An ultrastructural, cytochemical and immunofluorescence study of postmeiotic development of plasmodial tapetum inTradescantia virginiana L. and its relevance to the pathway of sporopollenin secretion

Abstract

Establishment of a tapetal plasmodium in postmeiotic stages in anther locules ofTradescantia virginiana encloses the tetrads in membrane-limited compartments. The perispore membrane (PSM), around each tetrad, is derived from composite tapetal cell plasma membranes. The tapetum acquires an abundance of ER and ribosomes and by the late tetrad stage the PSM and its underlying cytoplasm exhibit specialized features, studied here by ZnIO impregnation, osmium maceration, application of indirect immunofluorescence employing antitubulin, conventional thin sectioning and the Thiery reaction. These features include: labyrinthine convolutions of the PSM resulting from migration of membranous sacs and their partial fusion to the PSM, an intimate relationship of tubular ER with the convoluted PSM, and microtubules underlying the PSM and among the membranous sacs. At the same time membrane-bound granules, comparable to but smaller and simpler than tapetal orbicules of secretory tapeta, form in the convolutions. It is postulated that the ER supplies precursors of sporopollenincontaining parts of the spore wall, that the PSM-associated microtubules stabilise the whole secretory apparatus at the tapetum-spore interface, and that the precursors are expelled into the lumen bounded by the PSM and then accreted upon the orbicule-like granules or the developing spore wall. With dissolution of the callosic wall, the plasmodium invades the intermicrosporal spaces of late tetrads, the PSM unfolding its elaborations and becoming closely appressed to the exinous surfaces of individual spores. Microtubules, although present during this phase of invasion, do not seem to propel the invasion processes and may have roles in shape maintenance. During pollen mitosis and enlargement the tapetal cytoplasm accumulates lipidic globules. A late phase of Golgi activity precedes accumulation of vesicles or vacuoles near the spores, these being bounded by single or multiple tripartite membranes. With anther desiccation, portions of plasmodium are deposited on the pollen surface in the form of tryphine, the deposits containing stacked membrane-like bilayers.