Structure and development of epiphylly in knox-transgenic tobacco.

Anatomical modifications and developmental patterns of tobacco (Nicotiana tabacum L.) plants transgenic for the barley (Hordeum vulgare L.) homeo box genes bkn-1 or bkn-3 were analysed and the morphogenetic processes interpreted. No appreciable difference between bkn-1 and bkn-3 transgenic tobacco was observed in the examined transgenic plants. They were dwarfed and developed epiphyllous appendages on all leaves except the cotyledons. Two major categories of epiphyllous structure were recognised: (i) vegetative shoots on juvenile heart-shaped leaves and (ii) inflorescences or flowers on adult lanceolate leaves. Both types of epiphyllous structure originated in the subepidermal layers, either directly from individual meristems or from an expanded meristematic "cushion". Structural analysis confirmed that the occurrence of epiphylls in the tobacco transformants represented a switch from determinate to indeterminate leaf growth and reproduced the phenotype of the barley Hooded mutant, which had earlier been shown to be associated with the bkn-3 gene [K.J. Muller et al. (1995) Nature 374:727-730].