{{short description|Biological process}} {{More citations needed|date=November 2015}} '''Chemotropism''' is defined as the growth of organisms navigated by chemical stimulus from outside of the organism. It has been observed in bacteria, plants and fungi.<ref>{{Cite journal|last1=Turrà|first1=David|last2=El Ghalid|first2=Mennat|last3=Rossi|first3=Federico|last4=Di Pietro|first4=Antonio|date=2015|title=Fungal pathogen uses sex pheromone receptor for chemotropic sensing of host plant signals|url=http://www.nature.com/articles/nature15516|journal=Nature|language=en|volume=527|issue=7579|pages=521–524|doi=10.1038/nature15516|pmid=26503056|bibcode=2015Natur.527..521T|s2cid=4449720|issn=0028-0836|url-access=subscription}}</ref> A chemical gradient can influence the growth of the organism in a positive or negative way. Positive growth is characterized by growing towards a stimulus and negative growth is growing away from the stimulus.<ref name=":0">{{Cite web|url=https://www.biologyonline.com/dictionary/chemotropism|title=Chemotropism Definition and Examples - Biology Online Dictionary|date=2019-10-07|website=Biology Articles, Tutorials & Dictionary Online|language=en-US|access-date=2020-03-23}}</ref>
Chemotropism is slightly different from Chemotaxis, the major difference being that chemotropism is related to growth, while chemotaxis is related to locomotion. A chemotropic process may have an underlying chemotactic component, as is the case with mating yeast.<ref name=":6" />
==Chemotropism in plants== thumb|PSM V77 D352 The course of the pollen tube in a rock rose
One prime example of chemotropism is seen in plant fertilization and pollen tube elongation of angiosperms, flowering plants.<ref>{{Cite web|url=https://study.com/academy/lesson/pollen-tube-growth-function-formation.html|title=Pollen Tube: Growth, Function & Formation - Video & Lesson Transcript|website=Study.com|language=en|access-date=2020-04-18}}</ref> Unlike animals, plants cannot move, and therefore need a delivery mechanism for sexual reproduction. Pollen, which contains the male gametophyte is transferred to another plant via insects or wind.<ref name=":3">{{Cite journal|last1=Krichevsky|first1=Alexander|last2=Kozlovsky|first2=Stanislav V.|last3=Tian|first3=Guo-Wei|last4=Chen|first4=Min-Huei|last5=Zaltsman|first5=Adi|last6=Citovsky|first6=Vitaly|author-link6=Vitaly Citovsky|date=2007-03-15|title=How pollen tubes grow|journal=Developmental Biology|language=en|volume=303|issue=2|pages=405–420|doi=10.1016/j.ydbio.2006.12.003|pmid=17214979|issn=0012-1606|doi-access=free}}</ref> If the pollen is compatible it will germinate and begin to grow.<ref name=":3" /> The ovary releases chemicals that stimulates a positive chemotropic response from the developing pollen tube.<ref>{{cite journal|last1=Reger|first1=BJ|last2=Chaubal|first2=R|last3=Pressey|first3=R|date=1992|title=Chemo-tropic responses by pearl millet pollen tubes|url=https://pubag.nal.usda.gov/download/15080/PDF|archive-url=https://web.archive.org/web/20180208004550/https://pubag.nal.usda.gov/download/15080/PDF|archive-date=February 8, 2018|journal=Sexual Plant Reproduction|volume=5|issue=1|pages=47–56|doi=10.1007/BF00714557|s2cid=4592052|access-date=7 February 2018|url-access=subscription}}</ref> In response the tube develops a defined tip growth area that promotes directional growth and elongation of the pollen tube due to a calcium gradient.<ref name=":3" /> The steep calcium gradient is localized in the tip and promotes elongation and orientation of the growth.<ref name=":3" /> This calcium gradient is essential for the growth to occur; it has been shown that inhibiting the formation of the gradient results in no growth.<ref name=":3" /> As the pollen tube continues to grow towards the ovules, the male sperm remains in the apical region and is transported to the female ovule.<ref>{{Cite journal|last1=Hepler|first1=Peter K.|last2=Vidali|first2=Luis|last3=Cheung|first3=Alice Y.|author-link3=Alice Cheung|date=2001-11-01|title=Polarized Cell Growth in Higher Plants|journal=Annual Review of Cell and Developmental Biology|volume=17|issue=1|pages=159–187|doi=10.1146/annurev.cellbio.17.1.159|pmid=11687487|issn=1081-0706}}</ref> The pollen tube elongates at a rate comparable to neurite development
