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Intro to phototropism and phytochromes experiment: Phytochromes also have the ability to sense light, and cause the plant to grow towards the light this is called phototropism[8]. Janoudi and his fellow coworkers wanted to see what phytochrome was responsible for causing phototropism to occur. So they preformed 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[8]. 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.[8] Two of these mutant were very important for this study they are phyA-101 and phyB-1.[8] 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.[8] Whereas, phytochrome B is more sensitive to the red light.[8]

The experimental breakdown:

The experiment consisted in the wild-type form of Arabidopsis, phyA-101, phyB-1, a mutant that expresses more than the normal amount of phytochrome A, and one that expresses more than normal of phytochrome B.[8] They were then exposed to white light as a control blue and red light at different fluences of light, the curvature was measured 70 minutes after exposure to blue light and a photographic enlarger was implemented to measure the curvature.[8] 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.[8] However, the fluence that causes phyB-1 to exhibit the same curvature as the wild-type is identical to that of the wild-type.[8] 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 10 umol-m-2 the curvature was similar to the wild-type.[8] 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.[8]

Experimental Results:

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.[8] 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.[8] This is because for phyA-101 the threshold for curvature occurred at higher fluences, but curvature also occurs at low fluence values.[8] 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.[8] 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.[8] 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.[8] 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.[8] Overall they concluded that phytochrome A controls curvature at low fluences of light.[8]

[8] is the reference to the scientific paper