Orbicule

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]

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]

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]

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]

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]

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]

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]