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Phototropism is the growth of organisms in response to a light stimulus. It is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs. This causes the plant to have elongated cells on the farthest side from the light. Phototropism is one of the many plant tropisms or movements which respond to external stimuli. Growth towards a light source is called positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, and rearrange their chloroplasts in the leaves to maximize photosynthetic energy and promote growth.〔Goyal, A., Szarzynska, B., Fankhauser C. (2012). Phototropism: at the crossroads of light-signaling pathways. Cell 1-9.〕〔Sakai, T., Kagawa, T., Kasahara, M., Swartz, T.E., Christie, J.M., Briggs, W.R., Wada, M., Okada, K. (2001). Arabidopsis nph1 and npl1: Blue light receptors that mediate both phototropism and chloroplast relocation. PNAS 98(12), 6969-6974.〕 Roots usually exhibit negative phototropism, although gravitropism may play a larger role in root behavior and growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. The combination of phototropism and gravitropism allow plants to grow in the correct direction.〔Liscum, E. (2002). Phototropism: Mechanisms and Outcomes. Arabidopsis Book 1-21.〕 ==Mechanism== There are several signaling molecules that help the plant determine where the light source is, and this activates several genes, which change the hormone gradients allowing the plant to grow towards the light. The very tip of the plant known as the coleoptile is necessary in light sensing.〔 The middle portion of the coleoptile is the area where the shoot curvature occurs. The Cholodny-Went hypothesis, developed in the early 20th century, predicts that in the presence of asymmetric light, auxin will move towards the shaded side and promote elongation of the cells on that side to cause the plant to curve towards the light source.〔Christie, J.M., and Murphy, A.S. (2013). Shoot phototropism in higher plants: New light through old concepts. American Journal of Botany 100(1), 35-46.〕 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 break bonds in the cell wall structure, making the cell walls less rigid. In addition, the acidic environment causes disruption of hydrogen bonds in the cellulose that makes up the cell wall. The decrease in cell wall strength causes cells to swell, exerting the mechanical pressure that drives phototropic movement. A second group of genes, ''PIN'' genes, have been found to play a major role in phototropism. They are auxin transporters, so it is thought that they are responsible for the polarization of auxin. Specifically ''PIN3'' has been identified as the primary auxin carrier.〔Ding, Z., Galván-Ampudia, C.S., Demarsy, E., Langowski, L., Kleine-Vehn, J., Fan, Y., Morita, M.T., Tasaka M., Fankhauser, C., Offringa, R., Friml, J. (2011). Light-mediated polarization of the PIN3 auxin transporter for the phototropic response in Arabidopsis Nature Cell Biology 13(4), 447-453.〕 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.〔Friml, J., Wisniewska, J., Benkova, E., Mendgen, K., Palme, K. (2002). Lateral relocation of auxin efflux regulator PIN3 mediates tropism in Arabidopsis. Nature 415, 806-809.〕 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.〔Haga, K., and Sakai, T. (2012). PIN Auxin Efflux Carriers Are Necessary for Pulse-Induced But Not Continuous Light-Induced Phototropism in Arabidopsis. Plant Physiology 160, 763-776.〕 There are phototropins that are highly expressed in the upper region of coleoptiles. The two main phototropins 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.〔 The amounts of ''PHOT1'' and ''PHOT2'' present are different depending on the age of the plant and the intensity of the light. There is a high amount of ''PHOT2'' present in mature ''Arabidopsis'' leaves and this was also seen in rice orthologs. The expression of ''PHOT1'' and ''PHOT2'' changes depending on the presence of blue or red light. There was a downregulation of ''PHOT1'' mRNA in the presence of light, but upregulation of PHOT2 transcript. The levels of mRNA and protein present in the plant were dependent upon the age of the plant. This suggests that the phototropin expression levels change with the maturation of the leaves.〔Labuz, J., Sztatelman, O., Banas, A. K., Gabrys H. (2012). The expression of phototropins in Arabidopsis leaves: developmental and light regulation. Journal of Experimental Botany 63(4), 1763-1771.〕 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.〔 The ''nph1'' and ''npl1'' double mutants were found to have reduced phototropic responses. In fact, the two genes are both redundant in determining the curvature of the stem. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「phototropism」の詳細全文を読む スポンサード リンク
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