Pollen: Difference between revisions

{{redirect-distinguish|Exine|Exene Cervenka{{!}}Exene}}

[[File:Misc pollen colorized.jpg|thumb|Colorized [[scanning electron microscope]] image of pollen grains from a variety of common plants: sunflower (''[[Common sunflower|Helianthus annuus]]''), morning glory (''[[Ipomoea purpurea]]''), prairie hollyhock (''[[Sidalcea malviflora]]''), oriental lily (''[[Lilium auratum]]''), evening primrose (''[[Oenothera fruticosa]]''), and castor bean (''[[Ricinus communis]] communis'').]]

[[File:Pollen Tube.svg|thumb|Pollen Tubetube Diagramdiagram]]

'''Pollen''' is a powdery substance produced by most types of flowers of [[SpermatophyteSeed plant|seed plants]] for the purpose of sexual reproduction.<ref>{{cite web |title=Best and Worst Flowers for People With Allergies |url=https://www.webmd.com/allergies/best-worst-plants |website=WebMD.com |access-date=2023-10-30}}</ref> It consists of pollen grains (highly reduced [[microgametophyteGametophyte#Heterospory|microgametophytes]]s), which produce male [[gamete]]s (sperm cells). Pollen grains have a hard coat made of [[sporopollenin]] that protects the gametophytes during the process of their movement from the [[stamen]]s to the [[gynoeciumGynoecium|pistil]] of flowering plants, or from the male [[Conifer cone|cone]] to the female cone of [[gymnosperm]]s. If pollen lands on a compatible pistil or female cone, it [[germinateGermination|germinates]]s, producing a [[pollen tube]] that transfers the [[sperm]] to the [[ovule]] containing the female gametophyte. Individual pollen grains are small enough to require magnification to see detail. The study of pollen is called [[palynology]] and is highly useful in [[paleoecology]], [[paleontology]], [[archaeology]], and [[Forensic science|forensics]].

Pollen in plants is used for transferring [[Ploidy#Haploid and monoploid|haploid]] male genetic material from the [[Stamen|anther]] of a single flower to the [[Stigma (botany)|stigma]] of another in cross-pollination.<ref name=EB1911>{{cite EB1911 |wstitle=Pollination |volume=22 |pages=2–5}}</ref> In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower.<ref name=EB1911/>

Pollen is infrequently used as food and [[Dietary supplement|food supplement]]. Because of agricultural practices, it is often contaminated by agricultural pesticides.<ref name="Tosi-2018">{{Cite journal|title=A survey of honey bee-collected pollen reveals widespread contamination by agricultural pesticides|journal=The Science of the Total Environment|volume=615|pages=208–218|doi=10.1016/j.scitotenv.2017.09.226|pmid=28968582|year=2018|last1=Tosi|first1=S.|last2=Costa|first2=C.|last3=Vesco|first3=U.|last4=Quaglia|first4=G.|last5=Guido|first5=G.|s2cid=19956612}}</ref>

Pollen comesgrains come in manya differentwide variety of shapes, sizes, and surface markings characteristic of the species (see [[electron micrograph]], right). Pollen grains of [[pine]]s, [[fir]]s, and [[spruce]]s are winged. The smallest pollen grain, that of the [[forget-me-not]] (''Myosotis'' spp.),{{which|date=February 2017}} is 2.5–5&nbsp;[[micrometre|μm]] (0.005&nbsp;mm) in diameter.<ref name="sporomex">{{Cite web | url=http://www.sporomex.co.uk/technology/51-pollenspores |title = Spores and Pollens}}</ref> Corn pollen grains are large, about 90–100&nbsp;μm.<ref>{{Cite journal| pmc = 59743 | doi = 10.1073/pnas.211287498 | year = 2001 | title = Corn pollen deposition on milkweeds in and near cornfields | volume = 98 | issue = 21 | pages = 11919–24 | pmid = 11559840 | journal = Proceedings of the National Academy of Sciences of the United States of America | last1 = Pleasants| first1 = J. M.| last2 = Hellmich| first2 = R. L.| last3 = Dively| first3 = G. P.| last4 = Sears| first4 = M. K.| last5 = Stanley-Horn| first5 = D. E.| last6 = Mattila| first6 = H. R.| last7 = Foster| first7 = J. E.| last8 = Clark| first8 = P.| last9 = Jones| first9 = G. D. | bibcode = 2001PNAS...9811919P | doi-access = free}}</ref> Most grass pollen is around 20–25&nbsp;μm.<ref name="sporomex"/> Some pollen grains are based on [[geodesic polyhedra]] like a [[soccer ball]].<ref>{{cite journal |first1=Kleber |last1=Andrade|first2=Sara |last2=Guerra |first3=Alexis |last3=Debut |title=Fullerene-Based Symmetry in Hibiscus rosa-sinensis Pollen |journal=PLOS ONE |date=2014 |volume=9 |issue=7 |pages=e102123 |doi=10.1371/journal.pone.0102123 |pmid=25003375 |pmc=4086983 |bibcode=2014PLoSO...9j2123A |doi-access=free }} See also [https://www.newscientist.com/article/mg25934510-500-time-for-your-close-up-vivid-images-of-nature-loom-larger-than-life/ this picture] by Igor Siwanowicz of a [[morning glory]] pollen grain.</ref>

