thumb|Orbicules and pollen in the anthers of ''Cubanola domingensis'' thumb|Orbicules in the anthers of ''Argostemma africanum'' thumb|Orbicules in the anthers of ''Ixora borboniae''
'''Orbicules''' (syn. Ubisch bodies, con-peito grains) are small acellular structures of sporopollenin that might occur on the inner tangential and radial walls of tapetal cells. The ornamentation of the orbicule surface often resembles that of the pollen sexine. Different hypotheses about their function have been proposed, including them just being a by-product of pollen wall sporopollenin synthesis.
==Discovery==
In 1865, Rosanoff published observations on anthers of Fabaceae species in which he noticed small granules on the inner locule wall that were resistant to concentrated sulphuric acid.<ref name=Rosanoff1865/> Von Ubisch and Kosmath independently provided the first records of species with and without orbicules and indicated that orbicules are restricted to a secretory tapetum type.<ref name=vonUbisch1927/><ref name=Kosmath1927/> Von Ubish concluded that orbicules are homologous with the pollen exine, as both showed the same reaction to chemicals and stains and they developed synchronously.<ref name=vonUbisch1927/> Both von Ubisch and Kosmath are considered as pioneers in orbicule research.<ref name=Huysmans1998/>
==Name==
Rosanoff used the terms ''Körnchen und Tröpfchen'',<ref name=Rosanoff1865/> while von Ubisch used ''Plättchen''<ref name=vonUbisch1927/> and Kosmath used ''kutikulaähnliche Tapetumzellmembran''.<ref name=Kosmath1927/>
The term ''Ubisch body'' was introduced by Rowley.<ref name=Rowley1962/> This name was however later rejected by Heslop-Harrison because they were not discovered by von Ubisch.<ref name=Heslop1971/> In early Japanese literature, they are sometimes called ''con-peito grains''.<ref name=Huysmans1998/> However, the most commonly used name is ''orbicule'', which was coined by Erdtman and colleagues.<ref name=Erdtman1961/>
==Morphology==
Orbicules are morphologically variable. Their size ranges from < 1 μm to 15 μm, but they are usually smaller than 1 μm.<ref name=Huysmans1998/><ref name=Verstraete2011/><ref name=Verstraete2014/> Within a single species, orbicule size may vary.<ref name=Verstraete2011/> There is also variation in the shape of orbicule; they can be spherical, irregular, doughnut-shaped, etc.<ref name=Verstraete2011/> The orbicular wall can be smooth or ornamented (e.g. with microgranules or microspines) and this ornamentation often shows a striking similarity with the exine ornamentation of the pollen grain.<ref name=Verstraete2011/><ref name=Vinckier2003/><ref name=Oak2022/> Orbicules are resistant to acetolysis and react to histochemical staining in a similar way as the pollen exine, indicating that they are composed of sporopollenin.<ref name=Huysmans1998/>
==Development==
Orbicules originate as lipid droplets (i.e. pro-orbicules) within the cytoplasm of tapetal cells, most likely from the rough endoplasmic reticulum.<ref name=Huysmans1998/><ref name=ElGhazaly1986/> After exocytosis, the pro-orbicules nest on the tapetal plasmalemma and get a sporopollenin coat synchronously with the developing pollen exine.<ref name=Christensen1972/>
==Tapetum==
There is a positive correlation between the presence of orbicules and a parietal/secretory tapetum type,<ref name=vonUbisch1927/><ref name=Kosmath1927/> although species with parietal tapetal cells but lacking orbicules exist as well.<ref name=Huysmans1998/> Parietal tapeta are the dominant type in land plants and occur in the extant ‘basal’ angiosperm groups and in most fossil taxa; it is therefore considered as the plesiomorphic condition in angiosperms.<ref name=FurnessRudall2001/> Hence, the presence of orbicules represents the plesiomorphic character state for angiosperms.<ref name=Huysmans1998/>
==Distribution==
Orbicules are present in 123 of 150 investigate angiosperm families.<ref name=Verstraete2014/><ref name=Moon2018/> The presence or absence of orbicules is rather constant at the family level: only 24 angiosperm families have both positive and negative observations; the Rubiaceae family is one of them.<ref name=Verstraete2014/>
==Function==
The function of orbicules remains enigmatic. In general, there are two views: either orbicules play an active role or they are just a by-product.<ref name=Verstraete2014/>
==References== <references>
