Abstract
Local, efflux-dependent auxin gradients and maxima mediate organ and tissue development in plants. The auxin-efflux pattern is regulated by dynamic expression and asymmetric subcellular localization of PIN auxin-efflux proteins during plant organogenesis. Thus, the question of how the expression and subcellular localization of PIN proteins are controlled goes to the heart of plant development. It has been shown that PIN expression and polarity are established not only through a self-organizing auxin-mediated polarization mechanism, but also through other means such as cell-fate determination. We found that the Arabidopsis NO VEIN (NOV) gene, encoding a novel, plant-specific nuclear factor, is required for leaf vascular development, cellular patterning and stem-cell maintenance in the root meristem, and cotyledon outgrowth and separation. NOV function underlies cell-fate decisions associated with auxin gradients and maxima, thereby establishing cell-type-specific PIN expression and polarity. We propose that NOV mediates cell acquisition of the competence to undergo auxin-dependent coordinated cell specification and patterning, thereby educing context-dependent auxin-mediated developmental responses.