Phototropism 433801 225523129 2008-07-14T03:34:28Z SJFriedl 1132104 Minor linkage fixes [[Image:Arabidopsis thaliana.jpg|thumb|The Thale Cress (''[[Arabidopsis thaliana]]'') is regulated by blue to UV light ([http://www.plantphys.net/article.php?id=266 plantphys.net])]] [[Image:Phycomyces3.JPG|thumb|right|''[[Phycomyces]]'', a fungus, also exhibit phototropism]] '''Phototropism''' is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimuli. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. Phototropism is one of the many plant [[tropism]]s or movements which respond to external stimuli. Growth towards a light source is a '''positive phototropism''', while growth away from light is called '''negative phototropism''' (or '''Skototropism'''). Most plant shoots exhibit positive phototropism, while 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. Phototropism in plants such as ''[[Arabidopsis thaliana]]'' is regulated by blue light receptors called [[phototropin]]s. Other photosensitive receptors in plants include '''[[phytochrome]]s''' that sense red light and '''[[cryptochrome]]s''' 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. Phototropism is enabled by '''[[auxins]]'''. Auxins are plant hormones that have many functions. In this respect, auxins are responsible for expelling H+ ions (creating proton pumps) which decreases pH in the cells on the dark side of the plant. This acidification of the cell wall region activates enzymes known as [[Expansin|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. Phototropism relates to photosynthesis. == Other light responses == *[[Etiolation]] is the response of a plant when light is nearly (or completely) absent. *[[Heliotropism]] is the [[Day|diurnal]] motion of plant parts (flowers or leaves) in response to the direction of the sun. It is not a phototropism since it does not involve growth. *[[Nastic movements|Photonasty]] involves the movement of plant parts that does not involve growth but is triggered by light. The plant movement is not determined by the direction of light so it is not a phototropism. Photonasty in prayer plant (''Maranta leuconeura'') involves the downward movement of leaves when they receive light in the morning. *[[Phototaxis]] is movement of an entire organism in which the direction of movement is determined by the direction of light. It occurs in some motile microbes such as [[Euglena]] and algae. It is not a phototropism because growth is not required. *Photo-orientation occurs within a plant cell when [[chloroplasts]] change their positions depending upon light intensity. This was discovered in 1987 by Chelsea Polevy and Kelsey Joyce when experimenting in their laboratory. When the light intensity is high, chloroplasts move to the edge of the cell to reduce [[photobleaching]] (destruction of [[chlorophyll]]).<ref>Takagi, Shingo 23 December 2002, [http://jeb.biologists.org/cgi/content/full/206/12/1963 Actin-based photo-orientation movement of chloroplasts in plant cells], Journal of Experimental Biology 206: 1963-1969</ref> In low light, chloroplasts tend to spread out within the protoplasm to maximize their capture of light energy. Photo-orientation is also not a phototropism. ==References== {{reflist}} ==See also== *[[Scotobiology]] ==External links== * [http://plantsinmotion.bio.indiana.edu/ Time lapse films], Plants-In-Motion {{Tropism}} [[Category:biology]] [[cs:Fototropismus]] [[da:Fototropisme]] [[el:Φωτοτροπία]] [[fr:Phototropisme]] [[nl:Fototropie]] [[pl:Fototropizm]] [[pt:Fototropismo]] [[zh:向光性]]