Prions

plants are cloaked in hundreds of tiny projections, known as trichomes. These structures vary in function, shape, size and spacing from species to species, and it’s not known whether similar cellular differentiation programs are responsible for making trichomes in different plants. Trichomes are clearly visible in the scanning electron micrograph of a wild-type Nicotiana tabacum seedling above (bottom left). Petal epidermis is textured by different projections, called floral papillae, which are shown above at middle left. When the transcription factor MIXTA (which regulates papillae in Antirrhinum majus) is over-expressed in Nicotiana, petal papillae elongate to produce trichome-like structures and seedling leaves produce extra trichomes, as shown in the right-hand images. Although these specialized epidermal cells have nearly opposite functions — floral papillae attract pollinators by enhancing pigment perception, whereas leaf trichomes probably discourage hungry insects — these results suggest that, in Nicotiana, similar pathways regulate their development. For more details, see Payne T. et al., Development 1999, 126:671-682. Images reproduced with permission from Development and provided by Thomas Payne, Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712, USA. Quick guide