ULTRASTRUCTURE OF MICROSPOROGENESIS AND MICROGAMETOGENESIS IN ARABIDOPSIS-THALIANA (L) HEYNH ECOTYPE WASSILEWSKIJA (BRASSICACEAE)

The process of microsporogenesis and microgametogenesis was studied at the ultrastructural level in wild-type Arabidopsis thaliana ecotype W assilewskija to provide a basis for comparison with nuclear male-steri le mutants of the same ecotype. From the earliest stage studied to mat ure pollen just prior to anther dehiscence, microsporocyte/microspore/ pollen development follows the general pattern seen in most angiosperm s. The tapetum is of the secretory type with loss of the tapetal cell walls beginning at about the time of microsporocyte meiosis. Wall loss exhibits polarity with the tapetal protoplasts becoming located at a distance from the inner tangential walls first, followed by an increas e in distance from the radial walls beginning al the interior edge and progressing outward. The inner tangential and radial tapetal walls ar e completely degenerated by the microspore tetrad stage. Unlike other members of the Brassicaceae that have been studied, the tapetal cells of A. thaliana Wassilewskija also lose their outer tangential walls, a nd secretion occurs from all sides of the cells. Exine wall precursors are secreted from the tapetal cells in a process that appears to invo lve dilation of individual endoplasmic reticulum cisternae that fuse w ith the tapetal cell membrane and release their contents into the locu le. Following completion of the exine, the tapetal cell plastids devel op membrane-bound inclusions with osmiophilic and electron-transparent regions. The plastids undergo ultrastructural changes that suggest br eakdown of the inclusion membranes followed by release of their conten ts into the locule prior to the complete degeneration of the tapetal c ells.