In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, four groups offertile sporogenous cells, the '''archespore''', form within the anther. The fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac. Some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. The archespore cells divide by mitosis and differentiate to form '''pollen mother cells''' (microsporocyte, [[meiocyte]]).

In a process called '''microsporogenesis''', four haploid [[microspore]]s are produced from each diploid sporogenous cell (microsporocyte, pollen mother cell or [[meiocyte]]), after [[meiotic division]]. After the formation of the four microspores, which are contained by [[callose]] walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record. Two basic types of microsporogenesis are recognised, simultaneous and successive. In simultaneous microsporogenesis meiotic steps I and II are completed before [[cytokinesis]], whereas in successive microsporogenesis cytokinesis follows. While there may be a continuum with intermediate forms, the type of microsporogenesis has systematic significance. The predominant form amongst the [[monocots]] is successive, but there are important exceptions.<ref name=Furness2001>{{cite journal |last1=Furness|first1=Carol A. |last2=Rudall|first2=Paula J.|author-link2=Paula Rudall|title=Pollen and anther characters in monocot systematics|journal=Grana|date=January 2001|volume=40|issue=1–2|pages=17–25|doi=10.1080/00173130152591840|doi-access=free|bibcode=2001Grana..40...17F }}</ref>

During microgametogenesis, the unicellular microspores undergo mitosis and develop into mature [[microgametophyte]]s containing the gametes.<ref>[{{Cite web |url=http://www2.le.ac.uk/departments/biology/people/twell/lab/pollenis/development |title=Pollen Development — University of Leicester<!-- Bot generated title -->] |access-date=2013-12-12 |archive-date=2014-10-06 |archive-url=https://web.archive.org/web/20141006193005/http://www2.le.ac.uk/departments/biology/people/twell/lab/pollenis/development/ |url-status=dead }}</ref> In some flowering plants,{{which|date=February 2017}} [[germination]] of the pollen grain may begin even before it leaves the microsporangium, with the generative cell forming the two sperm cells.

The [[U.S. Food and Drug Administration]] (FDA) has not found any harmful effects of bee pollen consumption, except fromfor the usual allergies. However, FDA does not allow bee pollen marketers in the United States to make health claims about their produce, as no scientific basis for these has ever been proven. Furthermore, there are possible dangers not only from allergic reactions but also from contaminants such as pesticides<ref name="Tosi-2018" /> and from fungi and bacteria growth related to poor storage procedures. A manufacturers's claim that pollen collecting helps the bee colonies is also controversial.<ref>{{cite web| url=http://edis.ifas.ufl.edu/AA158 |access-date=2007-08-30 |title=Producing Pollen |last=Sanford |first=Malcolm T. |publisher=University of Florida, Institute of Food and Agricultural Sciences |archive-url=https://web.archive.org/web/20010429230017/http://edis.ifas.ufl.edu/AA158 |archive-date=2001-04-29}} Document ENY118. Original publication date November 1, 1994. Revised February 1, 1995. Reviewed May 1, 2003.</ref>

