Orbicule

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

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, 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 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 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

References

1. (1972). "Pollen wall and tapetal orbicular wall development in ''Sorghum bicolor'' (Gramineae)". American Journal of Botany.
2. (1986). "Studies of the development of wheat (''Triticum aestivum'') pollen. I. Formation of the pollen wall and ubisch bodies". Grana.
3. (1961). "An introduction to a Scandinavian pollen flora". Grana Palynologica.
4. (2001). "The tapetum in basal angiosperms: early diversity". International Journal of Plant Sciences.
5. (1971). "Pollen development and physiology". Butterworths.
6. (1998). "Orbicules in angiosperms: morphology, function, distribution, and relation with tapetum types". The Botanical Review.
7. (2018). "The phylogenetic potential of orbicules in angiosperms". Korean Journal of Plant Taxonomy.
8. (1927). "Studien über das Antherentapetum". Österreichische Botanische Zeitschrift.
9. (2022). "Systematic palynology in Korean Piperales with special focus on its exine surface ornamentation and orbicule morphology". Scientific Reports.
10. (1865). "Zur Kenntnis des Baues und der Entwicklungsgeschichte des Pollens der Mimoseae". Jahrbuch für wissenschaftliche Botanik.
11. (1962). "Nonhomogeneous sporopollenin in microspores of ''Poa annua'' L.". Grana Palynologica.
12. (1927). "Zur Entwicklungsgeschichte der Antheren". Planta.
13. (2011). "Phylogenetic signal of orbicules at family level: Rubiaceae as case study". Taxon.
14. (2014). "Orbicules in flowering plants: a phylogenetic perspective on their form and function". The Botanical Review.
15. (2003). "Morphological and ultrastructural diversity of orbicules in Gentianaceae". Annals of Botany.