Orbicules (syn. Ubisch bodies, con-peito grains) are small acellular structures of

sporopollenin 270px, SEM image of pollen grains Sporopollenin is one of the most chemically inert biological polymers. It is a major component of the tough outer (exine) walls of plant spores and pollen grains. It is chemically very stable and is usually well ...

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 Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

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. 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. 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. Both von Ubisch and Kosmath are considered as pioneers in orbicule research.

Name

Rosanoff used the terms ''Körnchen und Tröpfchen'', while von Ubisch used ''Plättchen'' and Kosmath used ''kutikulaähnliche Tapetumzellmembran''. The term ''Ubisch body'' was introduced by Rowley. This name was however later rejected by Heslop-Harrison because they were not discovered by von Ubisch. In early Japanese literature, they are sometimes called ''con-peito grains''. However, the most commonly used name is ''orbicule'', which was coined by Erdtman and colleagues.

Morphology

Orbicules are morphologically variable. Their size ranges from < 1 μm to 15 μm, but they are usually smaller than 1 μm. Within a single species, orbicule size may vary. There is also variation in the shape of orbicule; they can be spherical, irregular, doughnut-shaped, etc. 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. 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.

Development

Orbicules originate as lipid droplets (i.e. pro-orbicules) within the cytoplasm of tapetal cells, most likely from the rough endoplasmic reticulum. After

exocytosis Exocytosis () is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell ('' exo-'' + ''cytosis''). As an active transport mechanism, exocytosis requires the use o ...

, the pro-orbicules nest on the tapetal plasmalemma and get a sporopollenin coat synchronously with the developing pollen exine.

Tapetum

There is a positive correlation between the presence of orbicules and a parietal/secretory tapetum type, although species with parietal tapetal cells but lacking orbicules exist as well. 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 In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

condition in angiosperms. Hence, the presence of orbicules represents the plesiomorphic character state for angiosperms.

Distribution

Orbicules are present in 123 of 150 investigate angiosperm families. 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 The Rubiaceae are a family of flowering plants, commonly known as the coffee, madder, or bedstraw family. It consists of terrestrial trees, shrubs, lianas, or herbs that are recognizable by simple, opposite leaves with interpetiolar stipules ...

family is one of them.

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.

References

{{reflist, refs= {{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, pages=43–58, doi=10.2307/2441229 {{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 {{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 {{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, pages=375–392, doi=10.1086/319580 {{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 {{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, pages=240–272, doi=10.1007/BF02856566 {{cite journal, vauthors=Moon HK, year=2018, title=The phylogenetic potential of orbicules in angiosperms, journal=Korean Journal Plant Taxonomy, volume=48, pages=9–23, doi=10.11110/kjpt.2018.48.1.9 {{cite journal, vauthors=Kosmath L, year=1927, title=Studien über das Antherentapetum, journal=Österreichische Botanische Zeitschrift, volume=76, pages=235–241, doi=10.1007/BF01246254 {{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, pages=4142, doi=10.1038/s41598-022-08105-3 {{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 {{cite journal, vauthors=Rowley JR, year=1962, title=Nonhomogeneous sporopollenin in microspores of ''Poa annua'' L., journal=Grana Palynologica, volume=3, pages=3–19, doi=10.1080/00173136209429101 {{cite journal, vauthors=von Ubisch G, year=1927, title=Zur Entwicklungsgeschichte der Antheren, journal=Planta, volume=3, pages=490–495, doi=10.1007/BF01916485 {{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 {{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, pages=107–134, doi=10.1007/s12229-014-9135-1 {{cite journal, vauthors=Vinckier S, Smets E, year=2003, title=Morphological and ultrastructural diversity of orbicules in Gentianaceae, journal=Annals of Botany, volume=92, pages=657–672, doi=10.1093/aob/mcg187 Plant morphology Plant sexuality Plant reproductive system Pollination