<ref name=Christensen1972>{{cite journal|vauthors=Christensen JE, Horner Jr HT, Lersten NR|year=1972|title=Pollen wall and tapetal orbicular wall development in ''Sorghum bicolor'' (Gramineae)|journal=American Journal of Botany|volume=59|issue=1 |pages=43–58|doi=10.2307/2441229 |jstor=2441229 }}</ref> <ref name=ElGhazaly1986>{{cite journal|vauthors=El-Ghazaly G, Jensen WA|year=1986|title=Studies of the development of wheat (''Triticum aestivum'') pollen. I. Formation of the pollen wall and ubisch bodies|journal=Grana|volume=25|pages=1–29|doi=10.1080/00173138609429929|doi-access=free}}</ref> <ref name=Erdtman1961>{{cite journal|vauthors=Erdtman G, Berglund B, Praglowski J|year=1961|title=An introduction to a Scandinavian pollen flora|journal=Grana Palynologica|volume=2|issue=3|pages=3–86|doi=10.1080/00173136109428945|bibcode=1961GranP...2....3E }}</ref> <ref name=FurnessRudall2001>{{cite journal|vauthors=Furness CA, Rudall PJ|year=2001|title=The tapetum in basal angiosperms: early diversity|journal=International Journal of Plant Sciences|volume=162|issue=2 |pages=375–392|doi=10.1086/319580|bibcode=2001IJPlS.162..375F |s2cid=84338737 }}</ref> <ref name=Heslop1971>{{cite book|last1=Heslop-Harrison|first1=J|editor-last1=Heslop-Harrison|editor-first1=J|year=1971|chapter=The pollen wall: structure and development|title=Pollen development and physiology|publisher=Butterworths|publication-place=London|pages=75–98|isbn=9780390437402}}</ref> <ref name=Huysmans1998>{{cite journal|vauthors=Huysmans S, El-Ghazaly G, Smets E|year=1998|title=Orbicules in angiosperms: morphology, function, distribution, and relation with tapetum types|journal=The Botanical Review|volume=64|issue=3 |pages=240–272|doi=10.1007/BF02856566|bibcode=1998BotRv..64..240H |s2cid=38183780 |url=https://lirias.kuleuven.be/handle/123456789/59362 |url-access=subscription}}</ref> <ref name=Moon2018>{{cite journal|vauthors=Moon HK|year=2018|title=The phylogenetic potential of orbicules in angiosperms|journal=Korean Journal of Plant Taxonomy|volume=48|issue=1 |pages=9–23|doi=10.11110/kjpt.2018.48.1.9|doi-access=free|bibcode=2018KJPT...48....9M }}</ref> <ref name=Kosmath1927>{{cite journal|vauthors=Kosmath L|year=1927|title=Studien über das Antherentapetum|journal=Österreichische Botanische Zeitschrift|volume=76|issue=3 |pages=235–241|doi=10.1007/BF01246254|bibcode=1927PSyEv..76..235K |s2cid=36194266 }}</ref> <ref name=Oak2022>{{cite journal|vauthors=Oak MK, Yang S, Choi G, Song JH|year=2022|title=Systematic palynology in Korean Piperales with special focus on its exine surface ornamentation and orbicule morphology|journal=Scientific Reports|volume=12|issue=1 |article-number=4142|doi=10.1038/s41598-022-08105-3|pmid=35264735 |pmc=8907175 |bibcode=2022NatSR..12.4142O }}</ref> <ref name=Rosanoff1865>{{cite journal|vauthors=Rosanoff S|year=1865|title=Zur Kenntnis des Baues und der Entwicklungsgeschichte des Pollens der Mimoseae|journal=Jahrbuch für wissenschaftliche Botanik|volume=4|pages=441–450|url=https://www.biodiversitylibrary.org/page/6994345}}</ref> <ref name=Rowley1962>{{cite journal|vauthors=Rowley JR|year=1962|title=Nonhomogeneous sporopollenin in microspores of ''Poa annua'' L.|journal=Grana Palynologica|volume=3|issue=3 |pages=3–19|doi=10.1080/00173136209429101|doi-access=free|bibcode=1962GranP...3....5R }}</ref> <ref name=vonUbisch1927>{{cite journal|vauthors=von Ubisch G|year=1927|title=Zur Entwicklungsgeschichte der Antheren|journal=Planta|volume=3|issue=2–3 |pages=490–495|doi=10.1007/BF01916485|bibcode=1927Plant...3..490V |s2cid=40848249 }}</ref> <ref name=Verstraete2011>{{cite journal|vauthors=Verstraete B, Groeninckx I, Smets E, Huysmans S|year=2011|title=Phylogenetic signal of orbicules at family level: Rubiaceae as case study|journal=Taxon|volume=60|issue=3|pages=742–757|doi=10.1002/tax.603010|bibcode=2011Taxon..60..742V |url=https://lirias.kuleuven.be/handle/123456789/311526 |url-access=subscription}}</ref> <ref name=Verstraete2014>{{cite journal|vauthors=Verstraete B, Moon HK, Smets E, Huysmans S|year=2014|title=Orbicules in flowering plants: a phylogenetic perspective on their form and function|journal=The Botanical Review|volume=80|issue=2 |pages=107–134|doi=10.1007/s12229-014-9135-1|bibcode=2014BotRv..80..107V |s2cid=255562584 }}</ref> <ref name=Vinckier2003>{{cite journal|vauthors=Vinckier S, Smets E|year=2003|title=Morphological and ultrastructural diversity of orbicules in Gentianaceae|journal=Annals of Botany|volume=92|issue=5 |pages=657–672|doi=10.1093/aob/mcg187|pmid=14500324 |pmc=4244851 }}</ref>
</references>
Category:Plant morphology Category:Plant sexuality Category:Plant reproductive system Category:Pollination