Phytochrome

Phytochromes are a class of photoreceptor in plants, bacteria and fungi use 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 plants ability to germinate. Phytochromes are a class of photoreceptor in plants, bacteria and fungi use 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 plants 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. Phytochromes consist of a protein, covalently linked to a light-sensing bilin chromophore. The protein part comprises two identical chains (A and B). Each chain has a PAS domain, GAF domain and PHY domain. Domain arrangements in plant, bacterial and fungal phytochromes are comparable insofar, as the three N-terminal domains are always PAS, GAF and PHY domains. However C-terminal domains are more divergent. The PAS domain serves as a signal sensor and the GAF domain is responsible for binding to cGMP and also senses light signals. Together, these subunits form the phytochrome region, which regulates physiological changes in plants to changes in red and far red light conditions. In plants, red light changes phytochrome to its biologically active form, while far red light changes the protein to its biologically inactive form. Phytochromes are characterized by a red/far-red photochromicity. Photochromic pigments change their 'color' (spectral absorbance properties) upon light absorption. In the case of phytochrome the ground state is Pr, the r indicating that it absorbs red light particularly strongly. The absorbance maximum is a sharp peak 650–670 nm, so concentrated phytochrome solutions look turquoise-blue to the human eye. But once a red photon has been absorbed, the pigment undergoes a rapid conformational change to form the Pfr state. Here fr indicates that now not red but far-red (also called 'near infra-red'; 705–740 nm) is preferentially absorbed. This shift in absorbance is apparent to the human eye as a slightly more greenish color. When Pfr absorbs far-red light it is converted back to Pr. Hence, red light makes Pfr, far-red light makes Pr. In plants at least Pfr is the physiologically active or 'signalling' state. 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. Janoudi and his fellow coworkers wanted to see what phytochrome was responsible for causing phototropism to occur. 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. The experiment consisted in the wild-type form of Arabidopsis, phyA-101(phytochrome A (phyA) null mutant), phyB-1 (phytochrome B deficient mutant). They were then exposed to white light as a control blue and red light at different fluences of light, the curvature was measured. It was determined that in order to achieve a phenotype of that of the wild-type phyA-101 must be exposed to four orders of higher magnitude or about 100umol m−2 fluence. However, the fluence that causes phyB-1 to exhibit the same curvature as the wild-type is identical to that of the wild-type. The phytochrome that expressed more than normal amounts of phytochrome A it was found that as the fluence increased the curvature also increased up to 10umol-m−2 the curvature was similar to the wild-type. The phytochrome expressing more than normal amounts of phytochrome B exhibited curvatures similar to that of the wild type at different fluences of red light up until the fluence of 100umol-m−2 at fluences higher than this curvature was much higher than the wild-type. Thus, the experiment resulted in the finding that another phytochrome than just phytochrome A acts in influencing the curvature since the mutant is not that far off from the wild-type, and phyA is not expressed at all. Thus leading to the conclusion that two phases must be responsible for phototropism. They determined that the response occurs at low fluences, and at high fluences. This is because for phyA-101 the threshold for curvature occurred at higher fluences, but curvature also occurs at low fluence values. Since the threshold of the mutant occurs at high fluence values it has been determined that phytochrome A is not responsible for curvature at high fluence values. Since the mutant for phytochrome B exhibited a response similar to that of the wild-type, it had been concluded that phytochrome B is not needed for low or high fluence exposure enhancement. It was predicted that the mutants that over expressed phytochrome A and B would be more sensitive. However, it is shown that an over expression of phy A does not really effect the curvature, thus there is enough of the phytochrome in the wild-type to achieve maximum curvature. For the phytochrome B over expression mutant higher curvature than normal at higher fluences of light indicated that phy B controls curvature at high fluences. Overall, they concluded that phytochrome A controls curvature at low fluences of light.