A histological study of microsporogenesis in Tarenna gracilipes (Rubiaceae)

We studied the microsporogenesis in Tarenna gracilipes (Hayata) Ohwi, with special attention to the mode of exine deposition and tapetum development. We based this research on light (LM), scanning electron (SEM) and transmission electron microscopic (TEM) observations of developing anthers of T. gracilipes, from the microspore mother cell stage towards anther dehiscence. Evidence is supplied that the microsporogenesis in T. gracilipes can be considered as simultaneous. Columellae, foot layer and tectum develop in a fibrillar matrix. Similar with earlier studies in Rubiaceae species, a single white line formed near the plasmalemma in the extra-apertural region. The developing endexine dilated into several white line centered lamellae at the apertures. An annulus is formed around the inner surface of each pore. In the mature intine two strata can be distinguished. At the apertures thick onci are formed protruding through the apertures thereby forming papillae, a common feature in Rubiaceae. In Rubiaceae species amoeboid as well as secretory tapeta are reported. In T. gracilipes it is shown that the tapetum cells possess in all developmental stages characteristics of the secretory type. During microsporogenesis the tapetum cytoplasm undergoes considerable changes which may indicate cycles of hyperactivity. Sporopollenin deposition on the pre-orbicules is mediated by white lines showing correlations with endexine, annulus and columellae ontogeny. These findings corroborate the idea that orbicule wall development can represent a model to study sporopollenin deposition. At anther dehiscence Ca-oxalate crystals are released out of the ruptured septum cells into the locule, providing a possible visual signal for pollinators.