Expression of ASK1-like genes in arrested stamens of female Silene latifolia plants.

Silene latifolia is a dioecious plant in which sex is determined by heteromorphic sex chromosomes. In female plants, stamen development is arrested before microspore mother cells are formed. In this study, we isolated four cDNAs (SlSKP1-1 to 4) encoding ASK1-like protein as expression markers to reveal when expression levels are reduced in arrested stamens of female flowers. Expression patterns of the SlSKP1 genes were analyzed by in-situ hybridization. We use the flower development classification of Grant et al. (in Plant J 6:471-480, 1994). SlSKP1 genes were highly expressed in primary parietal cells and primary sporogenous cells that develop into microspore mother cells in male flowers. Expression levels started to be reduced in the external stamens of the female flowers when stamen development was arrested at stage 7. Although microspore mother cells could not be developed in female flowers and SlSKP1 expression may be unnecessary in arrested stamens, SlSKP1 genes were still expressed in sporogenous cells of degenerated stamens at stage 8. Parietal cells stopped differentiating earlier than sporogenous cells in arrested stamens. These results suggest that not all types of cell are arrested simultaneously at a particular stage of stamen development during stamen suppression of S. latifolia.