DEVELOPMENTAL BASIS OF HOMEOSIS IN PRECOCIOUSLY GERMINATING BRASSICA-NAPUS EMBRYOS - PHASE-CHANGE AT THE SHOOT APEX

Precociously germinating Brassica napus (oilseed rape) embryos produce extra cotyledons or chimeric organs with sectors of cotyledon and lea f tissue, rather than leaves, at the shoot apex, To investigate this p henomenon in more detail, scanning electron microscopy was used to exa mine the development of organ primordia at the shoot apex, In situ hyb ridizations with molecular markers of the embryonic phase were used to assess the status of individual cells in the shoot apex with regard t o the transition between embryonic and vegetative phases, The results indicate that, under conditions that support precocious germination, p rimordia develop at the shoot apex in the mode characteristic of postg erminative growth, i,e. they arise sequentially in a spiral phyllotaxy , Cells in the rest of the embryo, however, can continue to express mo lecular markers of the embryonic phase for several weeks after the sta rt of culture, When patterns of gene expression and the fate of indivi dual primordia were compared, a strong correlation was found between o rgan identity and the status of cells in the vicinity of the meristem with regard to phase, Primordia that develop in situations where neigh boring cells are in the embryonic phase always produce organs with cot yledon morphology. Primordia that develop in situations where neighbor ing cells have exited the embryonic phase produce leaves. Based on an examination of situations where chimeric organs are produced, I propos e that short range interactions or signalling are likely to be involve d in communicating information about phase to developing primordia.