An example of positive and negative chemotropism is shown by a plant's roots; the roots grow towards useful minerals displaying positive chemotropism, and grow away from harmful acids displaying negative chemotropism.<ref>{{cite journal|last1=Henke|first1=Michael|last2=Sarlikioti|first2=Vaia|title=Exploring root developmental plasticity to nitrogen with a three-dimensional architectural model.|journal=Plant Soil|date=3 August 2014|volume=385|issue=1–2|pages=49–62|doi=10.1007/s11104-014-2221-7|s2cid=17630453}}</ref><ref>{{cite journal|last1=Newcombe|first1=Frederick C.|last2=Rhodes|first2=Anna L. | name-list-style = vanc |title=Chemotropism of Roots|journal=Botanical Gazette|date=1904|volume=37|issue=1|pages=22–35|jstor=2465652|doi=10.1086/328441|s2cid=84789317|url=https://zenodo.org/record/1431377}}</ref>
==Chemotropism in animals==
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In more complex organisms an example of chemotropic movement includes the growth of individual neuronal cell axons in response to extracellular signals.<ref name=":1">{{Cite journal|last1=Song|first1=Hong-jun|last2=Poo|first2=Mu-ming|date=March 2001|title=The cell biology of neuronal navigation|url=https://www.nature.com/articles/ncb0301_e81|journal=Nature Cell Biology|language=en|volume=3|issue=3|pages=E81–E88|doi=10.1038/35060164|pmid=11231595|s2cid=7449327|issn=1476-4679|url-access=subscription}}</ref> Secreted protein can either repel or attract specific neurons. Some signal proteins such as netrins, semaphorins, neurotrophins and fibroblast growth factors have been identified in aiding neuronal growth.<ref name=":1" /> These signals guide the developing axon to innervate the correct target tissue.<ref>{{cite journal|last1=Tessier-Lavigne, Placzek, Lumsden, Dodd, Jessell|title=Chemotropic guidance of developing axons in the mammalian central nervous system|journal=Nature|volume=336|issue=6201|pages=775–8|pmid=3205306|year=1988|doi=10.1038/336775a0|bibcode=1988Natur.336..775T|s2cid=4247407}}</ref> The neuronal growth cones are guided by gradients of chemoattractant molecules released from their intermediate or final targets. There is evidence that the axons of peripheral neurons are guided by chemotropism and the directed growth of some central axons is also a chemotropic response, it remains to be determined whether chemotropism also operates in the central nervous system. Evidence has also been noted in neuronal regeneration, where chemotropic substances guide the ganglionic neurites towards the degenerated neuronal stump.<ref name="pmid7649810">{{cite journal | vauthors = Gu X, Thomas PK, King RH | title = Chemotropism in nerve regeneration studied in tissue culture | journal = Journal of Anatomy | volume = 186 | issue = 1| pages = 153–63 | year = 1995 | pmid = 7649810 | pmc = 1167281 }}</ref>
