A CYTOLOGICAL ANALYSIS OF MICROSPORES OF TRITICUM-AESTIVUM (POACEAE) DURING NORMAL ONTOGENY AND INDUCED EMBRYOGENIC DEVELOPMENT

Uninucleate microspores of Triticum aestivum cv. Pavon can be induced in vitro to alter their development to produce embryoids rather than p ollen. Microspores expressed their embryogenic capacity through one of two division pathways. In the more common route, the first sporophyti c division was asymmetric and produced what appeared to be a typical b icellular pollen grain. Here the generative cell detached from the int ine, migrated to a central position in the pollen grain, and underwent a second haploid mitosis as the vegetative cell divided to give rise to the embryoid. In the second pathway, the first division was symmetr ic and both nuclei divided repeatedly to form the embryoid. This compa rative analysis of normal pollen ontogeny and induced embryogenesis pr ovided no evidence for the existence of predetermined embryogenic micr ospores in vitro or in vivo. Instead, microspores are induced at the t ime of culture, and embryogenesis involves continued metabolic activit y associated with the gradual cessation of the gametophytic pathway an d a redifferentiation into the sporophytic pathway. In conjunction wit h a previous study, it appears that embryogenic induction of wheat mic rospores involves switching off gametophytic genes and derepressing sp orophytic genes.