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.[1] 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.[2][3] 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.[2] Both von Ubisch and Kosmath are considered as pioneers in orbicule research.[4]

Name

Rosanoff used the terms Körnchen und Tröpfchen,[1] while von Ubisch used Plättchen[2] and Kosmath used kutikulaähnliche Tapetumzellmembran.[3]

The term Ubisch body was introduced by Rowley.[5] This name was however later rejected by Heslop-Harrison because they were not discovered by von Ubisch.[6] In early Japanese literature, they are sometimes called con-peito grains.[4] However, the most commonly used name is orbicule, which was coined by Erdtman and colleagues.[7]

Morphology

Orbicules are morphologically variable. Their size ranges from < 1 μm to 15 μm, but they are usually smaller than 1 μm.[4][8][9] Within a single species, orbicule size may vary.[8] There is also variation in the shape of orbicule; they can be spherical, irregular, doughnut-shaped, etc.[8] 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.[8][10][11] 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.[4]

Development

Orbicules originate as lipid droplets (i.e. pro-orbicules) within the cytoplasm of tapetal cells, most likely from the rough endoplasmic reticulum.[4][12] After exocytosis, the pro-orbicules nest on the tapetal plasmalemma and get a sporopollenin coat synchronously with the developing pollen exine.[13]

Tapetum

There is a positive correlation between the presence of orbicules and a parietal/secretory tapetum type,[2][3] although species with parietal tapetal cells but lacking orbicules exist as well.[4] 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.[14] Hence, the presence of orbicules represents the plesiomorphic character state for angiosperms.[4]

Distribution

Orbicules are present in 123 of 150 investigate angiosperm families.[9][15] 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.[9]

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.[9]

References

  1. 1 2 Rosanoff S (1865). "Zur Kenntnis des Baues und der Entwicklungsgeschichte des Pollens der Mimoseae". Jahrbuch für wissenschaftliche Botanik. 4: 441–450.
  2. 1 2 3 4 von Ubisch G (1927). "Zur Entwicklungsgeschichte der Antheren". Planta. 3 (2–3): 490–495. doi:10.1007/BF01916485. S2CID 40848249.
  3. 1 2 3 Kosmath L (1927). "Studien über das Antherentapetum". Österreichische Botanische Zeitschrift. 76 (3): 235–241. doi:10.1007/BF01246254. S2CID 36194266.
  4. 1 2 3 4 5 6 7 Huysmans S, El-Ghazaly G, Smets E (1998). "Orbicules in angiosperms: morphology, function, distribution, and relation with tapetum types". The Botanical Review. 64 (3): 240–272. doi:10.1007/BF02856566. S2CID 38183780.
  5. ↑ Rowley JR (1962). "Nonhomogeneous sporopollenin in microspores of Poa annua L." Grana Palynologica. 3 (3): 3–19. doi:10.1080/00173136209429101.
  6. ↑ Heslop-Harrison, J (1971). "The pollen wall: structure and development". In Heslop-Harrison, J (ed.). Pollen development and physiology. London: Butterworths. pp. 75–98. ISBN 9780390437402.
  7. ↑ Erdtman G, Berglund B, Praglowski J (1961). "An introduction to a Scandinavian pollen flora". Grana Palynologica. 2 (3): 3–86. doi:10.1080/00173136109428945.
  8. 1 2 3 4 Verstraete B, Groeninckx I, Smets E, Huysmans S (2011). "Phylogenetic signal of orbicules at family level: Rubiaceae as case study". Taxon. 60 (3): 742–757. doi:10.1002/tax.603010.
  9. 1 2 3 4 Verstraete B, Moon HK, Smets E, Huysmans S (2014). "Orbicules in flowering plants: a phylogenetic perspective on their form and function". The Botanical Review. 80 (2): 107–134. doi:10.1007/s12229-014-9135-1. S2CID 255562584.
  10. ↑ Vinckier S, Smets E (2003). "Morphological and ultrastructural diversity of orbicules in Gentianaceae". Annals of Botany. 92 (5): 657–672. doi:10.1093/aob/mcg187. PMC 4244851. PMID 14500324.
  11. ↑ Oak MK, Yang S, Choi G, Song JH (2022). "Systematic palynology in Korean Piperales with special focus on its exine surface ornamentation and orbicule morphology". Scientific Reports. 12 (1): 4142. doi:10.1038/s41598-022-08105-3. PMC 8907175. PMID 35264735.
  12. ↑ El-Ghazaly G, Jensen WA (1986). "Studies of the development of wheat (Triticum aestivum) pollen. I. Formation of the pollen wall and ubisch bodies". Grana. 25: 1–29. doi:10.1080/00173138609429929.
  13. ↑ Christensen JE, Horner Jr HT, Lersten NR (1972). "Pollen wall and tapetal orbicular wall development in Sorghum bicolor (Gramineae)". American Journal of Botany. 59 (1): 43–58. doi:10.2307/2441229. JSTOR 2441229.
  14. ↑ Furness CA, Rudall PJ (2001). "The tapetum in basal angiosperms: early diversity". International Journal of Plant Sciences. 162 (2): 375–392. doi:10.1086/319580. S2CID 84338737.
  15. ↑ Moon HK (2018). "The phylogenetic potential of orbicules in angiosperms". Korean Journal Plant Taxonomy. 48: 9–23. doi:10.11110/kjpt.2018.48.1.9.
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