Initiation of primexine in freeze-substituted microspores of Brassica campestris

Abstract

The processes involved in initiating the primexine were investigated during development of tetrads of microspores in Brassica campestris anthers using rapid freeze-substitution technology. The first event is the appearance of the primexine matrix. The second event is convolution of the microspore plasma membrane, followed by insertion of an electron opaque “spacer” into the plasma membrane crypts. A convoluted microspore plasma membrane is only recorded in those species where the final exine pattern is reticulate, comprising foot layer, bacula, and tectum. Our hypothesis is that the spacers demarcate the future interbacular cavities of the exine, so that the membrane peaks are the sites for probacula formation.