and phytochrome -interacting factor (PIF) was used in 2002 to control gene transcription in yeast. This was the first example of using photoproteins from another organism for controlling a biochemical pathway

3. Phytochromes' effect on Phototropism Phytochromes also have the ability to sense light, and cause the plant to grow towards the light this is called  phototropism . So they performed a series of experiments to figure this out that had to start at the beginning. They found that blue light causes the plant Arabidopsis thaliana to exhibit a phototropic response, this curvature is heightened with the addition of red light .   They found that five phytochromes are present in the plant, they also found a variety of mutants in which the phytochromes do not function properly .  Two of these mutant were very important for this study they are phyA-101 and phyB-1 .  These are the mutants of phytochrome A and B respectively. The normally functional phytochrome A causes a sensitivity to far red light, and it causes a regulation in the expression of curvature toward the light .  Whereas phytochrome B is more sensitive to the red light .

4. Phytochromes are present in bacteria, cyanobacteria, fungi, algae, and land plants, and while in all cases they can perceive light, their photochemical properties vary largely among phyla. In land plants, phytochromes are red and far-red light receptors that exist in two forms. They are synthesized in the inactive Pr state, which upon light absorption converts to the active Pfr conformation. Pfr is inactivated upon far-red (FR) light absorption or through thermal relaxation, which depends on temperature, a process known as dark or thermal reversion. Phytochromes act as dimers, resulting in three possible phytochrome species: Pr – Pr , Pfr – Pr , and Pfr – Pfr . Pr and Pfr have different absorption maxima, but due to overlapping spectra both conformers are always present in the light while only prolonged darkness returns all phytochrome to Pr. Given that phytochrome responses depend on the proportion of Pfr conformers, signaling is influenced by a combination of light quantity, color, and temperature.

5. Phytochrome effect on root growth Phytochromes can also affect root growth. It has been well documented that gravitropism is the main tropism in roots. However, a recent study has shown that phototropism also plays a role. A red light induced positive phototropism has been recently recorded in an experiment that used Arabidopsis to test where in the plant had the most effect on a positive phototropic response. The experimenters utilized an apparatus that allowed for root apex to be zero degrees so that gravitropism could not be a competing factor. When placed in red light, Arabidopsis roots displayed a curvature of 30 to 40 degrees. This showed a positive phototropic response in the red light. They then wanted to pinpoint exactly where in the plant light is received. When roots were covered there was little to no curvature of the roots when exposed to red light. In contrast, when shoots were covered, there was a positive phototropic response to the red light. This proves that lateral roots is where light sensing takes place. In order to further gather information regarding the phytochromes involved in this activity, phtochrome A, B, D and E mutants, and WT roots were exposed to red light. Phytochrome A and B mutants were severely impaired. There was no significant difference in the response of  phyD  and  phyE  compared with the wildtype , proving that  phyA  and  phyB  are responsible for positive phototropism in roots.   

6. 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 . Many flowering plants use it to regulate the time of flowering based on the length of day and night and to set circadian rhythms. It also regulates including the germination of seeds, elongation of seedling, and the size , shape and number of leaves the synthesis of chlorophyll and the straightening of the epicotyl or hypocotyl hook of dicot seedlings.

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