Secretory events in the freeze-substituted tapetum of the orchid Pterostylis concinna

Abstract

Three distinct secretory events in tapetal cells correspond to key processes of microspore development: microspore release and expansion, exine development, and pollen-coat formation and dispersal. First, at the tetrad stage, the lower part of the radial and the inner tangential tapetal walls contain protein and are markedly thick and fibrillar. The subsequent absence of this extra wall after microspore release indicates that it may contain 1,3-β glucanase, active in callose dissolution. Using freeze-substitution to investigate tapetal development demonstrates that the secretory tapetal cell of Pterostylis concinna is functional until exine formation. Tapetal cell material then reorganizes for two further secretory events: second, the exine of the young microspore is developed, tapetal cells lose their characteristic shape, the plasma membrane is no longer detectable, and the tapetal contents flow out into the locule. Third and final tapetal release is electron-opaque pollen-coat material occurring before microspore mitosis.