The growing industries in pollen harvesting for human and bee consumption rely on harvesting pollen baskets from honey bees as they return to their hives using a ''pollen trap''.<ref>{{cite web | url=https://www.youtube.com/watch?v=JBP9pw2rNk4 | archive-url=https://ghostarchive.org/varchive/youtube/20211104/JBP9pw2rNk4| archive-date=2021-11-04 | url-status=live| title=How a Pollen Trap Works (Bee Pollen)| website=[[YouTube]]}}{{cbignore}}</ref> When this pollen has been tested for parasites, it has been found that a multitude of viruses and eukaryotic parasites are present in the pollen.<ref name=Graystock2013>{{cite journal |last1=Graystock |first1=Peter |last2=Yates |first2=Kathryn |last3=Evison |first3=Sophie E. F. |last4=Darvill |first4=Ben |last5=Goulson |first5=Dave |last6=Hughes |first6=William O. H. |s2cid=3937352 |title=The Trojan hives: pollinator pathogens, imported and distributed in bumblebee colonies |journal=Journal of Applied Ecology |date=July 2013 |volume=50 |issue=5 |pages=1207–1215 |doi=10.1111/1365-2664.12134|bibcode=2013JApEc..50.1207G }}</ref><ref name=Singh2010>{{cite journal |last1=Singh |first1=Rajwinder |last2=Levitt |first2=Abby L. |last3=Rajotte |first3=Edwin G. |last4=Holmes |first4=Edward C. |last5=Ostiguy |first5=Nancy |last6=vanEngelsdorp |first6=Dennis |last7=Lipkin |first7=W. Ian |last8=dePamphilis |first8=Claude W. |last9=Toth |first9=Amy L. |last10=Cox-Foster |first10=Diana L. |last11=Traveset |first11=Anna |title=RNA Viruses in Hymenopteran Pollinators: Evidence of Inter-Taxa Virus Transmission via Pollen and Potential Impact on Non-Apis Hymenopteran Species |journal=PLOS ONE |date=22 December 2010 |volume=5 |issue=12 |pages=e14357 |doi=10.1371/journal.pone.0014357 |pmid=21203504 |pmc=3008715|bibcode=2010PLoSO...514357S|doi-access=free}}</ref> It is currently unclear if the parasites are introduced by the bee that collected the pollen or if it is from the flower.<ref name="Singh2010" /><ref name=ParaBloom>{{cite journal |last1=Graystock |first1=Peter |last2=Goulson |first2=Dave |last3=Hughes |first3=William O. H. |title=Parasites in bloom: flowers aid dispersal and transmission of pollinator parasites within and between bee species |journal=Proceedings of the Royal Society B: Biological Sciences |date=5 August 2015 |volume=282 |issue=1813 |pages=20151371 |doi=10.1098/rspb.2015.1371 |pmid=26246556 |pmc=4632632}}</ref> Though this is not likely to pose a risk to humans, it is a major issue for the bumblebee rearing industry that relies on thousands of tonnes of honey bee collected pollen per year.<ref>{{cite journal |last1=Graystock |first1=Peter |last2=Blane |first2=Edward J. |last3=McFrederick |first3=Quinn S. |last4=Goulson |first4=Dave |last5=Hughes |first5=William O.H. |title=Do managed bees drive parasite spread and emergence in wild bees? |journal=International Journal for Parasitology: Parasites and Wildlife |volume=5 |issue=1 |pages=64–75 |date=October 2015 |doi=10.1016/j.ijppaw.2015.10.001 |pmid=28560161 |pmc=5439461}}</ref> Several sterilization methods have been employed, though no method has been 100% effective at sterilisation without reducing the nutritional value of the pollen <ref name=steralization>{{cite journal |last1=Graystock |first1=P. |last2=Jones |first2=J.C. |last3=Pamminger |first3=T. |last4=Parkinson |first4=J.F. |last5=Norman |first5=V. |last6=Blane |first6=E.J. |last7=Rothstein |first7=L. |last8=Wäckers |first8=F.|last9=Goulson |first9=D. |last10=Hughes |first10=W.O.H. |title=Hygienic food to reduce pathogen risk to bumblebees |journal=Journal of Invertebrate Pathology |date=May 2016 |volume=136 |pages=68–73 |doi=10.1016/j.jip.2016.03.007 |pmid=26970260}}</ref>