== Chemotropism in Fungi== thumb|Chemotropism seen in Yeast. Haploid Yeast cells release a- and α-factors that bind to the receptors of another haploid Yeast cell. The two Yeast cells fuse together to form an a/α diploid zygote. Fungal chemotropism was first reported over 100 years ago by Anton de Bary.<ref name=":4">{{Cite journal|last1=Turrà|first1=David|last2=Nordzieke|first2=Daniela|last3=Vitale|first3=Stefania|last4=El Ghalid|first4=Mennat|last5=Di Pietro|first5=Antonio|date=2016-09-01|title=Hyphal chemotropism in fungal pathogenicity|url=http://www.sciencedirect.com/science/article/pii/S1084952116301240|journal=Seminars in Cell & Developmental Biology|series=Transcriptional Enhancers|language=en|volume=57|pages=69–75|doi=10.1016/j.semcdb.2016.04.020|pmid=27150623|issn=1084-9521|url-access=subscription}}</ref> One example of fungi using chemotropism is seen in Yeast.Yeast release chemical pheromones in order to attract mates.<ref>{{Cite journal|last=Martin|first=Sophie G.|date=2019-06-01|title=Molecular mechanisms of chemotropism and cell fusion in unicellular fungi|url=https://jcs.biologists.org/content/132/11/jcs230706|journal=Journal of Cell Science|language=en|volume=132|issue=11|pages=jcs230706|doi=10.1242/jcs.230706|issn=0021-9533|pmid=31152053|doi-access=free}}</ref> Each haploid yeast cells express specific haploid genes; haploid α-cells express α-genes and haploid a-cells express a-genes.<ref>{{Cite journal|last1=Lodish|first1=Harvey|last2=Berk|first2=Arnoldaa|last3=Zipursky|first3=S. Lawrence|last4=Matsudaira|first4=Paul|last5=Baltimore|first5=David|last6=Darnell|first6=James|date=2000|title=Cell-Type Specification and Mating-Type Conversion in Yeast|url=https://www.ncbi.nlm.nih.gov/books/NBK21657/|archive-url=https://web.archive.org/web/20210309045214/https://www.ncbi.nlm.nih.gov/books/NBK21657/|archive-date=March 9, 2021|journal=Molecular Cell Biology. 4th Edition|language=en}}</ref> Each cell type releases a unique pheromone: a- or α-factor.<ref name=":2">{{Cite journal|last=Arkowitz|first=Robert A.|date=August 2009|title=Chemical Gradients and Chemotropism in Yeast|journal=Cold Spring Harbor Perspectives in Biology|volume=1|issue=2|article-number=a001958|doi=10.1101/cshperspect.a001958|issn=1943-0264|pmc=2742094|pmid=20066086}}</ref> By secreting these factors a chemical gradient is formed that attracts the other type of yeast cell during mating. In order for the yeast to sense the gradient they have to have proper receptors that bind a- or α-factor: Ste3 and Ste2 respectively.<ref name=":5">{{Cite journal|last1=Merlini|first1=Laura|last2=Dudin|first2=Omaya|last3=Martin|first3=Sophie G.|date=March 2013|title=Mate and fuse: how yeast cells do it|journal=Open Biology|volume=3|issue=3|article-number=130008|doi=10.1098/rsob.130008|doi-access=free|issn=2046-2441|pmc=3718343|pmid=23466674}}</ref> The receptors to detect the pheromones are seven transmembrane G-protein coupled receptor (GPCR).<ref name=":2" /> Once activated, there is a signaling cascade that results in the activation of transcription factors for mating-specific genes such as those involved in cell cycle arrest, directional polarization towards the chemical gradient, and sexual hyphae formation.<ref name=":4" /><ref name=":5" /> When fungal hyphae touch, the cells merge to form a diploid zygote.<ref name=":5" /> Mating pairs of budding yeast cells may polarize away from each other, but are able to adjust the location of polarity to enable successful alignment and fusion.<ref name=":6">{{Cite journal|last1=Ghose|first1=Debraj|last2=Jacobs|first2=Katherine|last3=Ramirez|first3=Samuel|last4=Elston|first4=Timothy|last5=Lew|first5=Daniel|date=2021-06-01|title=Chemotactic movement of a polarity site enables yeast cells to find their mates|journal=Proceedings of the National Academy of Sciences|language=en|volume=118|issue=22|article-number=e2025445118|doi=10.1073/pnas.2025445118|issn=0027-8424|pmid=34050026|pmc=8179161|bibcode=2021PNAS..11825445G |s2cid=235242389|doi-access=free }}</ref>
==See also== * Chemotaxis * Chemoreceptor * Mechanism
==References== {{reflist|2}} {{Tropism}}
Category:Tropism Category:Pollination