TRANSFER CELL INDUCTION IN COTYLEDONS OF VICIA-FABA L

Immediately prior to seed fill, a dermal transfer cell complex, compri sed of epidermal and subepidermal cells, differentiates on the abaxial surface of the cotyledons in seed of Vicia faba. Over the period of d ifferentiation of this complex in vivo, the principal sugars of the se ed apoplasmic sap change from hexoses, glucose and fructose, to sucros e. Cotyledons were removed from seeds before differentiation of the tr ansfer cell complex and cultured for 6 days on an agar-based medium in the dark with their abaxial surface in contact with a medium containi ng either 100 mM hexoses (glucose and fructose in equimolar concentrat ions) or 100 mM sucrose. On both media, cotyledon growth rate was main tained throughout the culture period at, or above, that of in vivo gro wn cotyledons of equivalent developmental age. When cotyledons were cu ltured on a medium containing glucose and fructose, epidermal cells of both the ab-and adaxial surfaces developed wall ingrowths on their ou ter periclinal walls and their cytoplasm became dense, vesicular, and rich in mitochondria. Extensive ingrowth deposition also occurred on w alls of the subepidermal cells and several rows of underlying storage cells where they abutted intercellular spaces. This latter ingrowth de velopment was apparent on both cotyledon surfaces, but extended into m ore of the underlying cell layers on the abaxial surface at the funicu lar end of the cotyledon. In in vivo grown cotyledons, such ingrowth d evelopment is restricted to the subepidermal cells of the abaxial surf ace. Ingrowth morphology was commensurate with that of transfer cells of in vivo grown cotyledons. In contrast to the observed induction on a medium containing glucose and fructose, cotyledons cultured with suc rose as the sole sugar source exhibited no ingrowth deposition or smal l wall ingrowths in some abaxial epidermal cells. While the potential sugar signalling mechanism is unknown, this culture system offers an e xciting opportunity to explore the molecular biology of transfer cell development.