Phototropism

There are several signaling molecules that help the plant determine where the light source is coming from, and these activate several genes, which change the hormone gradients allowing the plant to grow towards the light. The very tip of the plant is known as the coleoptile, which is necessary in light sensing. Auxins activate proton pumps, decreasing the pH in the cells on the dark side of the plant. This acidification of the cell wall region activates enzymes known as expansins which disrupt hydrogen bonds in the cell wall structure, making the cell walls less rigid. In addition, increased proton pump activity leads to more solutes entering the plant cells on the dark side of the plant, which increases the osmotic gradient between the symplast and apoplast of these plant cells. Water then enters the cells along its osmotic gradient, leading to an increase in turgor pressure. The decrease in cell wall strength and increased turgor pressure above a yield threshold causes cells to swell, exerting the mechanical pressure that drives phototropic movement. Proteins encoded by a second group of genes, PIN genes, have been found to play a major role in phototropism. They are auxin transporters, and it is thought that they are responsible for the polarization of auxin location. Specifically PIN3 has been identified as the primary auxin carrier. It is possible that phototropins receive light and inhibit the activity of PINOID kinase (PID), which then promotes the activity of PIN3. This activation of PIN3 leads to asymmetric distribution of auxin, which then leads to asymmetric elongation of cells in the stem. pin3 mutants had shorter hypocotyls and roots than the wild-type, and the same phenotype was seen in plants grown with auxin efflux inhibitors. Using anti-PIN3 immunogold labeling, movement of the PIN3 protein was observed. PIN3 is normally localized to the surface of hypocotyl and stem, but is also internalized in the presence of Brefeldin A (BFA), an exocytosis inhibitor. This mechanism allows PIN3 to be repositioned in response to an environmental stimulus. PIN3 and PIN7 proteins were thought to play a role in pulse-induced phototropism. The curvature responses in the "pin3" mutant were reduced significantly, but only slightly reduced in "pin7" mutants. There is some redundancy among "PIN1", "PIN3", and "PIN7", but it is thought that PIN3 plays a greater role in pulse-induced phototropism. There are phototropins that are highly expressed in the upper region of coleoptiles. There are two main phototropism they are phot1 and phot2. phot2 single mutants have phototropic responses like that of the wild-type, but phot1 phot2 double mutants do not show any phototropic responses. Mature leaves contain chloroplasts that are essential in photosynthesis. Chloroplast rearrangement occurs in different light environments to maximize photosynthesis. There are several genes involved in plant phototropism including the NPH1 and NPL1 gene. They are both involved in chloroplast rearrangement. ;Second model In the second model light inhibits auxin biosynthesis on the light side of the plant, thus decreasing the concentration of auxin relative to the unaffected side. ;Third model In the third model there is a horizontal flow of auxin from both the light and dark side of the plant. Incoming light causes more auxin to flow from the exposed side to the shaded side, increasing the concentration of auxin on the shaded side and thus more growth occurring. ;Fourth model In the fourth model it shows the plant receiving light to inhibit auxin basipetal down to the exposed side, causing the auxin to only flow down the shaded side. ;Fifth model Model five encompasses elements of both model 3 and 4. The main auxin flow in this model comes from the top of the plant vertically down towards the base of the plant with some of the auxin travelling horizontally from the main auxin flow to both sides of the plant. Receiving light inhibits the horizontal auxin flow from the main vertical auxin flow to the irradiated exposed side. And according to the study by Sakai and Haga, the observed asymmetric auxin distribution and subsequent phototropic response in hypocotyls seems most consistent with this fifth scenario. ==Effects of wavelength==

Phototropism in plants such as Arabidopsis thaliana is directed by blue light receptors called phototropins. Other photosensitive receptors in plants include phytochromes that sense red light and cryptochromes that sense blue light. Different organs of the plant may exhibit different phototropic reactions to different wavelengths of light. Stem tips exhibit positive phototropic reactions to blue light, while root tips exhibit negative phototropic reactions to blue light. Both root tips and most stem tips exhibit positive phototropism to red light. Cryptochromes are photoreceptors that absorb blue/ UV-A light, and they help control the circadian rhythm in plants and timing of flowering. Phytochromes are photoreceptors that sense red/far-red light, but they also absorb blue light; they can control flowering in adult plants and the germination of seeds, among other things. The combination of responses from phytochromes and cryptochromes allow the plant to respond to various kinds of light. Together phytochromes and cryptochromes inhibit gravitropism in hypocotyls and contribute to phototropism. ==Gallery==

File:Arabidopsis thaliana.jpg|The Thale Cress (Arabidopsis thaliana) is regulated by blue to UV light File:Phycomyces3.JPG|Phycomyces, a fungus, also exhibit phototropism File:Phototropism.jpg|Example on a Phalaenopsis File:Azuki Bean phototropism.ogv|Example on Azuki beans File:Plantes entre deux immeubles.jpg|Ravenalas growing between two buildings in Kinshasa, Democratic Republic of Congo. The plane (here perpendicular to the north–south axis) of these two plants is orientated to maximize daylight absorption ==See also==